Atomic spectroscopy methods are used for mass, fast, selective and fairly accurate determinations of small element contents.
For the simultaneous determination of several elements, as well as qualitative analysis, the best methods are atomic emission and X-ray fluorescence spectroscopy.
The use of various sources of atomization makes it possible to determine both the main and impurity components by atomic emission spectroscopy, and to analyze solutions and solid samples. The most important advantage of X-ray fluorescence spectroscopy is that it is a non-destructive method of analysis, which is very important, for example, when analyzing works of art, archaeological objects, etc. X-ray emission, photo and Auger electron spectroscopy methods are used for local analysis and analysis of the surface of solids.
The method of atomic absorption spectroscopy has also found wide application. This method can determine about 6 - 70 elements, mainly metals, at very low concentrations. However, atomic absorption spectroscopy is advisable to use only for single-element analyses. Ionometry (direct potentiometry) differs from other physicochemical methods in the simplicity of its methods and the low cost of measuring instruments. Modern portable ionometers make it possible to determine a variety of ions and dissolved gases not only in the laboratory, but also in the field. The areas of application of the methods are very diverse. This is an analysis of environmental objects, food, medicines, products of the metallurgical, construction, glass industries, and geological samples. Atomic spectroscopy is also used in the field (using portable X-ray fluorescence spectrometers).
Atomic spectroscopy methods are based on transitions of valence (Fig. 1.1, a-c) or internal (Fig. 1.1, d-g) electrons of atoms from one state to another.
Figure 1.1. Schemes of processes underlying spectroscopy methods: a atomic emission; b-atomic absorption; c - atomic fluorescent; g - X-ray photoelectron; d - Auger electron; e-X-ray fluorescence analysis; x-ray emission analysis. Electron energy levels: a-b -valence; Ms - internal emission of one or more electrons by an atom (ionization)
Therefore, in the methods of atomic spectroscopy, it is possible to register both electromagnetic and electronic spectra - distributions, respectively, of photons and emitted electrons according to their energies.
One of the features of atomic spectra is their line structure. The positions of the lines are individual for each element and can be used for qualitative analysis. Quantitative analysis is based on the dependence of the spectral line intensity. The likelihood of lines of different elements overlapping is relatively low. Therefore, many atomic spectroscopy techniques can be used to detect and determine several elements simultaneously.
Depending on the wavelength range of electromagnetic radiation used and the nature of the corresponding electronic transitions, atomic spectroscopy methods are divided into optical and x-ray.
Optical spectroscopy methods use radiation from the visible and UV regions of the optical range. It corresponds to a change in the energy of the valence electrons. To obtain optical atomic spectra, preliminary atomization of the sample is necessary—transferring it into a gaseous atomic state. For this purpose, atomizers are used - sources of high temperature of various designs. The interaction of a substance with radiation in the optical range, as a rule, is not accompanied by ionization of atoms. Therefore, only electromagnetic radiation spectroscopy methods are typical for the optical range. These include methods of atomic emission (AES), atomic fluorescence (AFS), and atomic absorption (AAS) spectroscopy.
X-ray spectroscopy methods use X-ray radiation corresponding to a change in the energy of internal electrons. The structures of the energy levels of internal electrons in the atomic and molecular states are very similar. Therefore, in X-ray methods, sample atomization is not required.
However, the interaction of a substance with X-ray radiation is always accompanied by ionization of atoms.
Such ionization occurs under the influence of an external source of X-ray radiation or a beam of high-energy electrons. The electron emitted by an atom due to ionization is called a photoelectron or, accordingly, a secondary electron. As a result of intra-atomic electronic transitions, the emission of another electron, called an Auger electron, is possible. When using X-ray radiation, it is possible to record both electromagnetic and electronic spectra.
X-ray methods of spectroscopy of electromagnetic radiation include X-ray emission analysis (XEA), X-ray fluorescence (XRF) and X-ray absorption (RAA) analysis, and methods of electron spectroscopy include X-ray photoelectron (XPS) and Auger electron (AES) spectroscopy.
Depending on the physical nature of the process of interaction of radiation with matter, methods of atomic spectroscopy of electromagnetic radiation (both optical and x-ray ranges) are divided into emission and absorption. In optical emission methods, to obtain the emission spectrum, it is necessary to first transfer the atoms to an excited state. For this purpose, devices called excitation sources, high temperature sources (for optical methods), streams of high-energy particles (for X-ray methods), and electromagnetic radiation are used. Emission optical methods, in which the excitation of atoms occurs under the influence of high temperature, are called atomic emission spectroscopy (AES) methods. In these methods, the atomizer is the source of excitation. If the source of excitation is electromagnetic radiation, the methods are called fluorescent - atomic fluorescence spectroscopy (AFS), X-ray fluorescence analysis (XRF).
In absorption methods, excitation of atoms is not required; there are no excitation sources.
The classification of the main methods of atomic spectroscopy is given in Table. 1
Table 1. Classification of the main spectroscopy methods
|
Electromagnetic range radiation |
|||||
|
atomization |
excitement |
registration |
|||
|
Nuclear emission (NPP) |
Optic |
Photon emission |
High temperature |
High temperature |
Electromagnetic |
|
Atomic fluorescence (AFS) |
Electromagnetic radiation (UV-visible) |
||||
|
Atomic absorption (AAS) |
Photon absorption |
Not required |
|||
|
X-ray emission (REA) |
X-ray |
Photon emission |
Not required |
Electron flow |
|
|
X-ray fluorescence (XRF) |
|||||
|
X-ray absorption (RAA) |
Photon absorption |
Not required |
|||
|
X-ray photoelectron (XPS) |
Registration of electronic spectrum with kinetic energy up to 1500 eV |
Electron emission |
Electromagnetic radiation (X-ray) |
Electronic |
|
|
Auger electron (OES) |
Electron flow |
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PHYSICAL EFFECT
Atomic emissionspectroscopy, NPP or atomic emission spectral analysis - a set of elemental analysis methods based on the study of the emission spectra of free atoms and ions in the gas phase in the wavelength range 150-800 nm.
Atomic emission spectral analysis is a method for determining the chemical composition of a substance from the emission spectrum of its atoms under the influence of an excitation source (arc, spark, flame, plasma).
Atoms are excited when one or more electrons move to a more distant energy level. In the normal state (unexcited), the atom has the lowest energy E 0 . An atom can be in an excited (unstable) state for a very short time (? 10 -7 - 10 -8 sec) and always strives to occupy a normal, unexcited state. In this case, the atom gives off excess energy in the form of photon radiation.
where E 2, E 1 - energy of the upper and lower levels; n - frequency; c is the speed of light; l is the radiation wavelength; h is Planck's constant.
In order for an atom to move to a higher energy level, it needs to transfer energy called excitation potential. The minimum energy required to remove its outer valence electron from an unexcited atom is the ionization potential (excitation energy).
A spectral line is radiation of any one wavelength, corresponding to a certain energy transition of an excited atom.
The intensity of the spectral line (I) is directly proportional to the number of excited particles (N *), because the excitation of atoms is of a thermal nature. Excited and unexcited atoms are in thermodynamic equilibrium with each other, which is described by the Boltzmann equation:
Where N 0 - number of unexcited atoms; g*, g 0 - static weights of excited and unexcited states of atoms; E - excitation energy; k is Boltzmann's constant; T - temperature.
Thus, at a constant temperature N * is directly proportional to N 0, i.e. in fact, the total number of given atoms in the sample. The total number of atoms is directly proportional to the concentration (c) of the element in the sample.
That is, the intensity of the emission spectral line can be used as an analytical signal to determine the concentration of the element:
where a is a coefficient depending on the process conditions.
In AESA, the correct choice of atomization conditions and measurement of the analytical signal is crucial, therefore, in real AESA conditions, the Lomakin-Shaibe formula is used:
where b is a constant coefficient depending on the energy transitions caused by the radiation of a given spectral line; determines the angle of inclination of the calibration graph of the controlled element.
Since the chemical composition of the samples is controlled over a wide range of concentrations, the Lomakin-Shaibe formula is used in logarithmic coordinates:
Graph 1 Calibration graph of the dependence of the intensity of the spectral line on the concentration of the element being determined
Graph 2 Graduated characteristic for the determination of sulfur S in high-chromium steels
PRINCIPAL DIAGRAM OF NPP CONDUCT
Spectral analysis is based on the study of the structure of light that is emitted or absorbed by the substance being analyzed. Let us consider the scheme of emission spectral analysis (Fig. 1). In order for a substance to emit light, it is necessary to transfer additional energy to it. The atoms and molecules of the analyte then pass into an excited state. Returning to their normal state, they give off excess energy in the form of light. The nature of the light emitted by solids or liquids usually depends very little on the chemical composition and therefore cannot be used for analysis. Radiation from gases has a completely different character. It is determined by the composition of the analyzed sample. In this regard, during emission analysis, before excitation of a substance, it must be evaporated.
Fig 1 Schematic diagram of emission spectral analysis: 1 -- Light source; 2 -- lighting condenser; 4 -- spectral apparatus; 5 -- spectrum registration; 6 -- determination of the wavelength of spectral lines or bands; 7 -- qualitative analysis of the sample using tables and atlases; 8 -- determination of the intensity of lines or stripes; 9 -- quantitative analysis of the sample using a calibration chart
Evaporation and excitation are carried out in sources Sveta, into which the analyzed sample is introduced. High-temperature flames or various types of electrical discharge in gases are used as light sources: arc, spark, etc. To obtain an electrical discharge with the required characteristics, use generators.
High temperatures (thousands and tens of thousands of degrees) in light sources lead to the disintegration of the molecules of most substances into atoms. Therefore, emission methods are used, as a rule, for atomic analysis and only very rarely for molecular analysis.
The radiation of the light source consists of the radiation of the atoms of all elements present in the sample. For analysis, it is necessary to isolate the radiation of each element. This is carried out using optical instruments - spectral devices , in which light rays of different wavelengths are separated in space from each other. The radiation of a light source, divided into wavelengths, is called a spectrum.
