SPECTRAL ANALYSIS, quality method. and quantities. definitions composition, based on the study of their emission, absorption, reflection, etc. spectra. There are atomic and molecular spectral analysis, the tasks of which are to determine the resp. elemental and molecular composition in-va. carried out by emission spectra, or, excited decomposition. methods, absorption spectral analysis - based on electromagnetic absorption spectra. radiation from analyzed objects (see). Depending on the purpose of the study, the properties of the analyzed substance, the specifics of the spectra used, the wavelength region and other factors, the course of analysis, equipment, methods of measuring spectra and metrology. the characteristics of the results vary greatly. In accordance with this, spectral analysis is divided into a number of independent ones. methods (see, in particular,).
Often, spectral analysis is understood only as atomic emission spectral analysis (AESA) - a method based on the study of emission spectra of free substances. and in the gas phase in the wavelength range 150-800 nm (see).
When analyzing solids max. Arc arcs are often used (permanent and alternating current) and spark discharges powered from a specially designed. stabilized generators (often electronically controlled). Universal generators have also been created, with the help of which discharges are obtained different types with variable parameters affecting the efficiency of the excitation processes of the samples under study. The electrically conductive solid can directly serve as an arc or spark; non-conducting solids and placed in coal recesses of one configuration or another. In this case, both the complete (spraying) of the analyzed substance and the fractional latter and the excitation of the components are carried out in accordance with their physical properties. and chem. St. you, which allows you to increase the sensitivity and accuracy of the analysis. To enhance the effect of fractionation, it is widely applied to the analyzed substance, promoting the formation of highly volatile compounds under high-temperature [(5-7)·10 3 K] coal arc conditions. ( , etc.) defined elements. For geological analysis. In this form, the method of sprinkling or blowing a carbon arc into the discharge zone is widely used.
When analyzing, along with spark discharges of various types, glow discharge light sources (Grim lamps, hollow discharge) are also used. Combinations have been developed. automated sources in which glow discharge lamps or electrothermal lamps are used for atomization. analyzers, and to obtain spectra, for example, high-frequency plasmatrons. In this case, it is possible to optimize the conditions and excitations of the elements being determined.
When analyzing liquid solutions best results are obtained using high-frequency (HF) and ultra-high-frequency (microwave) plasmatrons operating in inert conditions, as well as with flame photometric. analysis (see). To stabilize the discharge temperature at the optimal level, easily ionized substances are introduced, for example. . An HF discharge with an inductive coupling of a toroidal configuration is especially successfully used (Fig. 1). It separates the zones of RF energy absorption and spectral excitation, which makes it possible to dramatically increase the excitation efficiency and the useful analyte ratio. signal to noise and thus achieve very low detection limits for a wide range of elements. The excitation zone is injected using pneumatic or (less commonly) ultrasonic sprayers. When analyzed using HF and microwave plasmatrons and flame photometry, it relates. standard deviation is 0.01-0.03, which in some cases allows the use of AESA instead of accurate, but more labor-intensive and time-consuming chemical ones. methods of analysis.
Mixtures require special vacuum installations; the spectra are excited using RF and microwave discharges. Due to developments, these methods are rarely used.
Rice. 1. HF plasmatron: 1-outgoing torch; 2-spectrum excitation zone; 3-zone of HF energy absorption; 4-heat. inductor; 5-cooler input ( , ); 6-plasma-forming input (); 7-inlet atomized (carrier gas-argon).
When analyzing high purity, when it is necessary to determine elements whose content is less than 10 -5 -10%, as well as when analyzing toxic and radioactive substances pre-treated; for example, the elements being determined are partially or completely separated from the base and transferred to a smaller volume of solution or added to a smaller mass of a substance more convenient for analysis. To separate the components, fractional distillation of the base (less often impurities) is used. AESA using the listed chemicals. methods are usually called chemical spectral analysis. Additional operations of separation and determined elements significantly increase the complexity and duration of analysis and worsen its accuracy (relative standard deviation reaches values of 0.2-0.3), but reduces the detection limits by 10-100 times.
Specific The area of AESA is microspectral (local) analysis. In this case, a microvolume of the substance (crater depth from tens of microns to several microns) is usually evaporated by a laser pulse acting on a section of the sample surface with a diameter of several. tens of microns. To excite spectra, a pulsed spark discharge synchronized with a laser pulse is most often used. The method is used in research in metallurgy.
Spectra are recorded using spectrometers (quantometers). There are many types of these devices, differing in aperture, dispersion, resolution, and working spectral range. Large aperture is necessary for recording weak radiations, large dispersion is necessary for separation spectral lines with close wavelengths when analyzing materials with multiline spectra, as well as to increase the sensitivity of the analysis. Diffraction devices are used as devices that disperse light. gratings (flat, concave, threaded, holographic, profiled), having from several. hundreds to several thousand strokes per millimeter, much less often - quartz or glass prisms.
