Spectrochemical Analysis

Emission atomic spectrum analysis. Emission atomic spectrum analysis consists of the following basic steps:

(1) selection of a representative sample reflecting the average composition of the test material or the local distribution of the elements being determined in the material;

(2) introduction of the sample into a radiation source, in which vaporization of solid and liquid samples, dissociation of compounds, and excitation of atoms and ions occur;

(3) separation of the resulting radiation into a spectrum and recording or visual observation of the spectrum by means of a spectroscopic instrument;

(4) interpretation of the obtained spectra by means of tables and atlases of the spectral lines of the elements.

Qualitative analysis terminates with the fourth step. The best results are obtained when sensitive, or ultimate, spectral lines are used—that is, lines that persist in the spectrum to a very low concentration of the element being determined. The spectrograms are examined by means of measuring microscopes, comparators, and spectroprojectors. For qualitative analysis, it is sufficient to establish the presence or absence of the analytical lines of the elements being determined. With only visual examination, a rough estimate of the concentration of particular elements in the sample can be made from the brightnesses of the lines.

Quantitative analysis is carried out by comparing the intensities of two spectral lines in the spectrum of the sample. One of the lines belongs to the element being determined. The other, a comparison line, belongs to the principal element, whose concentration is known, in the sample or to an internal standard, that is, an element specially introduced into the sample at a known concentration.

Quantitative analysis is based on the following equation, which gives the relation between the concentration c of the test element and the ratio of intensities of the line of the test impurity (/[) and the comparison line (I₂):

I₁/I2 = ac_b

The constants a and b are determined experimentally. The relation is generally used in the form

log (I₁/I₂) = b log c + log a

By using at least three standard samples, a calibration curve, which is a graph of the dependence of log (I₁/I₂) on log ^c can be constructed (Figure 1); a and b can be determined from this curve. The values of I₁ and I₂ can be obtained directly by photoelectric recording or, in the case of photographic recording, by photometric measurement (measurement of the degree of blackening) of the line of the test impurity and the comparison line. The photometric measurements are carried out by means of a mi-crophotometer.

Figure 1. Calibration curve (three-standard method)

Various light sources and, accordingly, various methods of introducing samples into them are used to excite the spectra in atomic spectrochemical analysis. The choice of a source depends on the specific conditions of analysis of particular substances. The various techniques of atomic spectrochemical analysis differ in the type of source and in the method of introducing the sample.

The first artificial light source used in atomic spectrum analysis was the flame of a gas burner, which is a very convenient source for the rapid and accurate determination of many elements. The flame temperature of combustible gases is not high and ranges from 2100°K for a mixture of hydrogen and air to 4500°K for the rarely used mixture of oxygen and cyanogen. About 70 elements can be determined by flame photometry from their analytical lines and also from the molecular bands of compounds formed in the flames.

Electrical sources of light are widely used in emission atomic spectrum analysis. The simultaneous determination of tens of elements can be carried out with a DC arc between electrodes made of specially purified carbon. The electrodes may be of various shapes. The substance being investigated is placed, in a finely powdered state, in a cavity at the end of one electrode. The arc heats the electrodes to a relatively high temperature and provides favorable conditions for the excitation of atoms of the sample in the arc plasma. The accuracy of this method, however, is low owing to the instability of the discharge. The accuracy can be improved by increasing the voltage to 300–400 volts (V) or by using a high-voltage arc operating at 3,000–4,000 V.

Stabler excitation conditions are produced by an AC arc. In modern generators of AC arcs (see, for example, Figure 2) various excitation modes can be obtained, such as low-voltage spark,

Figure 2. Schematic of double-feed AC arc: (A) ammeter, (R,) and (R¡) rheostats, (Tr) step-up transformer, (IC) inductance coil, (AG) analytical gap, (G) auxiliary gap, (C₁) and (C₂) capacitors

high-frequency spark, AC arc, and pulsed discharge. Such light sources with different modes of operation are used for the determination of metals and of elements that are difficult to excite, for example, carbon, the halogens, and gases contained in metals. A high-voltage condensed spark (Figure 3) is used mainly as the light source in the analysis of metals. The stability of a spark discharge provides a high reproducibility of the analysis. The complex processes occurring on the surfaces of the test electrodes, however, lead to changes in the composition of the discharge plasma. To eliminate this effect, preburning of the samples and standardization of the shape and size of the samples and standards are required.