The main parts of the spectral device (Fig. 2) are: entrance slit S, illuminated by the radiation being studied; collimator lens ABOUT 1, in the focal plane of which the entrance slit is located S; dispersing device D, operating in parallel beams of rays; focusing lens ABOUT 2, creating in its focal surface R monochromatic images of the entrance slit, the totality of which forms the spectrum. As a rule, either prisms or diffraction gratings are used as a dispersing element.
Fig 2 Schematic optical diagram of a spectral device (l 1< л 2 <л 3)
Spectral devices are designed in such a way that light vibrations of each wavelength entering the device form one line. How many different waves were present in the radiation of a light source, so many lines are obtained in the spectral apparatus.
Atomic spectra of elements consist of individual lines, since the radiation of atoms contains only some specific waves (Fig. 3, A). The radiation from hot solids or liquids contains light of any wavelength. Individual lines in the spectral apparatus merge with each other. Such radiation has a continuous spectrum (Fig. 3, V). In contrast to the line spectrum of atoms, the molecular emission spectra of substances that have not decayed at high temperatures are striped (Fig. 3, b). Each stripe is formed by a large number of closely spaced lines.
Rice. 3 Types of spectra
Spectrum types:
A-- ruled; b-- striped; the individual lines that make up the strip are visible; V--solid.
The darkest places in the spectrum correspond to the highest light intensity (negative image)
Light, decomposed into a spectrum in a spectral apparatus, can be viewed visually or recorded using photography or photoelectric devices. The design of the spectral apparatus depends on the method of spectrum registration. Spectroscopes - styloscopes and stylometers - are used for visual observation of spectra. Spectra are photographed using spectrographs . Spectral devices - monochromators - make it possible to isolate light of one wavelength, after which it can be recorded using a photocell or other electrical light receiver.
In a qualitative analysis, it is necessary to determine which element is emitted by a particular line in the spectrum of the sample being analyzed. To do this, you need to find the wavelength of the line by its position in the spectrum, and then, using tables, determine its belonging to one or another element. To view an enlarged image of the spectrum on a photographic plate and determine the wavelength, measuring microscopes, spectroprojectors and other auxiliary instruments are used.
The intensity of the spectral lines increases with increasing concentration of the element in the sample. Therefore, to carry out a quantitative analysis, it is necessary to find the intensity of one spectral line of the element being determined. The intensity of the line is measured either by its blackening in a photograph of the spectrum (spectrogram) or immediately by the magnitude of the light flux emerging from the spectral apparatus. The amount of blackening of the lines in the spectrogram is determined using microphotometers.
The relationship between the intensity of the line in the spectrum and the concentration of the element in the analyzed sample is established using standards - samples similar to those being analyzed, but with a precisely known chemical composition. This relationship is usually expressed in the form of calibration graphs.
Below are the emission spectra of Fe and H:
Rice. 4.1 Fe emission spectrum
Rice. 4.2 H emission spectrum
RESEARCH METHODOLOGY
The process of atomic emission spectral analysis consists of the following main parts:
1. Sample preparation (sample preparation)
2. Evaporation of the analyzed sample (if it is not gaseous);
3. Dissociation - atomization of its molecules;
4. Excitation of radiation from atoms and ions of sample elements;
5. Decomposition of excited radiation into a spectrum;
6. Spectrum registration;
7. Identification of spectral lines - in order to establish the elemental composition of the sample (qualitative analysis);
8. Measurement of the intensity of analytical lines of sample elements to be quantified;
9. Finding the quantitative content of elements using pre-established calibration dependencies.
A sample of the test substance is introduced into a radiation source, where it evaporates, dissociates molecules and excites the resulting atoms (ions). The latter emit characteristic radiation, which enters the recording device of the spectral instrument.
In qualitative AESA, the spectra of samples are compared with the spectra of known elements given in the corresponding atlases and tables of spectral lines, and thus the elemental composition of the analyzed substance is established. In quantitative analysis, the concentration of the desired element in the analyzed substance is determined by the dependence of the magnitude of the analytical signal (blackening density or optical density of the analytical line on a photographic plate; luminous flux to a photoelectric receiver) of the desired element on its content in the sample. This dependence is determined in a complex way by many difficult-to-control factors (the bulk composition of samples, their structure, dispersion, parameters of the source of excitation of the spectra, instability of recording devices, properties of photographic plates, etc.). Therefore, as a rule, to establish it, a set of samples is used for calibration, which in terms of gross composition and structure are as close as possible to the substance being analyzed and contain known quantities of the elements being determined. Such samples can be specially prepared metal alloys, mixtures of substances, solutions, including standard samples produced by industry. To eliminate the influence on the analysis results of the inevitable differences in the properties of the analyzed and standard samples, different techniques are used; for example, the spectral lines of the element being determined and the so-called reference element, which is close in chemical and physical properties to the element being determined, are compared. When analyzing materials of the same type, you can use the same calibration dependencies, which are periodically adjusted using verification samples.
The sensitivity and accuracy of AESA depend mainly on the physical characteristics of the sources of excitation of spectra - temperature, electron concentration, residence time of atoms in the zone of excitation of spectra, stability of the source mode, etc. To solve a specific analytical problem, it is necessary to select a suitable radiation source and optimize its characteristics using various techniques - the use of an inert atmosphere, the application of a magnetic field, the introduction of special substances that stabilize the discharge temperature, the degree of ionization of atoms, diffusion processes at an optimal level, etc. Due to the variety of mutually influencing factors, methods of mathematical planning of experiments are often used.
The results presented below include drawings that illustrate how different the spectra of different elements (in this example, aluminum, copper, tungsten and iron) are from each other.
The ordinate axis shows intensity I in arbitrary units. The abscissa shows the wavelength l in nanometers, the spectral range is 172-441 nm. The spectra were taken on a spark spectrometer:
Rice. 5.1 AL emission spectrum
Rice. 5.1 Emission spectrum of Cu
Rice. 5.1 Emission spectrum of W-alloy
Rice. 5.1 Fe emission spectrum
CLASSIFICATION OF NPP METHODS
After receiving the spectrum, the next step is its apolitical assessment, which can be carried out using an objective or subjective method. Objective methods can be divided into indirect and direct. The first group covers spectrographic, and the second - spectrometric methods. In the spectrographic method, photoemulsion allows one to obtain an intermediate characteristic of the line intensity, while the spectrometric method is based on direct measurement of the intensity of the spectral line using a photoelectric light detector. In the subjective evaluation method, the sensitive element is the human eye.
Spectrographicanalysis
The spectrographic method consists of photographing the IR spectrum of suitable plates or film using an appropriate spectrograph. The resulting spectrograms can be used for qualitative, semi-quantitative and quantitative analyses. When excitation and photographing spectra of samples of various materials, it is necessary to strictly adhere to the relevant instructions. Organizational issues of creating and operating a spectrographic laboratory should also be discussed.
Spectrographic methods of spectral analysis are of particular importance. This is mainly due to the high sensitivity of the photographic emulsion and its ability to integrate light intensity, as well as the enormous amount of information contained in the spectrum and the ability to store this information for a long time. The necessary instruments and other equipment are relatively inexpensive, the cost of materials is low, the method is simple and easy to standardize. Spectrographic spectral analysis is suitable for routine analysis and scientific research. Its disadvantage is that, due to the laboriousness of photographic operations, it is not suitable for rapid analysis, and its accuracy is lower, for example, than the accuracy of spectrometric or classical chemical analysis. This is not always the case when determining trace elements. It is hoped that spectrographic analysis will receive great development, especially in the field of processing the huge amount of useful information contained in the spectrum, using an automatic microphotometer connected to a computer.
Spectrometricanalysis
The spectrometric analytical method differs from the spectrographic method essentially only in the method of measuring the spectrum. While in spectrographic analysis the intensity of the spectrum is measured through an intermediate photography step, spectrometric analysis is based on direct photometry of the intensity of spectral lines. Direct intensity measurement has two practical advantages: due to the absence of time-consuming processing of photographed spectra and associated sources of error, both the speed of analysis and the reproducibility of its results significantly increase. In spectrometric analysis, the operations of sampling, preparation and excitation of spectra are identical to the corresponding operations of the spectrographic method. The same applies to all processes occurring during excitation, and spontaneous or artificially created effects. Therefore they will not be discussed further here. The optical setup used in the spectrometric method, including the radiation source, its display, the entire dispersive system and spectrum acquisition, is almost identical to the spectrographic setup. However, a significant difference that deserves separate discussion is the method of supplying the light energy of spectral lines to the photoelectric layer of the photomultiplier. The final operation of analysis, namely measurement, is completely different from the corresponding operation of the spectrographic method. Therefore, this stage of analysis requires detailed discussion.
Visualanalysis
The third group of emission spectral analysis methods includes visual methods, which differ from spectrographic and spectrometric methods in the way the spectrum is assessed and, except in rare cases, the spectral region used. Spectrum evaluation method subjective as opposed to the objective ways of the other two methods. In visual spectroscopy, the light receiver is the human eye and uses the visible region of the spectrum from approximately 4000 to 7600 A*.
In visual methods of spectral analysis, the preliminary preparation of samples and the excitation of their spectra are essentially no different from similar operations of other methods of spectral analysis. At the same time, the decomposition of light into a spectrum is carried out exclusively using a spectroscope. Finally, due to the subjectivity of the assessment method, visual techniques differ significantly from spectrographic and especially spectrometric techniques. This also means that of the three spectral analysis methods, visual has the least accuracy.
The detection limit of the visual method is relatively high. The most sensitive lines of elements, with the exception of alkali and alkaline earth elements, are in the ultraviolet region of the spectrum. Only relatively weak lines of the most important heavy metals are located in the visible region. Therefore, their detection limit by the visual method is usually ten to a hundred times worse. Except in very rare cases, the visual method is not suitable for identifying non-metallic elements, since their lines in the visible region are especially weak. In addition, excitation of non-metallic elements requires special complex equipment and the intensity of the light source is insufficient to evaluate the spectral lines with the naked eye.