(Fig. 2), recording spectra on special. or (less often) on , preferable for high-quality AESA, since they allow you to study the entire spectrum of the sample at once (in work area device); however, they are also used for quantities. analysis due to compare. low cost, availability and ease of maintenance. The blackening of spectral lines is not measured using microphotometers (microdensitometers). The use of computers or microprocessors provides automatic measurement mode, processing of their results and output final results analysis.
Fig.2. Optical design: 1-entry slit; 2-turn mirror; 3-spherical mirror; 4-diffraction lattice; 5-light scale lighting; 6-scale; 7-photo plate.
Rice. 3. Quantometer diagram (out of 40 recording channels, only three are shown): 1-polychromator; 2-diffraction gratings; 3-output slots; 4-PMT; 5-entry slots; 6 - with light sources; 7 - generators of spark and arc discharges; 8- electronic recording device; 9 - the manager will calculate. complex.
In spectrometers photoelectricity is carried out. registration analyt. signals using photomultiplier tubes (PMTs) with automatic data processing on a computer. Photovoltaic multichannel (up to 40 channels or more) polychromators in quantometers (Fig. 3) allow simultaneous recording of the analyte. lines of all defined elements provided by the program. When using scanning monochromators, multi-elementanalysis is ensured by high-speed scanning across the spectrum in accordance with a given program.
For the determination of elements (C, S, P, As, etc.), the most intense analytes. the lines of which are located in the UV region of the spectrum at wavelengths less than 180-200 nm; vacuum spectrometers are used.
When using quantum meters, the duration of the analysis is determined in the mean. least procedures for preparing the source material for analysis. A significant reduction in sample preparation time is achieved by automation. long stages - bringing solutions to a standard composition, grinding and selecting a given mass. In plural In cases, multi-element AESA is performed over a period of several. minutes, for example: when analyzing solutions using automatic measurement. photovoltaic spectrometers with RF plasmatrons or during analysis during the melting process with automatic feeding into the radiation source.
In black and color, express semi-quantitative (relative standard deviation 0.3-0.5 or more) methods for determining the content of the main or most important ones are common. characteristic components, e.g. when marking them, when sorting scrap metal for its disposal, etc. For this purpose, simple, compact and cheap visual and photoelectric devices are used. instruments (stylo-scopes and stylometers) in combination with spark generators. The range of determined contents of elements is from several. tenths of a percent to tens of percent.
AESA is used in scientific research; with its help they discovered chemistry. elements are studied archaeologically. objects, establish the composition of celestial bodies, etc. AESA is also widely used to control technology. processes (in particular, to determine the composition of starting materials, technological and finished products), research of objects, etc. Using AES, it is possible to determine almost all elements of periodic. systems in a very wide range of contents - from 10 -7% (pkg/ml) to tens of percent (mg/ml). Advantages of NPP: possiblethe ability to simultaneously determine a large number of elements (up to 40 or more) in a small sample of a substance with sufficiently high accuracy (see table), the versatility of the method. techniques for analyzing various in-in, expressiveness, comparative simplicity, accessibility and low cost of equipment.
, ed. H.I. Zilbershteina, L., 1987; Kuzyakov Yu.Ya., Semenenko K.A., Zorov N.B., Methods of spectral analysis, M., 1990. Yu.I. Korovin,
Introduction……………………………………………………………………………….2
Radiation mechanism………………………………………………………………………………..3
Energy distribution in the spectrum……………………………………………………….4
Types of spectra……………………………………………………………………………………….6
Types of spectral analyzes………………………………………………………7
Conclusion………………………………………………………………………………..9
Literature……………………………………………………………………………….11
Introduction
Spectrum is the decomposition of light into its component parts, rays of different colors.
Research method chemical composition various substances are called by their line emission or absorption spectra spectral analysis. A negligible amount of substance is required for spectral analysis. Its speed and sensitivity have made this method indispensable both in laboratories and in astrophysics. Since each chemical element of the periodic table emits a characteristic only for it line spectrum emission and absorption, this makes it possible to study the chemical composition of a substance. The physicists Kirchhoff and Bunsen first tried to make it in 1859, building spectroscope. Light passed into him through narrow gap, cut from one edge of the telescope (this pipe with a slit is called a collimator). From the collimator, the rays fell onto a prism covered with a box lined with black paper on the inside. The prism deflected the rays that came from the slit. The result was a spectrum. After that, they covered the window with a curtain and placed a lit burner at the collimator slit. Pieces of various substances were introduced alternately into the candle flame, and they looked through a second telescope at the resulting spectrum. It turned out that the hot vapors of each element produced rays strictly a certain color, and the prism deflected these rays to a strictly defined place, and therefore no color could mask the other. This allowed us to conclude that a radical new way chemical analysis– according to the spectrum of the substance. In 1861, based on this discovery, Kirchhoff proved the presence of a number of elements in the chromosphere of the Sun, laying the foundation for astrophysics.
Radiation mechanism
The light source must consume energy. Light is electromagnetic waves with a wavelength of 4*10 -7 - 8*10 -7 m. Electromagnetic waves are emitted by the accelerated movement of charged particles. These charged particles are part of atoms. But without knowing how the atom is structured, nothing reliable can be said about the radiation mechanism. It is only clear that there is no light inside an atom, just as there is no sound in a piano string. Like a string that begins to sound only after being struck by a hammer, atoms give birth to light only after they are excited.
