New Thinking and Applications of the Principles of Atomic Absorption Spectroscopy

Rethinking the Principles of Atomic Absorption Spectroscopy

The ability to emit characteristic X-ray spectral lines when an electron beam or X-ray white light irradiates a solid substance is the basic principle of electron microscope energy spectroscopy elemental analysis or X-fluorescence elemental analysis. These characteristic elemental spectra are related to the energy level difference of the electrons outside the nucleus of the element. These emitted spectra are X-rays with wavelengths from 0.1 to 1 nm and energies in the KeV range. Electron microscopy elemental analysis and XRF elemental analysis are independent of the valence state of the atoms, and the ratio of the characteristic peaks of iron in monolithic iron and iron oxide is the same.

The atoms of each element can not only emit a series of characteristic spectral lines, but also absorb the characteristic spectral lines with the same wavelength as the emission lines. When a characteristic wavelength of light emitted by the light source through the atomic vapor, that is, the frequency of the incident radiation is equal to the frequency of the energy required for the electron in the atom from the ground state to jump to a higher energy state (generally the first excited state), the outer layer of electrons in the atom will be selectively absorbed by the characteristic spectral lines emitted by the same element, so that the incident light is weakened. The extent to which the characteristic spectral lines are attenuated by absorption is proportional to the content of the element being measured.

The absorbance of the measured unknown specimen is analyzed quantitatively against a standard series of curves with known concentrations.

Since the atomic energy level is quantized, the absorption of radiation by atoms is selective in all cases. Due to the different atomic structure and arrangement of outer electrons of the elements, the energy absorbed by the elements during the jump from the ground state to the first excited state is different, and thus the resonance absorption lines of the elements have different characteristics. Atomic absorption spectra are located in the ultraviolet and visible regions of the spectrum.

We focus on the fact that atomic absorption spectra are located in the ultraviolet and visible regions of the spectrum, their frequencies are much lower than X-ray spectra, and their energies are much lower than X-ray spectra.

There is another major difference, atomic absorption analysis, the sample is not a solid state at room temperature, but in the high temperature gasification of the atomic state, at the same time, the same atoms and different elements combined with different combinations of valence states, the detection of the same elements of the atom corresponding to the atomic absorption spectra will produce a significant drift.

If the previous basic hypothesis is true, the energy level difference between the ground state and the first excited state of an element is not a constant quantity; this energy level difference is related to the state of the atom.

Therefore, the principles of atomic absorption spectroscopy still need to be improved, and the currently available principles are for reference only and also derive from a number of assumptions.

The development history of atomic absorption spectroscopy

1, the first stage - the discovery and scientific explanation of atomic absorption phenomenon

As early as 1802, Wollaston (W.H. Wollaston) in the study of the continuous spectrum of the sun, found the continuous spectrum of the sun appeared in the dark line. 1817, Fraunhofer (J. Fraunhofer) in the study of the continuous spectrum of the sun, again found these dark lines, because at that time do not understand the reasons for the production of these dark lines, so they will be called the dark lines of Fraunhofer. Fraunhofer lines. In 1859, G. Kirchhoff and R. Bunson, in studying the flame spectra of the alkali and alkaline-earth metals, found that the light emitted by sodium vapor caused absorption of sodium light when it passed through sodium vapor at a lower temperature, and concluded, on the basis of the fact that the sodium emission lines were in the same position in the spectra as the dark lines, that the dark lines in the solar continuum spectrum were none other than the The sodium atoms in the sun's peripheral atmosphere to the results of the absorption of sodium radiation in the solar spectrum.

2, the second stage - the generation of atomic absorption spectroscopy instruments

Atomic absorption spectroscopy as a practical analytical method began in 1955. This year, Australia's Walsh (A. Walsh) published his famous paper “atomic absorption spectroscopy in chemical analysis” laid the foundation of atomic absorption spectroscopy. the late 50s and early 60s, Hilger, Varian Techtron and Perkin-Elmer has launched the atomic absorption spectrometry commercial instruments, the development of Walsh's design ideas. The design ideas of Walshe were developed. By the mid-1960s, atomic absorption spectroscopy began to enter a period of rapid development.

3, Third stage - the generation of electrothermal atomic absorption spectrometry instruments

In 1959, the Soviet Union Rivov published the first paper on electrothermal atomization technology. Electrothermal atomic absorption spectrometry absolute sensitivity can reach 10-12-10-14g, so that atomic absorption spectrometry a step forward. The development of the background-deduction techniques of the Seeman effect and the self-absorption effect has made it possible to realize atomic absorption measurements at very high backgrounds. The application of matrix improvement technology, platform and probe technology and the application of stable temperature platform graphite furnace technology (STPF) developed on the basis of the application of many complex composition of the specimen can be effectively realized atomic absorption determination.