In contrast to the disadvantages noted above, the great advantage of the visual method is its simplicity, speed and low cost. The spectroscope is very easy to operate. Although spectrum estimation requires some training, basic analyzes can be learned quickly. Spectra can be assessed with the naked eye without the difficulties inherent in indirect methods. This method is express: it usually takes no more than a minute to determine one component. The cost of relatively simple ancillary equipment for the visual method is low, and the costs of sample processing instrumentation, counter electrode materials, and electrical energy are also negligible. The techniques are so simple that with some training, the tests can be performed by unqualified laboratory technicians. Due to the high speed of the method, labor costs per analysis are low. The economic efficiency of the method also increases due to the fact that the analysis can be carried out without destroying the analyzed sample and at the place where it is located. This means that portable instruments can be used to analyze, without on-site sampling, intermediate products (e.g. metal rods), finished products (e.g. machine parts) or already installed products (e.g. superheated steam boiler tubes). Tools and time are also saved, organizational work is simplified and there is no need for destructive sampling methods.
The most important area of application of the visual spectral analysis method is the monitoring of metal alloys and mainly alloy steels during their production for the purpose of sorting. The method is also used to classify metals or alloy steels when selecting valuable materials from scrap metal. In other areas, for example in the analysis of dielectric materials, the visual method does not yet play a significant role. However, it is assumed that after improvement it may find application in this and similar areas.
SOURCES OF EXCITATION OF SPECTRA
In the practice of atomic emission spectral analysis, flames, electric arcs of direct and alternating current, low- and high-voltage condensed spark, low-voltage pulse discharge, various forms of glow gas discharge, etc. are used as sources of excitation of spectra. Over the past 10-15 years, they have become widespread various types of high-frequency discharges: high-frequency inductively coupled plasma (ICP) in an atmosphere of inert gases at atmospheric pressure, ultra-high-frequency (microwave) discharge, etc.
1 Flame
Flame is used as an atomizer and source of excitation of spectra in the method of flame photometry, as well as one of the main methods of atomizing substances in the method of atomic absorption analysis. The most commonly used flames are air-acetylene mixtures (T=2100-2400 K) and nitrogen oxide(I)-acetylene (T=3000-3200 K), less often flames of air-propane mixtures (T=2000-2200 K) and nitrogen oxide (I) - propane (T = 3000 K).
Schemes of burners used in the flame photometry method are shown in Fig. 1. The liquid to be analyzed is usually introduced into the flame by pneumatic atomization. Sprayers are mainly used of two types: angular and concentric, operating due to the vacuum created above the opening of the spraying capillary (or around it), the second end of which is immersed in the solution of the analyzed sample. The liquid flowing from the capillary is sprayed by a stream of gas, forming an aerosol. The quality of the sprayer is assessed by the ratio of the amount of liquid and gas ( M AND /M D) spent per unit of time.
The flame temperature provides a fairly low detection limit for elements whose energy, excitation of resonance lines does not exceed 5 eV; their compounds are sufficiently atomized in the flame. The flame photometry method is of particular importance for determining microquantities of compounds of alkali and alkaline earth metals, for which the detection limit by this method is in the range of 0.0001-0.01 mg/l. The high spatiotemporal stability of flames ensures good reproducibility of the results obtained by this method. When using continuous spraying of solutions, the relative standard deviation characterizing reproducibility is not at the level of 0.01 for contents exceeding the detection limit by two orders of magnitude or more.
Rice. 6 Burners for flame atomic emission spectrometry: A) And b) conventional Mecker burner and improved burner: 1 - burner body; 2 - the surface on which the flame is formed; 3 -- openings for the exit of flammable gases; 4 -- supply of a mixture of flammable gases and aerosol; 5 - protrusion on the burner body with holes; V) combined burner with separation of evaporation zones - atomization and excitation of spectra: 1 -- main burner with a projection and holes in him; 3 -- second additional burner with the same type or higher temperature flame; 4 - flame; 5 -- radiation registration zone; 6 -- supplying a mixture of combustible gases to an additional burner; 7 -- supply of a mixture of flammable gases and aerosol to the main burner
The main limitations of the flame photometry method are: the need to transfer the analyzed samples into solution, the relatively high level of matrix effects and, as a rule, single-element analysis.
Electric arc
DC electric arc
A direct current electric arc (Fig. 2) is a higher temperature source than a flame. The analyzed sample, in crushed form, is placed in a recess (channel) in the lower electrode, which, as a rule, is included as an anode in the arc circuit.
Rice. 7 DC arc as a source of excitation of spectra: A) DC arc power circuit; b)volt-ampere characteristics of a DC arc discharge; V) diagram of the transfer of atoms from the carbon electrode channel: 1 -fraction of atoms participating in the formation of the analytical signal ( 1a- removal in a free state, 1b-- removal in a bound state in the condensed phase); 2 -- release of substance beyond the excitation zone; 3a, 3b-- diffusion into the walls and bottom, respectively; 4a, 4b -- transition of a substance into the excitation zone in the form of atoms or compounds from the walls and bottom of the electrode
The temperature of the arc plasma depends on the material of the electrodes and the ionization potential of the gas in the interelectrode gap. The highest plasma temperature (~7000 K) is achieved when carbon electrodes are used. For an arc between copper electrodes, it is? 5000 K. The introduction of salts of alkaline elements (for example, potassium) reduces the temperature of the arc plasma to 4000 K.
Under the action of the arc, the end of the anode is heated to approximately 3500 K (for carbon electrodes), which ensures the evaporation of solid samples placed in the anode crater. However, the temperature of the electrode in the direction from the end drops very quickly and already at a distance of 10 mm is only about 1000 K. By giving the electrode a special shape, it is possible to reduce heat removal and thereby increase the temperature of the electrode to some extent.
In a DC carbon arc, the spectra of almost all elements are excited, with the exception of some gases and non-metals, characterized by high excitation potentials. Compared to flame emission or absorption measurements, arc discharge provides approximately an order of magnitude reduction in element detection limit, as well as a significant reduction in matrix effects.
An arc discharge is unstable, one of the reasons for this is the continuous movement of the cathode spot, which actually provides thermionic emission necessary to maintain the discharge. To eliminate arc instability, a large ballast resistance is included in its circuit R. Current flowing through the arc according to Ohm's law
Here U-- voltage of the source feeding the arc; r-- arc gap resistance.
The greater the ballast resistance R, the less the influence of fluctuations r to change the electric current of the arc. For the same reason, it is beneficial to increase the arc supply voltage (you can take a higher R). In modern generators, the arc supply voltage is usually 350 V. The arc current is usually in the range of 6-10 A. To evaporate refractory materials (for example, Al 2 O 3), an increase in current strength to 25-30 A is required. Electronic means allow you to stabilize the arc current at 25 A with fluctuations of no more than 1% when the supply voltage changes within 200-240 V, and the use of magnetic amplifiers as a control element makes it possible to increase the efficiency of the arc generator up to 90% .
To improve the conditions for excitation of spectra, use controlled atmosphere(for example, argon or other gaseous media), stabilization of the position of the plasma in space by a magnetic field (in particular, rotating) or gas flow. The use of a controlled atmosphere makes it possible to get rid of the cyanogen bands observed in the region of 340-420 nm and overlapping many sensitive lines of different elements.
AC electric arc
An arc discharge can also be powered by alternating current, but such a discharge cannot exist independently. When the direction of the current changes, the electrodes quickly cool down, thermionic emission stops, the arc gap is deionized and the discharge goes out, therefore, to maintain the arc, special ignition devices are used: the arc gap is pierced with a high-frequency pulse of high voltage, but low power (Fig. 3).
Rice. 8 Low Voltage AC Activated Arc Circuit: I -- main circuit; II-- auxiliary circuit; R-- arc power rheostat; A -- ammeter; d -- arc working span; L-- secondary coil of the high-frequency transformer; WITH-- blocking capacitor (0.5-2 µF); Tr-- step-up transformer; La --primary coil of high-frequency transformer; Sa-- activator capacitor (3000 µF); RTp-- activator resistance; da -- bit gap of the activator
The diagram of such an arc can be divided into two parts: main and auxiliary. The main part of the circuit looks exactly the same as for a DC arc, except for the shunt capacitor WITH, preventing the penetration of high-frequency currents into the network.
In the activator, a step-up transformer (120/260/3000 V, 25 W) creates a voltage of ~3000 V on the secondary winding and charges the capacitor Ca. At the moment of breakdown of the auxiliary spark gap da V circuit consisting of a coil La, capacitor Ca and arrester da, high frequency oscillations appear. As a result, at the ends of the second (high-voltage) coil L an EMF of about 6000 V appears, breaking through the working gap d. These breakdowns serve to periodically ignite the arc fed through the main circuit.
The stability of the electrical and optical parameters of an alternating current arc depends on the stability of the voltage at which breakdown occurs. Controlling the ignition based on the breakdown of the auxiliary gap does not provide the required accuracy due to oxidation and other changes in the working surfaces of the spark gap over time. More stable arc operation can be ensured by regulating the discharge ignition phase using electronic devices. Such control circuits are used in most modern generators.
To some extent, the pulsed nature of the alternating current arc leads to the fact that the discharge temperature becomes slightly higher than in a direct current arc, and measurements of the intensities of spectral lines are characterized by better reproducibility. At the same time, the control circuit can be configured in such a way that the gap is broken down not every half-cycle, but after one, two, four, etc. This allows you to regulate the heating of the electrodes, which may be necessary, for example, when analyzing low-melting alloys.
To reduce the detection limits of elements and improve the reproducibility of analysis results when working with arc discharges, the addition of certain reagents to the analyzed samples is widely used in order to initiate various kinds of thermochemical reactions directly in the channels of the arc electrodes. These reactions make it possible to convert the impurities being determined into highly volatile compounds, and the matrix elements that interfere with the determination of impurities into non-volatile compounds.
Arc in the spill option
In addition to the traditional version of the arc with vertically positioned electrodes, when analyzing powder samples (for example, ores and minerals), an arc is used in the so-called version waking up (inflating), when a finely dispersed sample does not evaporate from the channel of the carbon electrode, but wakes up (injected) through the arc discharge plasma between two (or three - with three-phase power supply) horizontally located carbon electrodes.