In order for an atom to begin to radiate, energy must be transferred to it. When emitting, an atom loses the energy it receives, and for the continuous glow of a substance, an influx of energy to its atoms from the outside is necessary.
Thermal radiation. The simplest and most common type of radiation is thermal radiation, in which the energy lost by atoms to emit light is compensated by energy thermal movement atoms or (molecules) of the radiating body. The higher the body temperature, the faster the atoms move. When fast atoms (molecules) collide with each other, part of them kinetic energy converted into excitation energy of atoms, which then emit light.
The thermal source of radiation is the Sun, as well as an ordinary incandescent lamp. The lamp is a very convenient, but low-cost source. Only about 12% of the total energy released in the lamp electric shock, is converted into light energy. The thermal source of light is a flame. Grains of soot heat up due to the energy released during fuel combustion and emit light.
Electroluminescence. The energy needed by atoms to emit light can also come from non-thermal sources. During a discharge in gases, the electric field imparts greater kinetic energy to the electrons. Fast electrons experience collisions with atoms. Part of the kinetic energy of electrons goes to excite atoms. Excited atoms release energy in the form of light waves. Due to this, the discharge in the gas is accompanied by a glow. This is electroluminescence.
Cathodoluminescence. The glow of solids caused by the bombardment of electrons is called cathodoluminescence. Thanks to cathodoluminescence, the screens of cathode ray tubes of televisions glow.
Chemiluminescence. For some chemical reactions, coming with the release of energy, part of this energy is directly spent on the emission of light. The light source remains cold (it has a temperature environment). This phenomenon is called chemioluminescence.
Photoluminescence. Light incident on a substance is partially reflected and partially absorbed. The energy of absorbed light in most cases only causes heating of bodies. However, some bodies themselves begin to glow directly under the influence of radiation incident on them. This is photoluminescence. Light excites atoms of matter (increases their internal energy), after which they are highlighted themselves. For example, luminous paints, which are used to cover many Christmas decorations, emit light after being irradiated.
The light emitted during photoluminescence, as a rule, has a longer wavelength than the light that excites the glow. This can be observed experimentally. If you direct a light beam at a vessel containing fluoresceite (an organic dye),
passed through a violet light filter, this liquid begins to glow with green-yellow light, i.e. light of a longer wavelength than violet light.
The phenomenon of photoluminescence is widely used in fluorescent lamps. Soviet physicist S.I. Vavilov proposed covering inner surface discharge tube with substances capable of glowing brightly under the influence of short-wave radiation gas discharge. Fluorescent lamps are approximately three to four times more economical than conventional incandescent lamps.
The main types of radiation and the sources that create them are listed. The most common sources of radiation are thermal.
Energy distribution in the spectrum
On the screen behind the refracting prism, monochromatic colors in the spectrum are arranged in the following order: red (which has the largest wavelength visible light wavelength (k = 7.6 (10-7 m and the smallest refractive index), orange, yellow, green, cyan, indigo and violet (having the shortest wavelength in the visible spectrum (f = 4 (10-7 m and the largest index) refraction). None of the sources produces monochromatic light, that is, light of a strictly defined wavelength. We are convinced of this by experiments on the decomposition of light into a spectrum using a prism, as well as experiments on interference and diffraction.
The energy that light carries with it from the source is distributed in a certain way over the waves of all lengths that make up the light beam. We can also say that energy is distributed over frequencies, since there is a difference between wavelength and frequency. simple connection: v = c.
Flux density electromagnetic radiation, or intensity /, is determined by the energy &W attributable to all frequencies. To characterize the frequency distribution of radiation, it is necessary to introduce a new quantity: the intensity per unit frequency interval. This quantity is called the spectral density of radiation intensity.
The spectral radiation flux density can be found experimentally. To do this, you need to use a prism to obtain the radiation spectrum, for example, of an electric arc, and measure the radiation flux density falling on small spectral intervals of width Av.
You cannot rely on your eye to estimate energy distribution. The eye has selective sensitivity to light: its maximum sensitivity lies in the yellow-green region of the spectrum. It is best to take advantage of the property of a black body to almost completely absorb light of all wavelengths. In this case, radiation energy (i.e. light) causes heating of the body. Therefore, it is enough to measure the body temperature and use it to judge the amount of energy absorbed per unit time.
An ordinary thermometer is too sensitive to be successfully used in such experiments. More sensitive instruments are needed to measure temperature. You can take an electric thermometer in which sensing element made in the form of a thin metal plate. This plate must be coated with a thin layer of soot, which almost completely absorbs light of any wavelength.
The heat-sensitive plate of the device should be placed in one or another place in the spectrum. The entire visible spectrum of length l from red to violet rays corresponds to the frequency interval from v cr to y f. The width corresponds to a small interval Av. By heating the black plate of the device, one can judge the radiation flux density per frequency interval Av. Moving the plate along the spectrum, we find that most of energy falls on the red part of the spectrum, and not on the yellow-green, as it seems to the eye.