Figure 3. Schematic of condensed-spark generator with control gap: (AG) controlled analytical gap formed by vanadium electrodes, (R₁) rheostat, (Tr) supply transformer, (C) capacitor, (IC) inductance coil, (CG) control gap, (R₂) blocking resistor

A promising approach in atomic spectrum analysis is the use of stabilized forms of electrical discharges produced by plasmatrons of various designs, high-frequency induction discharges, ultra-high-frequency discharges produced by magnetron generators, and high-frequency torch discharges. Various methods of introducing the test substances into the plasma of these types of discharge—such as the blowing of powders and the atomization of solutions—permit an improvement in the relative accuracy of the analysis to 0.5–3 percent. This substantial improvement includes analyses of complex-sample components whose content is ten percent. In some important cases of the analysis of pure substances, the use of these types of discharge lowers the limits for the determination of impurities by one or two orders of magnitude to 10^–5–10^–6 percent.

The use of a hollow-cathode discharge and electrodeless high-frequency and ultrahigh-frequency discharges is very promising for the analysis of pure substances, radioactive materials, and gaseous mixtures, for isotopic analysis, and for the spectral determination of isotopes of gases in metals and solids. Lasers are also used as excitation sources (see).

Atomic absorption and fluorescence spectrochemical analysis. In the absorption- and fluorescence-spectrum methods, the sample is converted to a vapor in an atomizer (flame, graphite tube, or stabilized high-frequency or ultrahigh-frequency discharge plasma).

In the case of atomic absorption spectrum analysis, light from a line-emission source passes through the vapor and is attenuated; the concentration of the element being determined is found from the decrease in intensity of the lines. The analysis is carried out with special spectrophotometers. The techniques of atomic absorption spectrum analysis are considerably simpler than other methods. Characteristic of atomic absorption spectrum analysis is a high accuracy in the determination of both small and large concentrations of elements in the samples. Atomic absorption spectrochemical analysis is supplanting laborious and time-consuming chemical methods of analysis, since it is not inferior to these methods in accuracy.

In atomic fluorescence spectrum analysis, the atomic vapor of the sample is irradiated with light from a source of resonance radiation, and the fluorescence spectrum of the element being determined is recorded. For some elements, such as zinc, cadmium, and mercury, the relative limits of detection by this method are very low, ~10^–5–10^–6 percent.

Atomic spectrum analysis permits the measurement of isotopic composition. Some elements, such as hydrogen, helium, and uranium, have spectral lines with an easily resolvable structure. The isotopic composition of these elements can be measured with ordinary spectroscopic instruments by using light sources that produce narrow spectral lines, for example, hollow-cathode lamps and electrodeless high-frequency and ultrahigh-frequency lamps. The isotopic spectrum analysis of most elements requires high-resolution instruments, such as a Fabry-Perot étalon. Isotopic spectrum analysis can also be carried out on the basis of elec-tron-vibrational molecular spectra by measuring the isotopic band shifts, which in a number of cases are rather substantial.

Rapid methods of atomic spectrochemical analysis are widely used in industry, agriculture, geology, and many other areas. Atomic spectrum analysis plays an important role in, for example, nuclear engineering and the production of pure semiconductor materials and superconductors. More than three-fourths of all analyses performed in the metallurgical industry are done by the methods of atomic spectrum analysis. Rapid monitoring, requiring 2–3 minutes, of melting in open-hearth furnaces and converters can be performed by means of quantum counters. In geology and geological exploration, about 8 million analyses are made annually for the evaluation of deposits. Atomic spectrum analysis is used in, for example, environmental protection, soil analysis, criminalistics, medicine, the geology of the ocean floor, the study of the composition of the upper layers of the atmosphere, the separation of isotopes, and the determination of the age and composition of geological and archaeological objects.

REFERENCES

L. V. LIPIS

Molecular spectrochemical analysis is based on the qualitative and quantitative comparison of the measured spectrum of a test sample with the spectra of individual substances. A distinction is accordingly made between qualitative and quantitative molecular spectrum analysis. Various types of molecular spectra are used in molecular spectrum analysis: rotational spectra, which are spectra in the microwave and long-wavelength infrared regions; vibrational and vibrational-rotational spectra, which include absorption and emission spectra in the intermediate infrared region, Raman spectra, and infrared fluorescence spectra; and electronic, electronic-vibrational, and electronic-vibrational-rotational spectra, which include emission spectra in the visible and ultraviolet regions and fluorescence spectra. Molecular spectrum analysis techniques permit the analysis of small amounts—in some cases, fractions of a microgram or less—of substances in various states of aggregation.

The principal factors determining the suitability of the various methods of molecular spectrum analysis are as follows:

(1) the information yield of the method, which by convention is the number of spectroscopically resolvable lines or bands in a certain range of wavelengths or frequencies; this number is ~10⁵ for the microwave region and ~10³ for the intermediate-infrared region in spectra of solid and liquid substances;

(2) the number of measured spectra of individual compounds;

(3) the existence of general regularities relating the spectrum of a substance to its molecular structure;

(4) the sensitivity and selectivity of the method;

(5) the generality of the method;

(6) the simplicity and convenience of the spectrum measurements.