Rice. 9 Schematic diagram of introducing a powder sample into an arc discharge using the spill-injection method: 1 -- powder sample in a vibrating funnel; 2 -- arc electrodes; 3 -- cooling and plasma-forming air flows; 4 -- cylindrical air duct; 5 -- arc plasma; 6 -- window in the air duct for observing radiation from the working area of the arc plasma
The design and operating principle of such an arc are shown in Fig. 4. In terms of parameters and characteristics, a horizontal arc differs little from a vertical one, however, due to the fact that the sample is introduced into the arc by a gas flow (usually air), it stabilizes the shape and position of the arc plasma, which in itself helps to reduce random errors in the analysis compared to a conventional spatially unstabilized arc between vertical electrodes. In addition, with uniform injection of powders, the composition of the arc cloud remains unchanged over time, therefore, the main parameters of the arc plasma (concentration of atoms and electrons, temperature) also remain constant, which greatly simplifies the analysis. The main problems of analysis by the injection method are associated with incomplete evaporation of powder particles due to the short duration of their stay in the plasma (3 * 10-3 -5 * 10-3 s), which determines the dependence of the intensity of spectral lines on the size and composition of particles of powder samples.
Spark. Low voltage spark
Increasing the capacity of the shunt capacitor leads to the fact that the energy stored in it will play a noticeable role in the overall balance of the discharge. This type of discharge is called a low-voltage spark. Depending on the parameters of the low-voltage spark circuit, you can obtain different discharge modes: oscillatory ( CR 2 /4L<1), критический (CR 2 /4L>1), aperiodic ( CR 2 /4L?1).
The voltage on the capacitors of the discharge circuit usually varies in the range of 450-1000 V. By changing the capacitance of the capacitors, the resistance of the rheostats in the power circuit and the inductance of the secondary winding of the transformer, you can adjust the ratio between the discharge current of the capacitors and the current passing through the power circuit, and thereby smoothly change the discharge temperature in the desired direction (from soft arc mode to pure spark). Modern electronic means make it possible to stabilize the energy of single pulses with an accuracy of no worse than 0.1%.
High voltage spark
In the spectral analysis of metals and alloys, a high-voltage condensed spark is most often used as a light source (Fig. 5). Step-up transformer charges the capacitor WITH up to voltage 10-15 kV. The voltage value is determined by the resistance of the auxiliary gap IN, which in turn is always chosen to be larger than the resistance of the working gap A. At the moment of breakdown of the auxiliary gap, breakdown of the working gap also occurs simultaneously, the capacitor WITH discharges and then charges. Depending on the parameters of the circuit and the rate of deionization of the gap, the next breakdown can occur either in the same or in another half-cycle. The simplicity and reliability of this scheme ensured its successful operation.
Rice. 10 Diagram of a controlled condensed high-voltage spark: T-- step-up transformer 15000 V; WITH-- capacitor; L-- variable inductance; r-- blocking resistance; A-- working period; IN-- constant auxiliary interval; R-- adjustable resistance
At the moment of breakdown, atoms and molecules of nitrogen and oxygen in the air are excited and illuminated in a narrow spark channel; This is useless and even interfering radiation (background). However, its duration is short (10-8 s). At the next moment, the current (up to 50 A) passing through the channel heats up the small area (0.2 mm) of the electrode. The current density reaches 10 4 A/cm 2, and the electrode material is ejected into the discharge gap in the form of a torch of hot vapor, and, as a rule, not along the spark channel, but at some random angle to it.
Each new breakdown affects different areas of the surface of the sample, and after searching for the entire selected exposure time, a search spot appears on the sample with a diameter of up to 3-5 mm, but of insignificant depth (when working with a carbon counter electrode - only 20-40 microns). The total amount of solid sample evaporating during exposure is very small: for example, for steels it is usually about 3 mg.
The torch of emitted vapors has a temperature of about 10,000 K, i.e. sufficient not only to excite the spectra of metals, but also nonmetals and ions. The temperature immediately at the beginning of the spark reaches 30,000-40,000 K.
High Frequency Inductively Coupled Plasma
spectral atomic emission plasma
Thanks to the emergence of a new method for exciting spectra using a high-frequency inductively coupled plasma (ICP) source operating at atmospheric pressure, there has been a sharp leap in the development of physics, technology and practice of atomic emission spectral analysis. This source is a type of electrodeless high-frequency discharge maintained in a special burner consisting of concentrically located three (less often two) quartz tubes (Fig. 6). An external (cooling) gas flow (argon or molecular gas) is supplied into the gap between the outer and intermediate tubes, an intermediate flow (argon only) is supplied through the middle tube, and an aerosol of the analyzed solution is transported into the plasma through the central tube. The open end of the torch is surrounded by a water-cooled induction coil connected to an RF generator. To produce plasma, RF generators with a power consumption of 1.5-5 kW and an operating frequency in the range from 27 to 50 MHz are used.
Rice. 11 Burner diagram for high-frequency induction discharge: 1 -- analytical zone; 2 -- primary radiation zone; 3 -- discharge zone (skin layer); 4 -- central channel (preheating zone); 5 -- inductor; 6 -- a protective tube that prevents breakdown of the inductor (installed only on short burners); 7, 8, 9 -- external, intermediate, central tubes, respectively
To initiate a discharge, preliminary ionization of the gas is necessary, since the voltage across the inductor is significantly less than the breakdown voltage of the working gas. For this purpose, a high-voltage spark (Tesla coil) is most often used. A discharge occurs in the ionized gas, powered by a magnetic field. A high frequency current flowing through the solenoid coil creates an alternating magnetic field. Under its influence, a vortex electric field is induced inside the coil. The eddy electric current heats and ionizes portions of gas arriving from below due to Joule heat. The conductive plasma is analogous to the short-circuited secondary winding of a transformer, the magnetic field of which compresses the ring current into a torus (skin effect).
The argon flow supplied into the gap between the intermediate and outer tubes, on the one hand, serves as a plasma-forming gas, and on the other, it presses the hot plasma away from the burner walls, protecting them from overheating and destruction. The aerosol of the analyzed sample spreads along the central channel of the discharge, practically without touching the electrically conductive skin layer and without affecting its characteristics; This is one of the main features of the ICP discharge, which distinguishes it, for example, from arc plasma torches.
Typically, an aerosol formed by a solution of the sample in an aqueous or organic solvent is injected into the plasma. Along with this, the introduction of samples is used in the form of condensates formed during the evaporation of a sample in an electrothermal atomizer, arc, spark, laser torch plasma, as well as in the form of fine powders suspended in a gas or liquid flow. To introduce liquid samples, various designs of pneumatic sprayers are used (Meinhard concentric sprayer, corner sprayers, Babington sprayer, Hildebrand mesh sprayer, etc.), as well as ultrasonic sprayers. All types of nebulizers use a forced supply of sample solution using a peristaltic pump.
In ultrasonic atomizers, atomization occurs due to the energy of acoustic vibrations, and the gas flow serves only to transfer the aerosol to the burner. These nozzles produce a fine aerosol with a narrow particle size distribution. Their generation efficiency is at least 10-20 times greater than that of pneumatic sprayers, which allows for a better signal/background ratio and a lower detection limit.
We can highlight the following unconditional advantages of the ISP source in relation to problems of atomic emission spectral analysis (AESA):
1. Due to the ability to effectively excite both easily and difficultly excited lines, ICP is one of the most universal light sources in which almost all elements of the periodic table can be determined (detected). ICP is the most universal source not only in terms of the number of elements determined, but also in the type of compounds containing these elements;
2. in ICP it is possible to analyze both large masses of solutions, feeding them into the plasmatron in a continuous flow, and microvolumes (of the order of hundreds of microliters) when they are pulsedly introduced into the transport gas and pulsed recording of spectra;
3. The range of determined concentrations for most elements is 4-5 orders of magnitude, i.e. In ICP, it is possible to determine both small and medium, as well as large concentrations of a particular element, which is difficult for other sources of excitation of spectra. Calibration graphs for many elements are rectilinear, parallel to each other and have an inclination angle of about 45°, which simplifies calibration and reduces the likelihood of systematic analysis errors;
4. Due to the high excitation efficiency and low background, the detection limits of most elements are 1-2 orders of magnitude lower than in other sources of spectral excitation. The average detection limit when analyzing solutions for all elements is approximately 0.01 mg/l, decreasing for some of them to 0.001-0.0001 mg/l;
5. When all operating conditions are stabilized and optimized, the ICP torch has good spatiotemporal stability, which ensures high instrumental reproducibility of analytical signals, sometimes at the level of 0.5-1%.
The disadvantages of the ICP spectrometry method include the relatively high cost of operating spectrometers associated with a high argon flow rate (15-20 l/min). Determination of trace metal contents near the detection limit is complicated by the presence in the spectrum of molecular bands -NO and -OH in the region of 200-260 and 280-340 nm, which arise at the periphery of the discharge, at the point of contact with the atmosphere. To reduce the intensity of these bands, burners are used with an outer tube extended by 40-50 mm with a cut window for radiation output.
ICP discharge is characterized by very developed spectra, with a large number of lines belonging to atoms, as well as singly and doubly charged ions. In this regard, the use of this excitation source is complicated by the effects of spectral interference, which imposes higher requirements on the resolving power of spectral devices. Due to the lower brightness of the source, the role of scattered light in the device increases.
Spectral analysis methods are simple, easy to mechanize and automate, i.e. they are suitable for mass analysis. When special techniques are used, detection limits for individual elements, including some non-metals, are extremely low, making these techniques suitable for the determination of trace amounts of impurities. These methods are virtually non-destructive as only small amounts of sample material are required for analysis.
The accuracy of spectral analysis generally satisfies practical requirements in most cases of determining impurities and components. The cost of spectral analysis is low, although the initial investment is quite high. However, the latter quickly pays off due to the high productivity of the method and low requirements for materials and operating personnel.
Spectral analysis is not suitable for determining the types of connections between elements. Like all instrumental methods of analysis, quantitative spectral analysis is based on a comparative study of the analyzed sample and standard samples of known composition.
Spectral analysis can be considered as an instrumental research method that has found the greatest application. However, this method cannot fully satisfy the various analytical requirements that arise in practice. Thus, spectral analysis is only one laboratory method among a number of other research methods that serve different purposes. With reasonable coordination, different methods can perfectly complement each other and jointly contribute to their overall development.