Based on the results of these experiments, it is possible to construct a curve of the dependence of the spectral density of radiation intensity on frequency. The spectral density of radiation intensity is determined by the temperature of the plate, and the frequency is not difficult to find if the device used to decompose the light is calibrated, that is, if it is known what frequency a given part of the spectrum corresponds to.
Plotting along the abscissa axis the values of frequencies corresponding to the midpoints of the intervals Av, and along the ordinate axis spectral density radiation intensity, we get a number of points through which we can draw a smooth curve. This curve gives visual representation on the distribution of energy and the visible part of the spectrum of the electric arc.
Spectral analysis is a set of methods for qualitative and quantification composition of an object, based on the study of the spectra of interaction of matter with radiation, including the spectra of electromagnetic radiation, acoustic waves, distribution of masses and energies of elementary particles, etc.
Depending on the purposes of analysis and the types of spectra, several methods of spectral analysis are distinguished:
Emission spectral analysis is a physical method based on the study of the emission spectra of vapors of the analyzed substance (emission or emission spectra) arising under the influence strong sources excitations (electric arc, high-voltage spark); This method makes it possible to determine the elemental composition of a substance, that is, to judge which chemical elements are included in the composition of a given substance.
Flame spectrophotometry, or flame photometry, which is a type of emission spectral analysis, is based on the study of the emission spectra of the elements of the analyzed substance, arising under the influence of soft excitation sources. In this method, the solution to be analyzed is sprayed into a flame. This method makes it possible to judge the content of mainly alkaline and alkaline earth metals, as well as some other elements, such as gallium, indium, thallium, lead, manganese, copper, phosphorus.
Note. In addition to flame emission photometry, absorption photometry is also used, also called atomic absorption spectroscopy or atomic absorption spectrophotometry. It is based on the ability of free metal atoms in flame gases to absorb light energy at wavelengths characteristic of each element. This method can determine antimony, bismuth, selenium, zinc, mercury and some other elements that cannot be determined by flame emission photometry.
Absorption spectroscopy is based on the study of the absorption spectra of a substance, which is its individual characteristic. There is a spectrophotometric method based on determining the absorption spectrum or measuring light absorption (both in the ultraviolet and in the visible and infrared regions of the spectrum) at a strictly defined wavelength (monochromatic radiation), which corresponds to the maximum of the absorption curve of a given substance under study, as well as a photocolorimetric method, based on determining the absorption spectrum or measuring light absorption in the visible part of the spectrum.
Unlike spectrophotometry, the photocolorimetric method uses “white” light or “white” light previously passed through broadband filters.
Method of analysis using Raman spectra. The method uses a phenomenon discovered simultaneously Soviet physicists G. S. Landsberg and L. I. Mandelstam and Indian physicist C. V. Raman. This phenomenon is associated with the absorption of monochromatic radiation by a substance and the subsequent emission of new radiation that differs in wavelength from the absorbed one.
Turbidimetry is based on measuring the intensity of light absorbed by an uncolored suspension of a solid. In turbidimetry light intensity, absorbed by or passed through a solution, is measured in the same way as in photocolorimetry of colored solutions.
Nephelometry is based on measuring the intensity of light reflected or scattered by a colored or uncolored suspension of solid matter (sediment suspended in a given medium).
The luminescent or fluorescent method of analysis is based on measuring the intensity of visible light (fluorescence) emitted by substances when irradiated with ultraviolet rays.
10) Optical methods of analysis also include the refractometric method, based on measuring the refractive index, and the polarometric method, based on the study of the rotation of the plane of polarization.
Dark lines in spectral stripes have been noticed for a long time, but the first serious study of these lines was undertaken only in 1814 by Joseph Fraunhofer. In his honor, the effect was called “Fraunhofer lines”. Fraunhofer established the stability of the positions of the lines, compiled a table of them (he counted 574 lines in total), and assigned an alphanumeric code to each. No less important was his conclusion that the lines are not associated with either the optical material or the earth's atmosphere, but are a natural characteristic of sunlight. He discovered similar lines in artificial light sources, as well as in the spectra of Venus and Sirius.
It soon became clear that one of the clearest lines always appeared in the presence of sodium. In 1859, G. Kirchhoff and R. Bunsen, after a series of experiments, concluded: each chemical element has its own unique line spectrum, and according to the spectrum heavenly bodies conclusions can be drawn about the composition of their substance. From this moment on, spectral analysis appeared in science, powerful method remote determination of chemical composition.
To test the method, in 1868 the Paris Academy of Sciences organized an expedition to India, where a total solar eclipse was coming. There, scientists discovered: all the dark lines at the moment of the eclipse, when the emission spectrum replaced the absorption spectrum of the solar corona, became, as predicted, bright against a dark background.
The nature of each of the lines and their connection with chemical elements were gradually clarified. In 1860, Kirchhoff and Bunsen discovered cesium using spectral analysis, and in 1861, rubidium. And helium was discovered on the Sun 27 years earlier than on Earth (1868 and 1895, respectively).