4, the fourth stage - the development of atomic absorption analytical instruments

With the development of atomic absorption technology to promote the continuous updating and development of atomic absorption instruments, and other scientific and technological advances for the continuous updating and development of atomic absorption instruments to provide a technical and material basis. The use of continuous light source and the middle step grating, combined with the use of photoconductive photocells, diode array multi-element analysis detector, designed a microcomputer-controlled atomic absorption spectrophotometer, for the solution of multi-element simultaneous determination of a new prospect. The microcomputer-controlled atomic absorption spectrometry system simplifies the structure of the instrument, increases the degree of automation of the instrument, improves the accuracy of the determination, and makes the face of atomic absorption spectrometry undergo a significant change. Combined technology (chromatography - atomic absorption coupling, flow injection - atomic absorption coupling) is increasingly being emphasized. Chromatography-atomic absorption coupling, not only in solving the chemical form analysis of elements, but also in the determination of complex mixtures of organic compounds, have important uses, is a very promising development direction.

##Application of Atomic Absorption Spectroscopy

From the history of the development of the above can be seen, the detection of atoms are in the vapor state (gasification state), can not be multi-element analysis, atomic absorption method to determine an element has to be replaced by a hollow cathode lamp as a sharp light source, although, has developed into a new light source - multi-element lamps, but the stability of the multi-element lamps, the intensity of the light source is subject to certain limitations, the application is not very broad. Can not do structural analysis, and atomic emission like it can only do component analysis, can not do structural analysis. Refractory elements and non-metallic elements are difficult to determine.

Atomic absorption spectroscopy is an extremely important analytical method in the field of analytical chemistry, has been widely used in the metallurgical industry. Absorption atomic absorption spectrometry is a method of quantitative analysis using the degree of absorption of the characteristic radiation lines of the atoms in the ground state of the element to be measured. Both for some constant component determination, but also ppm, ppb level trace determination, can be carried out in the low content of iron and steel Cr, Ni, Cu, Mn, Mo, Ca, Mg, Als, Cd, Pb, Ad; raw materials, iron alloys, such as K2O, Na2O, MgO, Pb, Zn, Cu, Ba, Ca, and other elements of elemental analysis and some of the pure metals (such as Al, Cu) in the Detection of residual elements. Using the fire test gold enrichment separation method combined with atomic absorption spectrometry analysis can determine the content of trace precious metal elements in the raw ore.

Atomic Absorption Spectrophotometer Composition

Atomic absorption spectrophotometer consists of several parts such as light source, atomizer, spectrometer, detection system and so on. Among them, most of them use hollow cathode lamps and other sharp light sources. Realization of the atomization method, the most commonly used there are two kinds: one is the flame atomization method (flame atomizer), is the atomic spectrometry analysis of the earliest use of the atomization method, is still widely used; the other is a non-flame atomization method, the most widely used is the graphite furnace electrothermal atomization method.

##Atomic absorption spectrometer elemental analysis results of the source of error

Physical interference refers to the specimen in the transfer, evaporation process of any physical factor changes caused by the interference effect. Belong to this type of interference factors are: the viscosity of the test solution, the vapor pressure of the solvent, the pressure of the atomized gas. Physical interference is non-selective interference, the sample is basically similar to the effect of the elements.

Preparation and the test sample is similar to the standard sample, is to eliminate the physical interference of the common methods. In do not know the composition of the sample or can not match the sample, can be used to reduce and eliminate the physical interference standard addition method or dilution method.

Chemical interference refers to the interference effect caused by the chemical interaction between the element to be measured and other components, which mainly affects the atomization efficiency of the element to be measured, and is the main source of interference in atomic absorption spectrophotometry. It is due to the liquid or gas phase of the element to be measured between the atoms and the composition of the interfering substances to form thermodynamically more stable compounds, thereby affecting the dissociation of the element to be measured compounds and their atomization. Methods to eliminate chemical interferences include: chemical separation; use of high temperature flames; addition of releasing and protective agents; and use of matrix improvers.

Atomic ionization at high temperatures, so that the concentration of the base state of the atom is reduced, causing the atomic absorption signal is reduced, this kind of interference is called ionization interference. Ionization effect with the increase in temperature, ionization equilibrium constant increases and increases with the concentration of the element to be measured increases and decreases. Adding more easily ionized alkali metal elements can effectively eliminate ionization interference.

Spectral interferences include spectral line overlap, the presence of non-absorption lines within the spectral passband, DC emission within the atomization cell, molecular absorption, and light scattering. When the use of sharp line light source and AC modulation technology, the first three factors can generally not be considered, the main consideration of molecular absorption and light scattering, which is the main factor in the formation of the spectral background.

Molecular absorption interference is generated in the atomization process of gas molecules, oxides and salts molecules on the radiation absorption caused by the interference. Light scattering refers to the solid particles generated in the atomization process to scatter light, so that the scattered light deviates from the optical path and is not detected by the detector, resulting in high absorbance values.

Prospects for R&D of Atomic Absorption Spectrometer

(1) Replacing the hollow cathode light source with a tunable laser.

(2) Atomization of samples with lasers. Atomic absorption method has been widely used in various fields and plays a positive role in the development of industry, agriculture, medicine and health, teaching and research.