To select from the methods of spectral analysis the one that is most suitable for a given task, and to obtain correct results with the selected methods, appropriate theoretical and practical knowledge, very careful and accurate work is required.
Sampling must be carried out with extreme care. Because of the high sensitivity of spectral release, conclusions about the chemical composition of very large batches of material must often be made from the results of analysis of small quantities of a sample. Contamination of the analyzed sample can significantly distort the analysis results. Appropriate physical or chemical treatment of samples, such as fusion, dissolution or pre-enrichment, can often be very beneficial.
To excite spectra, different methods require substances in different physical states or in the form of different chemical compounds. Analysis performance can have a decisive influence on the selection of the most suitable radiation sources.
The intensity ratio of the lines of an analytical pair, even for the most careful sampling method and when using the most suitable radiation source, largely depends on the external physical and chemical parameters (experimental conditions) specified by the analysis method and changing during the excitation process. Knowledge of theoretical correlations and practical conclusions from them is of great importance for fully realizing the analytical capabilities of the method.
The excited emission spectrum of the sample is recorded using a spectrograph, spectrometer or spectroscope. Therefore, methods for assessing spectra in spectral analysis can be divided into three groups.
In spectrographic qualitative analysis, a conclusion about the nature of the elements in the analyzed sample can be made based on the wavelength of the spectral lines. In quantitative analysis, the blackening of lines generally serves as a measure of their intensity and, therefore, the desired quantitative composition of the sample
The spectrometric method, in which the line intensity is usually determined using a photomultiplier and electronic measuring equipment, refers to objective methods of quantitative analysis. This method of measuring intensities is more accurate and rapid compared to spectrographic, but requires expensive and difficult-to-maintain equipment.
Spectral analysis instruments for visual spectroscopy are relatively inexpensive and can be analyzed quickly. However, these methods are based solely on subjective methods of measuring line intensity. Therefore, the results obtained are always semi-quantitative.
To achieve higher sensitivity of determination, reproducibility and accuracy, it is necessary to process measurement results using mathematical statistics methods.
When carrying out spectral analysis, tables containing the corresponding physical constants and spectroscopic constants of the elements and their most important compounds, as well as tables for auxiliary calculations and operating instructions necessary for qualitative and quantitative determinations, are of great help.
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The practical goal of atomic emission spectral analysis is quality, semi-quantitative or quantitative determination of elemental composition analyzed sample. This method is based on recording the intensity of light emitted during transitions of the electrons of an atom from one energy state to another.
One of the most remarkable properties of atomic spectra is their discreteness (line structure) and the purely individual nature of the number and distribution of lines in the spectrum, which makes such spectra an identifying feature of a given chemical element. Qualitative analysis is based on this property of the spectra. In quantitative analysis, the concentration of the element of interest is determined by the intensity of individual spectral lines, called analytical.
To obtain an emission spectrum, the electrons contained in the particles of the analyte must be given additional energy. For this purpose, a spectrum excitation source is used, in which the substance is heated and evaporated, the molecules in the gas phase dissociate into neutral atoms, ions and electrons, i.e. the substance is transferred to the plasma state. When electrons collide with atoms and ions in a plasma, the latter go into an excited state. The lifetime of particles in an excited state does not exceed 10 "-10 s s. Spontaneously returning to a normal or intermediate state, they emit light quanta that carry away excess energy.
The number of atoms in an excited state at a fixed temperature is proportional to the number of atoms of the element being determined. Therefore, the intensity of the spectral line I will be proportional to the concentration of the element being determined WITH in the sample:
Where k- a proportionality coefficient, the value of which nonlinearly depends on temperature, ionization energy of the atom and a number of other factors that are usually difficult to control during analysis.
In order to eliminate to some extent the influence of these factors on the analysis results, in atomic emission spectral analysis it is customary to measure the intensity of the analytical line relative to the intensity of a certain comparison lines (internal standard method). An internal standard is a component whose content is the same in all standard samples, as well as in the analyzed sample. Most often, the main component is used as an internal standard, the content of which can be approximately considered equal to 100% (for example, when analyzing steels, iron can serve as an internal standard).
Sometimes a component that plays the role of an internal standard is deliberately introduced in equal quantities into all samples. As a comparison line, select a line in the spectrum of the internal standard whose excitation conditions (excitation energy, temperature effect) are as close as possible to the excitation conditions of the analytical line. This is achieved if the comparison line is as close as possible in wavelength to the analytical line (YES, homologous pair.
The expression for the relative intensity of the spectral lines of two elements can be written as
where index 1 refers to the analytical line; index 2 - to the comparison line. Considering the concentration of component C2, which plays the role of an internal standard, to be constant, we can assume that A is also a constant quantity and does not depend on the conditions for excitation of the spectrum.
At a high concentration of atoms of the element being determined in the plasma, the absorption of light by unexcited atoms of the same element begins to play a significant role. This process is called self-absorption or reabsorption. This leads to a violation of the linear dependence of line intensity on concentration in the region of high concentrations. The influence of self-absorption on the intensity of the spectral line is taken into account empirical Lomakin equation
Where b- a parameter characterizing the degree of self-absorption depends on the concentration and, as it increases, changes monotonically from 1 (no self-absorption) to 0. However, when working in a fairly narrow concentration range, the value b can be considered almost constant. In this case, the dependence of the intensity of the spectral line on the concentration in logarithmic coordinates is linear:
Lomakin's equation does not take into account the influence of matrix effects on the intensity of the spectral line. This influence is manifested in the fact that often the value of the analytical signal and, consequently, the result of the analysis depend not only on the concentration of the element being determined, but also on the content of accompanying components, as well as on the microstructure and phase composition of the analyzed materials.
The influence of matrix effects is usually minimized by using standard samples that are as close as possible in size, structure, and physicochemical properties to the substance under study. Sometimes, when analyzing microimpurities, matrix effects can be avoided by using the additive method and careful homogenization of all samples.
Sources of excitation of spectra. The main sources of excitation of spectra in atomic emission spectroscopy include flame, direct or alternating current arc, spark, and inductively coupled plasma.
The most important characteristic of the spectrum excitation source is its temperature. Temperature mainly determines the probability of particles transitioning to an excited state with subsequent emission of light and, ultimately, the magnitude of the analytical signal and the metrological characteristics of the technique.
Flame . A variant of atomic emission spectroscopy using flame spectra as an excitation source is called the method flame photometry.
Structurally, the flame excitation source is a gas burner in which the analyzed sample (solution) is introduced into the flame using a nozzle. The flame consists of two zones: internal (reductive) and external (oxidative). In the reduction zone, primary reactions of thermal dissociation and incomplete combustion of the components of the combustible mixture occur. This zone contains many excited molecules and free radicals that intensely emit light in almost the entire optical range, from the UV to the IR region of the spectrum. This radiation interferes with the spectral lines of the analyte and interferes with its determination. Therefore, the reduction zone is not used for analytical purposes.
In the oxidation zone, reactions of complete combustion of the components of the gas mixture occur. The main part of its radiation occurs in the IR range and therefore does not interfere with the determination of spectral lines in the UV and visible ranges. As a result, it is the oxidation zone that is used for analytical purposes. The temperature, composition and redox properties of the flame can be adjusted within certain limits by changing the nature and ratio of combustible gas and oxidizer in the mixture. This technique is often used to select optimal conditions for excitation of the spectrum.
Depending on the nature and composition of the combustible mixture, the flame temperature can vary in the range of 1500-3000°C. Such temperatures are optimal for determining only volatile and easily excitable elements, primarily alkali and alkaline earth metals. For them, the flame photometry method is one of the most sensitive (the detection limit is up to 10 "wt.%). For other elements, the detection limits are several orders of magnitude higher.
An important advantage of the flame as a source of spectrum excitation is its high stability and the associated good reproducibility of measurement results (the error does not exceed 5%).
Electric arc. In atomic emission spectroscopy, a direct or alternating current arc can be used as a source of spectrum excitation. An arc source is a pair of vertically located electrodes (most often carbon), between which an arc is ignited. The bottom electrode has a recess into which the sample is placed. When analyzing metals or alloys, the bottom electrode is usually made of the analyte. Thus, the arc discharge is most convenient for the analysis of solid samples. To analyze solutions, they are usually evaporated together with a suitable powdered collector, and the resulting precipitate is placed in the well of the electrode.
The temperature of the arc discharge is significantly higher than the flame temperature (3000-7000°C), and for an alternating current arc the temperature is slightly higher than for a direct current arc. Therefore, atoms of most elements are effectively excited in an arc, with the exception of the most difficult to excite nonmetals, such as halogens. In this regard, for most elements, the detection limits in an arc discharge are one to two orders of magnitude lower than in a flame.
Arc excitation sources (especially direct current), unlike flame ones, are not highly stable in operating mode. Therefore, the reproducibility of the results is low (the error is 10-20%). However, for semi-quantitative determinations this is quite sufficient. The optimal application of arc excitation sources is qualitative analysis based on the survey spectrum.
Electric spark. The spark excitation source is designed absolutely similarly to the arc source. The difference lies in the operating modes of the electronic circuit. Like the arc, the spark excitation source is intended primarily for the analysis of solid samples.
The peculiarity of a spark is that thermodynamic equilibrium does not have time to be established in its volume. Therefore, talking about the temperature of the spark discharge as a whole is not entirely correct. Nevertheless, it is possible to estimate the effective temperature, which reaches a value of the order of 10,000°C. This is quite enough to excite the atoms of all currently known chemical elements.
A spark discharge is much more stable than an arc discharge, so the reproducibility of results is higher.
Inductively coupled plasma (ISP). This is the most modern source of spectral excitation, which has the best analytical capabilities and metrological characteristics for a number of parameters.
It is a plasma torch consisting of coaxially arranged quartz tubes. Especially pure argon is blown through them at high speed. The innermost flow is used as a carrier of the sample substance, the middle one is plasma-forming, and the outer one serves to cool the plasma. The argon plasma is initiated by a spark discharge and then stabilized by a high frequency inductor located at the top of the torch. In this case, a ring current of charged particles (ions and free electrons) of the plasma appears. The plasma temperature varies with the burner height and can reach 10,000°C.