Principle of operation
The atoms of each chemical element have strictly defined resonant frequencies, as a result of which it is at these frequencies that they emit or absorb light. This leads to the fact that in the spectroscope the lines (dark or light) in the spectra are visible certain places, characteristic of each substance. The intensity of the lines depends on the amount of substance and its state. In quantitative spectral analysis, the content of the substance under study is determined by the relative or absolute intensities of lines or bands in the spectra.
Optical spectral analysis is characterized by relative ease of implementation, the absence of complex sample preparation for analysis, and a small amount of substance (10-30 mg) required for analysis big number elements.
Atomic spectra (absorption or emission) are obtained by transferring the substance into a vapor state by heating the sample to 1000-10000 °C. A spark or an alternating current arc are used as sources of excitation of atoms in the emission analysis of conductive materials; in this case, the sample is placed in the crater of one of the carbon electrodes. Flames or plasmas of various gases are widely used to analyze solutions.
Application
Recently, emission and mass spectrometric methods of spectral analysis, based on the excitation of atoms and their ionization in argon plasma of induction discharges, as well as in a laser spark, have become most widespread.
Spectral analysis is a sensitive method and is widely used in analytical chemistry, astrophysics, metallurgy, mechanical engineering, geological exploration and other branches of science.
In signal processing theory, spectral analysis also means the analysis of the energy distribution of a signal (for example, audio) over frequencies, wave numbers, etc.
Spectral analysis methods are based on the study of optical emission or absorption spectra. A distinction is made between atomic absorption spectral analysis (analysis based on absorption spectra) and emission spectral analysis (analysis based on emission spectra). Spectral analysis widely used for quality and quantitative analysis various substances. From the characteristic lines of the spectrum, the elemental composition of a substance can be determined, and the intensity of the spectral line is a measure of the concentration of the substance in the sample.
Emission spectroscopy
Atoms of elements in an excited state emit radiation with a strictly defined wavelength. The emission spectra (emission spectra) for each element are individual, they consist of a certain set characteristic lines by which the elemental composition of a substance and its concentration can be determined.
In emission spectral analysis, the test sample is evaporated or burned if it is liquid or solid, then exposed to high temperature or electric charge to transfer atoms to an excited state and record the spectrum. Qualitative emission analysis comes down to deciphering the lines in the spectrum of the analyzed sample. Quantitative analysis is based on comparing the intensity of the spectral lines of the sample with the intensity of the lines in the spectrum of a standard sample, the content of the element being determined in which is known.
Excitation sources can be flame, electric arc, spark, pulse or electric vacuum discharge. An arc discharge produces a temperature of 5000-7000 °C, at which atoms of most elements go into an excited state. In a high-voltage spark with a temperature of 7000-15000 °C, atoms of elements with a high excitation potential are excited. Pulse and electric vacuum discharges are used to excite inert gases.
According to the method of spectrum registration, several types of emission spectral analysis are distinguished. By visual analysis high-quality composition determined by direct observation visible spectrum. More accurate is photographic analysis, in which the spectrum is photographed on a photographic plate, which is then examined on a spectroprojector at qualitative definitions or photometered using a microphotometer for quantitative determinations. A fixed series of lines corresponding to the spectral lines of the sample under study are obtained on a photographic plate, the degree of blackening of which is proportional to the intensity of these lines.
Spectroprojectors are used to decipher spectrograms. The domestic industry produces the PS-18 spectroprojector, which makes it possible to obtain small sections of the spectrum enlarged 20 times on the screen, making it easier to decipher them during express qualitative or semi-quantitative analysis.
The density of blackening of lines on a photographic plate is measured using microphotometers. The light flux is passed through the unblackened part of the photographic plate, and then directed to a photocell with a galvanometer. The deflection of the galvanometer needle on the scale is noted. Then the light flux is passed through the blackened part of the plate and the deflection of the galvanometer needle is again noted. Blackening density is determined by the equation:
where I0 is the intensity of light passing through the unblackened part of the photographic plate; I is the intensity of light passing through the blackened part of the photographic plate.
Since the density of blackening is proportional to the concentration of the element, a calibration graph of the dependence of blackening on concentration is constructed based on the readings of the galvanometer. Using this graph, the content of the element is then determined. To determine the density of blackening of lines on a spectrogram, an MF-2 (or MF-4) microphotometer and an IFO-451 two-beam microphotometer are used.
In photoelectric emission analysis, analytical lines are recorded using photocells. The result of the analysis is indicated on the scale measuring instrument or fixed on the tape of a self-recording device.
Quartz spectrograph ISP-28. The ISP-28 spectrograph is used to obtain spectra in the wavelength range 200-600 nm. It conducts qualitative and quantitative analyzes of metals, alloys, ores, minerals and other materials. In Fig. 126 shows the optical diagram of the device. Light from source 1 (arc or spark) through a three-lens condenser 3-5, protected from metal splashes by a quartz plate 2, is directed into a slit 6 located at the focus of a mirror lens 8. A parallel beam of light reflected from this lens is directed to a quartz prism 9. The exposed dispersion light is focused by a quartz lens 10 on the emulsion of the photographic plate 11.