Qualitative molecular spectrum analysis. In qualitative molecular spectrum analysis the molecular composition of the test sample is established. A molecule is unambiguously characterized by its spectrum. The most specific spectra are those of substances in the gaseous state with resolved rotational structure, which are studied by means of high-resolution spectroscopic instruments. The most commonly used spectra are infrared absorption and Raman spectra of substances in the liquid and solid states and absorption spectra in the visible and ultraviolet regions. The widespread introduction of the Raman spectrum method has been facilitated by the use of laser radiation to excite the spectra.

To improve the efficiency of molecular spectrum analysis, the measurement of spectra is in some cases combined with other methods for the identification of substances. An example of an approach that is being increasingly used is the combination of chromatographic separation of mixtures with measurement of the infrared absorption spectra of the separated components.

Qualitative molecular spectrum analysis includes what is known as structural molecular analysis. Molecules having identical structural elements have been found to display common features in their absorption and emission spectra. This fact is most apparent in vibrational spectra. For example, the presence of a mercapto group (—SH) in the structure of a molecule leads to the appearance in the spectrum of a band in the range 2,565–2,575 cm^-1, and the nitrile group (—CN) is characterized by a band in the range 2,200–2,300 cm^–1. The presence of such characteristic bands in the vibrational spectra of compounds with common structural elements is explained by the specificity of the frequency and form of many molecular vibrations. In many cases, such features of vibrational spectra and, to a lesser extent, of electronic spectra permit determination of the structural type of a substance.

Qualitative analysis can be substantially simplified and speeded up by the use of electronic computers. In principle, qualitative analysis can be automated completely by feeding the readings of the spectroscopic instruments directly into the computer. The computer memory should store the characteristic spectral features of many substances. The computer would use this stored information in performing an analysis of the test substance.

Quantitative molecular spectrum analysis. Quantitative analysis by means of absorption spectra is based on the Bouguer-Lam-bert-Beer law: if I₀ is the intensity of the light incident on a substance, l is the intensity of the light transmitted through the substance, l is the thickness of the absorbing layer, and c is the concentration of the substance, then

I(I) = I₀e^–kcl

The coefficient K characterizes the absorptivity, for a given radiation frequency, of the component being determined. An important condition for the carrying out of quantitative analysis is that K must be not dependent on the concentration of the substance and must be constant within the frequency interval used in the measurement and determined by the slit width of the spectropho-tometer. Molecular spectrum analysis using absorption spectra is carried out mainly for liquids and solutions; it is considerably more complicated for gases.

In practical molecular spectrum analysis, the optical density D is usually measured:

D = ln (I₀/I) = kcl

If a mixture consists of n substances that do not react with each other, then the optical density of the mixture at the frequency v is additive:

This additivity permits the complete or partial analysis of multi-component mixtures to be carried out. The problem in this case reduces to measuring the values of the optical density at m points of the spectrum of the mixture (m ≥ n) and solving the resulting set of equations:

Quantitative molecular spectrum analysis generally uses spec-trophotometers that permit measurement of I(v) in a comparatively wide range of v. If the absorption band of the investigated substance is sufficiently isolated and bands of the other components of the mixture are not superposed on it, then the spectral region segment being studied can be separated out by means of, for example, an interference filter. Special-purpose analyzers constructed on this basis are widely used in industry.

In quantitative molecular spectrum analysis by means of Raman spectra, an external standard is most often used. Here, a line of the mixture component under investigation is compared, with respect to intensity, with some line of a standard substance measured under the same conditions. In other cases, an internal standard is used: a definite amount of a standard substance is added to the test substance.

Fluorescence and Shpol’skii spectra. Among the other methods of qualitative and quantitative molecular spectrum analysis, the most sensitive is fluorescence analysis, although, under ordinary conditions, it is less general and selective than vibrational spectroscopic methods. In quantitative analysis based on fluorescence spectra, the fluorescence of a solution of the test sample is compared with the fluorescence of a series of standard solutions of similar concentration.

Of particular interest is molecular spectrum analysis that uses the technique of frozen solutions in special solvents, for example, in paraffins. The spectra of substances in such solvents have a markedly individual character; they are very different for molecules of similar structure, even for isomers (Shpol’skii effect). This circumstance permits the identification of substances that, under ordinary conditions, cannot be determined from their fluorescence spectra. For example, the Shpol’skii method permits the qualitative and quantitative analysis of complex mixtures containing aromatic hydrocarbons. Qualitative analysis in this case is carried out by means of luminescence and absorption spectra, and quantitative analysis is accomplished by means of luminescence spectra through the use of internal and external standards. Because of the extremely narrow width of the lines in Shpol’skii spectra, this method permits achievement of a threshold sensitivity of ~10^–11 g/cm³ for the detection of some polyatomic aromatic compounds.

REFERENCES

V. T. ALEKSANIAN