The method of atomic emission spectroscopy using ICP is characterized by versatility (most elements are excited at plasma temperature), high sensitivity, good reproducibility and a wide range of detectable concentrations. The main factor limiting the widespread use of this method in analytical practice is the high cost of equipment and consumables (high-purity argon).
In Fig. Figure 9.1 presents a modern instrument for atomic emission spectral analysis with ICP as an excitation source.
Rice. 9.1.
Simultaneous measurement across the entire wavelength range ensures the highest accuracy and speed of analysis.
Methods for recording spectra. In atomic emission spectroscopy, single- and multi-channel methods for recording spectra are used. Mono- and polychromators are used to decompose the sample radiation into a spectrum. As a rule, atomic spectra contain a large number of lines, so the use of high-resolution equipment is necessary. In the flame photometry method, due to the small number of observed lines, light filters can be used instead of prism or diffraction monochromators.
The intensity of spectral lines can be measured visual, photochemical(photographic) and photovoltaic
ways. In the first case, the eye serves as a radiation receiver, in the second - a photoemulsion, in the third - a photodetector (photocell, photomultiplier, photodiode, etc.). Each method has its advantages, disadvantages and optimal areas of application.
Visual methods for recording spectra are used for massive semi-quantitative styloscopic and stylometric studies of the composition of materials, mainly metals. In the first case, a visual comparison is made of the intensities of the spectral lines of the element being determined and nearby lines of the internal standard. Due to the characteristics of the eye as a radiation receiver, with sufficient accuracy it is only possible to establish the equality of the intensities of neighboring lines, or to select the brightest line from the observed group.
Stylometric analysis differs from styloconic analysis in the presence of the possibility of controlled attenuation of the brighter line of the analytical pair. In addition, stylometers provide the possibility of bringing the compared lines closer together in the field of view. This makes it possible to more accurately estimate the ratio of the intensities of the analytical line and the comparison line.
The detection limit of elements visually is usually two orders of magnitude worse compared to other methods of recording spectra. The measurements themselves are quite tedious and not documented.
However, the great advantages of the visual method are its simplicity, high productivity and low equipment cost. It takes no more than 1 minute to determine one component. Therefore, the method is widely used for express analysis in cases where high accuracy of results is not required.
The most widely used method in atomic emission spectral analysis is the photographic method of recording spectra. It is quite simple in execution technique and is publicly available. The main advantages of photographic recording are documentary analysis, simultaneous recording of the entire spectrum, and low detection limits for many elements. In the automated version, this method acquires another advantage - enormous information content. It is not yet possible to simultaneously determine up to 75 elements in one sample by analyzing several hundred spectral lines using any other methods.
The properties of a photographic image depend on the total number of quanta absorbed by the photographic emulsion. This allows analysis to be carried out at a low signal level at the system output by increasing the exposure time. An important advantage of the method is the possibility of repeated statistical processing of photographs of spectra.
With the photographic recording method, the intensity of a spectral line is determined by the blackening (optical density) of the image of this line on a photographic plate (film). The main disadvantage of photographic materials is the nonlinear dependence of blackening on illumination, as well as the wavelength of light, development time, temperature of the developer, its composition and a number of other factors. Therefore, for each batch of photographic plates it is necessary to experimentally determine characteristic curve, i.e. dependence of the amount of blackening S from the logarithm of illumination E S =f(gE). To do this, they usually use a step attenuator, which is a quartz or glass plate coated on its surface with a set of translucent metal strips, usually made of platinum, with different but pre-known transmittance coefficients. If a photographic plate is exposed through such an attenuator, areas with varying amounts of blackening will appear on it. By measuring the amount of blackening of the area and knowing the transmittance for each of them, it is possible to construct a characteristic curve of the photographic plate. A typical view of this curve is shown in Fig. 9.2.
Rice. 9.2.
L - blackening threshold; LW - underexposure area; Sun- area of normal blackening;
CD- overexposure area
The shape of the curve does not depend on the choice of illumination units and does not change if illumination is replaced by radiation intensity, so it can be constructed by plotting the logarithms of the transmittance coefficients of the step attenuator on the abscissa axis.
The curve has a straight section Sun(area of normal blackening), within which the contrast factor
takes a constant and maximum value. Therefore, the relative intensity of two spectral lines within the region of normal blackening can be found from the relations
Photometry of spectral lines and processing of the resulting data are one of the most labor-intensive stages of atomic emission spectral analysis, which is also often accompanied by subjective errors. The solution to this problem is the automation, based on microprocessor technology, of the processes of processing photographs of spectra.
For photoelectric recording, photocells, photomultiplier tubes (PMTs) and photodiodes are used. In this case, the magnitude of the electrical signal is proportional to the intensity of the measured light flux. In this case, either a set of photodetectors is used, each of which records the intensity of only its specific spectral line (multichannel devices), or the intensity of spectral lines is sequentially measured by one photodetector when scanning the spectrum (single-channel devices).
Qualitative atomic emission analysis. Qualitative analysis is as follows:
The presence of an element in a sample is considered proven if at least four lines in the sample coincide in will length with the tabulated data for this element.
Length measurement, which is not very accurate, can be carried out using the scale of the device. More often, the resulting spectrum is compared with a known spectrum, which is usually the spectrum of iron, which contains a large number of well-studied spectral lines. To do this, the spectrum of the sample and the spectrum of iron are photographed in parallel on one photographic plate under identical conditions. There are atlases that show the spectra of iron indicating the position of the most characteristic lines of other elements, using which one can establish the nature of the elements in the sample (see work No. 34).
If the wavelengths of lines are known, for example in the spectrum of iron, between which there is a line with an unknown wavelength, the wavelength of this line can be calculated using the formula
Where X x - wavelength of the determined line, X t X Y distance from the line with wavelength l 1 to the determined line; x 2- distance from the line with wavelength l 2 to the determined line. This formula is only valid for a small range of wavelengths. The distance between lines in the spectrum is usually measured using a measuring microscope.
Example 9.1. In the spectrum of the sample between the iron lines X x = 304.266 nm and X 2 == 304.508 nm there is one more line. Let's calculate the wavelength of this line X x, if on the device screen it is removed from the first iron line by 1.5 mm, and from the second by 2.5 mm.
Solution. We use the above formula:
If the sample spectrum is not too complex, elements in the sample can be identified by comparing the sample spectrum with the spectra of standards.
Methods of quantitative analysis. Quantitative spectral analysis uses the three-standard method, the constant-graph method, and the additive method.
Using three standards method the spectra of at least three standards (samples of known concentration) are photographed, then the spectra of the analyzed samples are plotted and a calibration graph is constructed in coordinates "AS - lg C".
Example 9.2. When analyzing the contact material for chromium using the three standards method on an MF-2 microphotometer, the blackening of 5 lines of a homologous pair in the spectra of the standards and the sample under study was measured. Let's find the percentage of chromium C Cr according to the data from the table. 9.2.
Table 9.2
Data for Example 9.2
Solution. The three-standard method uses the difference dependence S blackening of lines of a homologous pair from the logarithm of the concentration of the element being determined. Under certain conditions, this dependence is close to linear. According to the readings of the measuring scale of the microphotometer, we find:
We determine the logarithms of concentrations: IgC, = -0.30; lgC 2 = 0.09; logC 3 = 0.62 and build a calibration graph in coordinates "AS- IgC" (Fig. 9.3).
Rice. 93.
Find D5 for the analyzed sample: D Sx= 0.61 - 0.25 = 0.36, and from the calibration graph we determine S l: lgC Cr = 0.35; C Cr = 2.24%.
Constant Schedule Method used for mass analyzes of homogeneous samples. In this case, knowing the contrast of photographic plates, they use the once constructed constant graph in the coordinates “D5/y - IgC”. When working in the area of normal blackening, this will be equivalent to the “lg” coordinates IJI- IgC." When working in the area of underexposure, using the characteristic curve of the photographic plate (5 = /(lg/)) for values 5 H and 5, lg/, and lg/ cp are found and a graph is plotted in the coordinates “lg/// p - IgC”. In the area of underexposure, to eliminate the curvature of the graph, it is necessary to subtract from the blackening of the lines the blackening of the background of the photographic plate, measured next to the line.
Example 9.3. To determine very small amounts of copper in a powdered material, an emission spectral analysis technique was used, which involves burning the sample three times in a direct current arc and determining the concentration from the intensity of the 3247 A copper line and from a constant “logC - log/” graph, taking into account the background.
To construct a characteristic curve of a photographic plate with sample spectra, the following data are available:
Solution. For three spectra, we calculate the difference between the copper lines and the background and find the average value:
Using the data given in the example condition, we construct the characteristic curve of the photographic plate in coordinates “D” S-lg I"(Fig. 9.4).
From the characteristic curve for 5 cp = 1.48 we find log/ = 1.38.
We build a calibration graph in “lg/ - IgC” coordinates (Fig. 9.5).
According to the calibration graph for log / = 1.38, we find logC = -3.74, which corresponds to a copper concentration in the sample of 1.8-10 4%.
Rice. 9.4.
Rice. 95.
Additive Method used in the analysis of single samples of unknown composition, when special difficulties arise associated with the preparation of standards, the composition of which must be exactly identical to the composition of the sample (matrix effect). In this method, the analyzed sample is divided into parts and the element being determined is introduced into each of them in a known concentration.
If the concentration of the determined mat element and the self-absorption effect can be neglected, then
In this case, one addition is sufficient:
If b 7^1 and I = аС b, at least two additives are needed: ( C x + WITH () And (C x + C 2). After photographing and measuring the blackening of the line on the photographic plate, a graph is drawn in coordinates "AS - lgС 7 ", where AS = 5 L - C p I = 1.2, is the concentration of the additive. By extrapolating this graph to zero, we can find the value C x.
In addition to the graphical method, the calculation method is used, especially if the number of additives is large.
Example 9.4. Let us determine the niobium content in the sample (%) using the addition method according to the data in Table. 9.3 and 9.4 (TI - comparison line).