Other spectrographs. Tabletop quartz laboratory spectrograph ISP-30 is used for qualitative analysis metals, alloys and ores; The glass three-prism spectrograph ISP-51 is used for the analysis of substances containing elements with a small number of spectral lines. To analyze substances containing elements with particularly complex spectra, the STE-1 spectrograph is used. For qualitative and quantitative analysis of metals, ores, minerals, etc., a long-focus spectrograph DFS-8 (three modifications) with diffraction gratings and a diffraction spectrograph DFS-452 are used.
Flame photometry
Flame photometry is one of the most accurate methods of emission spectral analysis. This method is widely used for the determination of alkali and alkaline earth metals. The essence of the flame photometry method is as follows.
The solution of the analyzed substance is sprayed with compressed air into the flame zone of a gas burner, in which acetylene, hydrogen, lighting or some other gas is burned. The burner flame also serves as a source of energy to excite atoms. Optical device selects the spectral line of the element being determined and measures its intensity using a photocell. The intensity of the spectral line is proportional to the salt concentration in the solution (within certain limits). The concentration of the element is determined using a calibration curve. Below are the composition of some flammable gas mixtures And average temperature obtained by burning them (in °C):
Portable flame photometer PPF-UNIZ. The schematic diagram of the PPF-UNIZ photometer is shown in Fig. 127. Combustible gas from a cylinder (or city network) passes through manostat 2, buffer bottle 3, filter 4 and enters through microfaucet 5 into mixer 7, which simultaneously performs the function of a droplet eliminator. The gas pressure after the manostat is maintained constant using a micro tap 5 and is measured by a U-shaped liquid pressure gauge 6. Excess gas exits into the laboratory burner 1 and is burned.
Compressed air from a compressor (without the use of oil lubrication) or from a cylinder enters a 3" buffer bottle, then into a filter 13. The air pressure is maintained constant using a microfaucet 12 and measured by a pressure gauge 11. The air enters the sprayer 8, where the analyzed solution is sucked from the glass 10. The solution in the form of a finely atomized aerosol enters mixer 7, where it is mixed with flammable gas. The gas-air mixture leaving the mixer, containing the element under study in a sprayed state, enters burner 20 through a droplet eliminator 14.
The wavelength of the yellow flame line of sodium is 589±5 µm, the red line of calcium is 615±5 µm, and the infrared line of potassium is 766±5 µm. The intensity of these lines is recorded by a photocell 16, equipped with replaceable interference filters 17 and diaphragms 18. When determining sodium and calcium, selenium photocells of the AFI-5 type with a sensitivity of 460-500 μA/lm are used, for the determination of potassium - a silver sulfur photocell of the FESS-UZ type with sensitivity 6000-9000 µA/lm. Photocells and light filters are protected from direct thermal radiation flame with a glass screen 19. The resulting photocurrents are recorded by a magnetoelectric microammeter 21 type M-95, to which two of the three photocells are connected according to a compensation circuit through an electric switch 15.
Before starting to work with the device, open door 10 (Fig. 128) and secure it with a latch. A rubber tube is connected to the drain tube 14 of the sprayer 12 and lowered into a vessel with a barrier liquid 20-25 cm high. A glass with a capacity of 25-30 ml of distilled water is placed under the suction tube 13 of the sprayer. A protective device (visor) 11 is installed on the door and the device is connected to an alternating current network of 220 V (50 Hz). Turn on the compressor to supply air and, by slowly rotating the handle of the micro-faucet “air” 4 counterclockwise, achieve a good atomization of distilled water, i.e. formation of highly dispersed aerosol. The optimal air pressure (4-8) * 10000 Pa (0.4-0.8 atm) should not change during the entire measurement time.
Slowly rotating the handle of the micro-faucet “gas” 5, supply gas to the burner and after 10-20 s, ignite it at the entrance to the burner and at the outlet of the manostat. The gas supply is adjusted so that the inner cone of the flame is colored green color, and the outer one is bluish-blue. Using handle 9, set the burner in a position in which the inner cone of the flame is lowered 5-6 cm below the edge of the diaphragm inlet.
Measurements begin after 20 minutes of warming up the photometric cell. During the heating period, the cell diaphragm must be completely open, the microammeter is turned on to low sensitivity (1.0 μA) and distilled water is introduced into the burner flame. After warming up the photoelectric cell, the diaphragm is closed, the handle of the microammeter 6 is switched to the highest sensitivity (0.1 μA) and the microammeter pointer is set to zero by rotating the corrector head located on the right side of the device.
To construct a calibration curve, a series of standard solutions is prepared. For cooking initial solution 2.385 g of potassium chloride KCl (reagent grade) is dissolved in a 500 ml volumetric flask and diluted with water to the mark. Pipette 5.00 ml of this solution into a 500 ml volumetric flask and dilute with distilled water to the mark (100-fold dilution). The resulting solution contains 25 mg of potassium in 1 ml; solutions containing 5, 10, 15 and 20 mg of potassium in 1 ml are prepared from it. To do this, pipet 20, 40, 60 and 80 ml of a solution containing 25 mg/ml potassium into 100 ml volumetric flasks and dilute the volume with water to the mark.
These solutions are sequentially introduced into the burner flame and the microammeter readings are recorded. When moving from one solution to another, the sprayer is washed with distilled water until the microammeter needle returns to zero. Based on the data obtained, a calibration graph is constructed: microammeter readings (along the abscissa axis) - concentration of the element being determined (along the ordinate axis) (in mg/ml).
To determine the concentration of an element in the test solution, it is introduced into the burner flame and the microammeter readings are recorded, from which, using a calibration graph, the concentration of the element being determined is found. During the entire analysis process, it is necessary to maintain constant air and gas pressure.
In addition to the method of determining concentration using a calibration curve, the method of limiting solutions is used, i.e. take readings of a microammeter when analyzing the solution under study and, in parallel, readings from the device when analyzing standard solutions: solutions with lower and higher concentrations. Potassium content (in mg/l) is calculated using the formula
where c1 is the potassium content in a more concentrated standard solution; c2 - potassium content in a less concentrated standard solution; I1 - microammeter readings when analyzing a standard solution with a higher concentration; I2 - microammeter readings when analyzing a standard solution with a lower concentration; Ix - microammeter readings when analyzing the test solution.
Flame photometer Flapho-4. Two-channel instrument for serial determination of sodium, potassium, calcium, lithium and lead with high sensitivity. Produced in the GDR.
The test solution of the sample is absorbed by flowing through; spray with compressed air and turns into an aerosol. The aerosol enters a special tank, where a flammable gas (acetylene or propane) is mixed with it, and the resulting mixture is supplied to a burner surrounded by purified air. In a gas flame, the substance under study evaporates and its atoms are excited. A metallized interference filter selects a monochromatic radiation component from the general flame spectrum, which falls on the selenium photocell. The resulting intermittent photocurrent is amplified and supplied to a measuring or recording device. The device diagram is shown in Fig. 129.
Other flame photometers: three-channel flame photometer FP-101 for determining the concentration of Na, K, Ca and Li; flame photometer PFM for quantitative determination of alkaline and alkaline earth elements, as well as magnesium, boron, chromium and manganese; flame photometric liquid analyzers PAZH-1 and BIAN-140 for determining microquantities of K, Na, Ca and Li in solutions, flame photometer for determining Na and K in biological fluids.
Atomic absorption spectrophotometry
Free atoms in an unexcited state located in the low-temperature flame zone have the ability to selectively absorb light. The wavelength of light absorbed by the atoms of an element is the same as the wavelength of light emitted by the atoms of that element. Consequently, using the characteristic lines of the absorption spectrum and their intensity, it is possible to analyze substances, determining their composition and the concentration of its constituent elements.
To carry out atomic absorption analysis the test substance is evaporated by feeding it into the low-temperature flame zone. The molecules of the evaporated substance dissociate into atoms. The flow of light, in the spectrum of which there is a line of light absorbed by the substance, passing through this flame, is weakened, and the greater the concentration of the analyzed substance, the more.
In Fig. 130 shows a schematic diagram of the installation for atomic absorption analysis. Light from the discharge tube 1 (hollow cathode) passes through the flame of the burner 2 and is focused on the slit of the monochromator 3. Then the radiation hits the photomultiplier, or photocell 4. The monochromator selects from the total luminous flux radiation with a wavelength absorbed by the element under study. The current is amplified in block 5 and recorded by measuring device 6.
The determination consists of measuring the ratio of the intensities of light passing through the flame with and without the analyte introduced into it. Since the intensity of the spectral line of the element under study in the burner flame turns out to be greater than their radiation intensity from the hollow cathode, the radiation of the latter is modulated. Modulation of radiation (changing the amplitude and frequency of oscillations) is carried out using a rotating disk with holes (modulator 7) located between the hollow cathode and the flame. Amplifier 5 must have a maximum gain for the same frequency with which the radiation of the hollow cathode is modulated.
Atomic absorption spectrophotometer AAS-1. Intended for absorption and emission spectral analysis. Allows you to define 65 elements.
Operating principle. The liquid sample is atomized using an oxidizing gas, mixed with a flammable gas (acetylene or propane) and burned in a burner flame. Radiation from a hollow cathode lamp passes through the burner flame. After selecting a suitable line by a diffraction monochromator, the radiation is directed to a photomultiplier. The direct current component caused by self-radiation is suppressed. The signal from the photomultiplier is amplified, rectified by a sensitive rectifier and recorded. The device is adjusted and controlled using standard solutions.
In Fig. 131 shows a diagram of the AAS-1 atomic absorption spectrophotometer.
Device design. The device has a fitting complex for supplying gases, a spraying and combustion system, a replaceable device for lamps with hollow cathodes, an optical system and a receiving device with an amplifier and indicator.