Table 9.3
Blackening of analytical lines
Solution. Using the data given in the example condition, we construct the characteristic curve of the photographic plate (Fig. 9.6).
Rice. 9.6.
According to the characteristic curve, using the blackening of spectral lines for niobium and titanium, we find log/ Nb, log/ Tj, log(/ N .,// Ti), / Nb // Ti) (Table 9.5).
Table 9.5
Calculations for Example 9.4
|
Sample parts |
Niobium concentration in the sample |
||||
|
Original |
|||||
|
With the first addition |
C x + 0,2 |
||||
|
With the second addition |
C g + 0,6 |
We build a graph of the dependence “/ Nb // Ti - C forehead” (Fig.
Rice. 9.7.
Continuation of the graph until it intersects the x-axis allows us to determine
coordinate of the intersection point: -0.12. Thus, the concentration of niobium
in the sample C x is 0.12%.
Metrological characteristics and analytical capabilities of atomic emission spectroscopy. Sensitivity. The detection limit in atomic emission spectrum analysis depends on the method of excitation of the spectrum and the nature of the element being determined and can change significantly when the analysis conditions change. For easily excitable and easily ionized elements (alkali and most alkaline earth metals), the best source of excitation of the spectra is a flame. For most other elements, the highest sensitivity is achieved using inductively coupled plasma. High detection limits in a spark discharge are due to the fact that it is localized in a very small region of space. Accordingly, the amount of evaporated sample is small.
Range of determined contents. The upper limit of the determined contents is determined mainly by the effect of self-absorption and the associated violation of the linearity of the calibration graph. Therefore, even when constructing a calibration graph in logarithmic coordinates, the range of determined contents is usually 2-3 orders of magnitude of concentrations. An exception is the method using ICP, for which the self-absorption effect is very weak, and therefore the range of linearity can reach 4-5 orders of magnitude.
Reproducibility. In atomic emission spectroscopy, the analytical signal is very sensitive to temperature fluctuations. Therefore, the reproducibility of the method is low. The use of the internal standard method can significantly improve this metrological indicator.
Selectivity is mainly limited by the effect of spectral line overlap. Can be improved by increasing the resolution of the equipment.
Non-state non-profit educational institution of secondary vocational education "Pokrovsky Mining College"
Test
Atomic emission spectral analysis
Completed:
Group student
"Laboratory Analyst"
Profession: OK16-94
Chemical analysis laboratory assistant
Introduction
2. Atomizers
3 Flame processes
4. Quantitative atomic emission analysis
5. Spectrographic analysis
6. Spectrometric analysis
7. Visual analysis
Conclusion
Bibliography
Introduction
The purpose of practical emission spectral analysis is the qualitative detection, semi-quantitative or precise quantification of elements in the analyte
Spectral analysis methods are, as a rule, simple, rapid, and easy to mechanize and automate, i.e., they are suitable for routine mass analyses. When special techniques are used, detection limits for individual elements, including some non-metals, are extremely low, making these techniques suitable for the determination of trace amounts of impurities. These methods, unless only a small amount of sample is available, are virtually non-destructive since only small quantities of sample material are required for analysis.
The accuracy of spectral analysis, in general, satisfies practical requirements in most cases of determining impurities and components, with the exception of the determination of high concentrations of the main components of alloys. The cost of spectral analysis is low, although the initial investment is quite high. However, the latter quickly pays off due to the high productivity of the method and low requirements for materials and operating personnel.
Goals of work:
1. familiarization with the theory of atomic emission spectral analysis;
2. learn to understand the main characteristics of NPP equipment;
3. study of AESA methods;
1. Atomic emission spectral analysis (AESA)
Analysis methods based on measuring any radiation from the substance being determined are called emission methods. This group of methods is based on measuring the wavelength of radiation and its intensity.
The atomic emission spectroscopy method is based on thermal excitation of free atoms or monoatomic ions and recording the optical emission spectrum of the excited atoms.
To obtain emission spectra of the elements contained in the sample, the analyzed solution is placed into a flame. The flame radiation enters the monochromator, where it is decomposed into individual spectral lines. In a simplified application of the method, a certain line is highlighted with a light filter. The intensity of the selected lines, which are characteristic of the element being determined, is recorded using a photocell or photomultiplier connected to a measuring device. Qualitative analysis is carried out by the position of the lines in the spectrum, and the intensity of the spectral line characterizes the amount of the substance.
The radiation intensity is directly proportional to the number of excited particles N*. Since the excitation of atoms is of a thermal nature, excited and unexcited atoms are in thermodynamic equilibrium with each other, the position of which is described by the Boltzmann distribution law (1):
(1)where N 0 is the number of unexcited atoms;
g* and g 0 - statistical weights of the excited and unexcited states; E - excitation energy;
k is Boltzmann's constant;
T - absolute temperature.
Thus, at a constant temperature, the number of excited particles is directly proportional to the number of unexcited particles, i.e. in fact, the total number of these atoms N in the atomizer (since in real conditions of atomic emission analysis the fraction of excited particles is very small: N*<< N 0). Последнее, в свою очередь, при заданных условиях атомизации, определяемых конструкцией и режимом работы прибора и рядом других факторов), пропорционально концентрации определяемого элемента в пробе С. Поэтому между интенсивностью испускания и концентрацией определяемого элемента существует прямо пропорциональная зависимость:
(2)Thus, the intensity of the emission spectral line can be used as an analytical signal to determine the concentration of the element. Coefficient a in equation (2) is a purely empirical value, depending on the process conditions. Therefore, in nuclear power plants, the correct choice of atomization conditions and measurement of the analytical signal, including calibration using reference samples, is crucial.
The method is widely used for analytical purposes in medical, biological, geological, and agricultural laboratories.
emission spectral atomization photometer
2. Atomizers
The main types of atomization and excitation sources are given in Table 1.
Table 1
The most important characteristic of any atomizer is its temperature. The physicochemical state of the analyte and, consequently, the magnitude of the analytical signal and the metrological characteristics of the technique depend on temperature.
Flame. The flame version of the method is based on the fact that the substance to be determined in the form of an aerosol, together with the solvent used, enters the flame of a gas burner. In a flame with the analyzed substance, a number of reactions occur and radiation appears, which is characteristic only of the substance under study and is in this case an analytical signal.
Schemes of burners used in the flame photometry method are shown in Fig. 1. The liquid to be analyzed is usually introduced into the flame by pneumatic atomization. Sprayers are mainly used of two types: angular and concentric, operating due to the vacuum created above the opening of the spraying capillary (or around it), the second end of which is immersed in the solution of the analyzed sample. The liquid flowing from the capillary is sprayed by a stream of gas, forming an aerosol. The quality of the sprayer is assessed by the ratio of the amount of liquid and gas (M F / M G) consumed per unit time.
Rice. 1. Burners for flame atomic emission spectrometry:
a) and b) a conventional Mecker burner and an improved burner: 1 - burner body; 2 - surface on which the flame is formed; 3 - holes for the exit of flammable gases; 4 - supply of a mixture of flammable gases and aerosol; 5 - protrusion on the burner body with holes; c) a combined burner with separation of zones of evaporation - atomization and excitation of spectra: 1 - main burner with a protrusion and holes in it; 3 - second additional burner with the same type or higher temperature flame; 4 - flame; 5 - radiation detection zone; 6 - supply of a mixture of combustible gases to the additional burner; 7 - supply of a mixture of flammable gases and aerosol to the main burner.
To form a flame, prepare a gas mixture consisting of a combustible gas and an oxidizing gas. The choice of components of a particular gas mixture is determined, first of all, by the required flame temperature.
Table 2 contains information about the temperatures of various tribes in atomic emission analysis and their main characteristics.
Table 2 Characteristics of tribes used in atomic emission analysis
There are certain analytical characteristics of the flame. The flame, of course, must be stable, safe, and the cost of components to maintain it must be low; it must have a relatively high temperature and slow propagation speed, which increases the efficiency of desolvation and vapor production, and results in large emission, absorption or fluorescence signals. In addition, the flame must provide a restorative atmosphere. Many metals tend to form stable oxides in a flame. These oxides are refractory and difficult to dissociate at ordinary temperatures in a flame. To increase the degree of formation of free atoms, they must be reduced. Reduction can be achieved in almost any flame by creating a flow rate of combustible gas greater than that required by combustion stoichiometry. Such a flame is called enriched. The rich flames produced by hydrocarbon fuels such as acetylene provide an excellent reducing atmosphere due to the large number of carbon-containing radical species.
Flame is the lowest temperature source of atomization and excitation used in nuclear power plants. The temperatures achieved in the flame are optimal for determining only the most easily atomized and excitable elements - alkali and alkaline earth metals. For them, the flame photometry method is one of the most sensitive - up to 10 -7% by mass. For most other elements, the limits of determination are several orders of magnitude higher. An important advantage of the flame as a source of atomization is its high stability and the associated good reproducibility of measurement results (S r - 0.01-0.05).
Modular unit 5. Atomic emission spectrometry SLIDE 1
Lecture 2: ATOMIC EMISSION SPECTROMETRY
Prevention.
1. Combating acute intestinal infections.
2. Avoidance of various intoxications.
3. Proper regular nutrition.
Annotation. The lecture discusses the theoretical foundations of the atomic emission spectroscopy method, the design and principle of operation of atomic emission spectrometers, the capabilities of the atomic emission spectrometry method using various radiation sources: flames, plasma, electric arc and electric spark, as well as with various dispersing devices.
Keywords: atomic emission spectrometry, thermal, flame, plasma, arc, spark, glow discharge lamp, monochromator, polychromator, prism, diffraction grating.
Issues covered:
1 question. Theoretical foundations of the atomic emission spectrometry method.
Question 2. Radiation sources used in atomic emission spectrometry.
Question 3. Spectrometers for atomic emission spectrometry.
Question 4. Possibilities of the atomic emission spectrometry method.
Goals and objectives of studying a modular unit. As a result of studying this modular unit, students must master the theoretical foundations of the atomic emission spectroscopy method, become familiar with the design and operating principle of atomic emission spectrometers, know the capabilities of the atomic emission spectrometry method using various radiation sources: flames, plasma, electric arc and electric spark , as well as with various dispersing devices.