The burner flame is powered by a mixture of acetylene or propane and compressed air. Gases enter the combustion system from conventional cylinders with adjusted (primary) pressure reducers. The supply of oil-free air is provided by a membrane compressor (16 l/min at a pressure of 3*100000 Pa (3 atm)). The device's valve complex has adjustable (secondary) gearboxes and flow meters to control the flow rate of each gas, as well as ceramic sintered dust filters and a bottle for additional acetylene rinsing. The safety valve automatically stops the access of flammable gas when the operating pressure of the compressed air decreases (for example, due to kinking or tearing of the supply hose); the valve eliminates the incorrect order of gas supply when igniting the flame.
The atomization and combustion system is located behind a removable laminated glass window allowing the system to be observed. The annular nozzle atomizer has a high atomization ratio and is characterized by low liquid flow (3.4 ml/min, or 0.5 ml during the entire analysis). The burner is equipped with replaceable nozzle heads - one slotted for absorption analysis (Fig. 132, a) and two multi-hole (Mecker burners with a mesh) for emission analysis (Fig. 132,6).
Adjustable holders for four hollow cathode lamps are located in a device that allows for quick lamp changes. After replacing one of the lamps, the holders do not need to be adjusted.
The optical system directs the lamp radiation in the form narrow beam to the flame. Due to the lateral displacement of the tube with the imaging system, radiation passes through the flame once or three times to increase the sensitivity of the analysis. A high-aperture diffraction monochromator selects the desired resonance line from the line spectrum of a given hollow-cathode lamp. The width of the monochromator slit is adjusted from 0 to 2 mm.
Precision diffraction grating with 1300 lines per 1 mm and an angular dispersion of 1.5 nm/mm, it has high resolution. The spectral range of the grating is from 190 to 820 nm.
The radiation receiver is a 12-stage photomultiplier. The instrumentation amplifier, hollow cathode lamp power supply and photomultipliers operate on transistors and are capable of compensating for mains voltage fluctuations from +10 to -15%.
The device readings are measured using a dial indicator that has three scales: a logarithmic scale of the extinction coefficient from 0 to 1.5; linear scale from 0 to 100 and operating voltage scale from 0 to 16 mV. A recording or computing device may be connected to the device to determine the concentration or to process the data. The sensitivity of determinations (in mg/l) is:
The device operates from an alternating current network of 220 V, 50 Hz. Produced in the GDR.
Other domestic atomic absorption spectrophotometers: atomic absorption spectrophotometer S-302 for determining trace amounts of iron, copper, zinc, cobalt, nickel, bismuth, calcium and other elements; automated atomic absorption spectrophotometer AA-A for the determination of calcium and copper with hypersensitivity; "Saturn" - flame atomic absorption semi-automatic recording spectrophotometer for the determination of 32 elements; "Spectrum-1" is an atomic absorption spectrophotometer for the rapid determination of more than 40 elements with a sensitivity of approximately 0.2 μg/ml.
The Perkin-Elmer atomic absorption spectrophotometer, model 603, is produced in England. The device is built using a two-beam scheme and combined with a microcomputer. Provides high accuracy and rapidity of definition. A flammable oxygen-acetylene mixture is used to ignite the flame.
Spectral analysis is a method for studying the chemical composition of various substances using their spectra.
Analysis carried out using emission spectra is called emission spectral analysis, and analysis carried out using absorption spectra is called absorption spectral analysis.
Emission spectral analysis is based on the following facts:
1. Each element has its own spectrum (differing in the number of lines, their location and wavelengths), which does not depend on the methods of excitation.
2. The intensity of spectral lines depends on the concentration of the element in a given substance.
To perform a spectral analysis of a substance with an unknown chemical composition, it is necessary to carry out two operations: somehow force the atoms of this substance to emit light with a line spectrum, then decompose this light into a spectrum and determine the wavelengths of the lines observed in it. By comparing the resulting line spectrum with the known spectra of chemical elements of the periodic table, it is possible to determine which chemical elements are present in the composition of the substance under study. By comparing the intensities of different lines in the spectrum, one can determine the relative content various elements in this substance.
Spectral analysis can be qualitative and quantitative.
If the substance under study is in a gaseous state, then a spark discharge is usually used to excite the atoms of the substance. A tube with two electrodes at the ends is filled with the gas under study. High voltage is applied to these electrodes and an electrical discharge occurs in the tube. Impacts of electrons accelerated electric field, lead to ionization and excitation of atoms of the gas under study. During transitions of excited atoms into normal condition quanta of light characteristic of a given element are emitted.
To determine the chemical composition of a substance located in a solid or liquid state, according to its emission spectrum, it is necessary to first transfer the substance under study into a gaseous state and somehow force this gas to emit light. Typically, an arc discharge is used to carry out spectral analysis of samples of a substance in the solid state. In the arc plasma, the substance is converted into vapor, and atoms are excited and ionized. The electrodes between which the arc discharge is ignited are usually made of the substance under study (if it is metal) or of graphite or copper. Carbon and copper are chosen because the emission spectra of their atoms in the visible region have a small number of lines and, therefore, do not create serious interference in observing the spectrum of the substance under study. The powder of the test substance is placed in the recess of the lower electrode.
Literature
Aksenovich L. A. Physics in high school: Theory. Tasks. Tests: Textbook. allowance for institutions providing general education. environment, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsiya i vyakhavanne, 2004. - P. 531-532.