2.1.1. Principle of the method.
Atomic emission spectrometry is a method of qualitative and quantitative elemental analysis based on obtaining and detecting line spectra resulting from the transition of the outer electrons of atoms to an excited state and the subsequent spontaneous transition of these electrons to lower and main levels with the emission (emission) of excess energy in in the form of quanta of electromagnetic radiation.
The line spectrum is specific for a given element, so proper selection of this line and its isolation using a dispersing system allows the analyst to check the presence of this element and determine its concentration.
1.1.2. Atomic emission spectra.
Each element of the periodic table has a certain number of electrons equal to its atomic number. Electrons are located with a certain probability at levels and sublevels around the nucleus in accordance with quantum theory. Quantum theory was created by Planck, who proposed that electromagnetic energy is absorbed or emitted in discrete quantities; this means that the energy is not continuous. The energy state of each electron in a free atom is characterized by four quantum numbers:
principal quantum number P(n takes values from 1 to 7 for atoms in the ground state).
· orbital quantum number l(l = 0,1,2,...,n- 1) corresponds to sublevels s, p, d, f.
magnetic quantum number m(any integer that satisfies the condition –l< m < +l).
spin quantum number s(s= ±1/2).
The total angular momentum of an electron from both the orbital and spin quantum numbers. To characterize the total angular momentum of the electron, another quantum number is introduced - the total or internal quantum number j. For an atom having one valence electron j = l + s = l± ½. If the orbital quantum number is greater than zero, then the internal quantum number has two values, corresponding to two different energy states.
If the charge of the atomic nucleus is small (less than 35), and the number of valence electrons is two or more, then for the totality of these valence electrons new quantum numbers are introduced, which are defined as the sum of the corresponding quantum numbers of individual electrons:
L= S l i; S= S s i; J= L+ S
A group of energy states characterized by the same quantities L And S, has similar energy and forms one term.
When writing a term symbol, first of all indicate its main characteristic: the quantum number of the total orbital momentum L. If L= 0, then a capital letter is written in the term symbol S, If L= 1, then they write R. L, equal to 2 and 3, are designated by letters D And F respectively. On the left, in the form of a superscript, the number of states that are close in energy that a given term forms, that is, its multiplicity, is indicated. Multiplicity is 2 S+ 1, where S – total spin of the atom. Thus, the multiplicity is one greater than the number of unpaired electrons in the atom. If the multiplicity of a term is 1, then it is called a single or singlet term. A term with a multiplicity equal to 2 is called doublet or doublet. Bottom right of letter designation L values are written as an index j. Before the term designation, indicate the value of the principal quantum number n. For completely filled electronic sublevels ( s 2 , p 6 , d 10) L+ S equals 0.
For example, in a sodium atom, the first and second energy levels are completely filled, so the terms of this atom are determined by its only valence electron. In the ground state this electron is located at 3 s-sublevel In this case, the term of the sodium atom is denoted as follows:
3 2 S 1/2. You should pay attention to the left superscript 2, which indicates the formal multiplicity of this term. In fact, all the terms S are singlet (single). When a sodium atom is excited, an electron from sublevel 3 s moves to higher sublevels. The first excited state corresponds to the transition of the electron to sublevel 3 R. In this case, the term of the sodium atom is written as 3 2 R 3/2, 1/2. This notation corresponds to the following values of quantum numbers: n= 3, l= 1, j=3/2 or ½. This term is a doublet. The energy sublevels of the sodium atom are shown in Fig. 1.1.
Rice. 1.1. Therms of the sodium atom. The arrows indicate the transitions that cause the appearance of a doublet with wavelengths of 588.996 and 588.593 nm in the sodium spectrum.
Each spectral line reflects the transition of an electron from one energy level to another. However, not all transitions are allowed. There are selection rules that indicate between which energy levels transitions are possible and between which they are not. Possible transitions are called allowed, and impossible ones are called forbidden. Let us list the basic selection rules:
1. Transitions are allowed in which the term changes by one. According to this rule, transitions are possible P-S, D-P, but transitions are not possible P-P, D-D or D-S.
2. The internal quantum number during the transition can only change by ±1 or not change at all. Transitions in which D J= ±2.
3. Transitions are allowed without changing multiplicity.
For example, in the sodium atom a transition from sublevel 3 is allowed R(doublet term 3 2 R 3/2,1/2) to sublevel 3 s(singlet term 3 2 S 1/2). This transition causes the appearance of a double yellow line (doublet) in the sodium spectrum. This transition fully complies with the selection rules. According to the first rule, transitions are allowed P–S. According to the second rule D J may be equal to ±1, as in the transition 3 2 R 3/2 - 3 2 S 1/2, or 0, as in the transition 3 2 R 1/2 - 3 2 S 1/2. The third rule is also not violated, since the formal multiplicity of the term is 3 2 S 1/2 is equal to 2.
The brightest line in the spectrum is the line caused by the transition from the first excited level to the ground level. Such a line is called resonant.
The spectrum of an atom of any element differs significantly from the spectrum of its ion due to a change in the number of optical electrons as a result of ionization. In tables of spectral lines, next to the symbol of a chemical element, a Roman numeral is given, by which one can judge the multiplicity of ionization. Number I refers to a neutral atom, figure II– to a singly ionized atom, i.e. cation with charge +1.
According to selection rules and possible excited levels, each element of the periodic table can exhibit a set of lines (spectrum) specific to that element. This explains why combinations of element lines allow for qualitative analysis.
Rice. 2.2. Ground and excited states of the aluminum atom and cation. Allowed optical transitions are shown.
For example, the aluminum atom (Fig. 2.2) has 46 electronic levels below the ionization energy, corresponding to approximately 118 lines in the range 176-1000 nm. There are 226 levels for the singly charged A1 ion, giving approximately 318 lines in the range 160-1000 nm. Particles A1 I and A1 II emit relatively simple spectra, i.e., with a limited number of lines. In the same wavelength range, uranium can emit several tens of thousands of lines, resulting in what is probably the most complex spectrum observed. However, while resonant lines can be observed in any radiation source, lines arising from highly excited states can only be observed with high-temperature radiation sources or under special excitation conditions.
The radiation emitted by a sample in which all components except the one being determined are present is called background radiation. . It consists of lines emitted by other (accompanying) elements and a continuum arising from non-quantized transitions.
2.1.3. Intensity of spectral lines.
Quantitative analysis is possible if the line intensity can be related to the concentration of the emitting particles. The intensity of the line is proportional to:
1) energy differences of the upper (Em) and lower (E k) transition levels;
2) electron population ( n m) top level (E t);
3) the number of possible transitions between E t n E k per unit of time. This quantity is expressed by the transition probability A; its definition was given by Einstein.
Thus, the line intensity I can be expressed by the relation
1~ (E t -E k)× A×p t
The relationship between the populations of different levels was described by Boltzmann. If we consider the population p t And n k levels E t And Ek Accordingly, their ratio is determined by the Boltzmann equation:
Where k- Boltzmann constant ( k= 1.380×10 -23 J/K = 0.695 cm -1 × K -1 = 0.8617× 10-4 eV/K), T- temperature of the radiation source and g- statistical weight (2 J + 1), J- quantum number of total electronic angular momentum.
Since the population of excited levels is proportional to the exponential value (- E), then with increasing E the population is decreasing very quickly. A possible way to overcome this limitation is to use high-temperature radiation sources, such as plasma. For the ground state E = 0 and:
To get attitude p t total population of levels of an atom (or ion) N
N= n 0 +n 1 + ... + n m + ...
you can sum members of type g t exp (-E t /kT) for all possible levels and determine the statistical sum by state (Z) in the following form:
Z= g 0 + g 1 exp(- E 1 /kT) + …+ g m exp(- E m /kT) + …
Boltzmann's equation takes the form:
The statistical sum over states is therefore a function of temperature. However, in the temperature range of most radiation sources used in analytical applications, i.e., 2000-7000 K, these changes are small or even negligible.
This means that the line intensity can be written as:
where Ф is the conversion factor taking into account isotropy over the solid angle of 4p steradians.
From this equation it is clear that the line intensity l proportional to the number of atoms N.
When the radiation source is sufficiently stable and maintains a constant temperature, the state partition function Z the number of atoms (ions) will also remain constant N will be proportional to concentration With. For a given line of a defined element g m, A, l and E t permanent. Therefore, the line intensity l proportional With, which allows for quantitative determination. In relative quantitative analysis, a number of comparison samples are used to construct a calibration graph, i.e. e. dependence of intensity on the concentration of the element being determined. The line intensity of the element being determined in an unknown sample is used to find its concentration using a calibration graph. Theoretically, it is also possible to perform an absolute quantitative analysis, i.e., an analysis without using a calibration procedure. However, absolute quantitative analysis requires knowledge of temperature, solid angle of emission, etc. These measurements are not easy to make in routine analysis.
It should be noted that in the case of a constant concentration of the element being determined, any small changes in the characteristics of the radiation source can lead to changes in temperature and subsequent changes in line intensity due to changes in the population of the excited level. When considering the resonance line Al I 396.15im (E t = 25347cm" 1) an increase in the temperature of the radiation source by 100 K corresponds to an increase in the exponential term (-E t /kT) by approximately 50% and 5% at 3000 K and 6000 K, respectively. This explains why high source stability is required to obtain good reproducibility and repeatability, as well as to avoid analytical signal drift.
In atomic emission spectrometry, the source actually plays a dual role: the first step consists of atomizing the sample to be analyzed in order to obtain free atoms, usually in the ground state; the second is in the excitation of atoms into higher energy states. An ideal source for emission spectrometry should exhibit excellent analytical and instrumental characteristics. Analytical performance includes the number of elements that can be determined, accuracy and precision, selectivity, absence of physical and chemical interference, long-term stability, concentration dynamic range, and detection limits. Moreover, the emission system must be able to handle any type of sample, regardless of its form (liquid, solid or gas), with the ability to use a limited amount of sample. Instrumental characteristics of interest include ease of operation and maintenance, automation, performance, reliability, and system size. Some consideration should also be given to capital investment and cost of operation.
