Spectral analysis

Spectral analysis

Emission spectral analysis is a method of studying a substance, which is based on the registration of electromagnetic radiation obtained as a result of the transitions of particles from high energy levels to lower energy levels.

During the electron transitions of the outer shells of the atom, the radiated lines belong to the optical region of the spectrum (50-1000 nm). Part of the optical region is visible to the human eye, this is the region of 390-780 nm.

When an electron passes to the internal energy level, an X-ray photon is emitted

During transitions between the energy levels of atomic nuclei, gamma radiation occurs. Low-energy gamma radiation is in the same wavelength range as hard X-rays. The name reveals the origin of the radiation

The methods of excitation of X-ray radiation and detection of X-ray spectra differ from optical ones, so this section is historically separate and it is commonly called X-ray fluorescence analysis. Optical emission spectral analysis is often referred to simply as spectral analysis for short.

It should be borne in mind that in addition to the spectra of electromagnetic radiation, physicists also study the spectra of acoustic waves, the mass and energy distributions of elementary particles, etc. And in addition to the emission method, there is also an absorption method of spectral analysis.

Optical Emission Spectral Analysis (OESA)

For many years, chemists have considered OES and other physical methods of analysis their fiefdom. Sometimes it seems that it was analytical chemistry that gave birth to this science. Of course, this is not the case.

Nevertheless, the first contribution to the analysis of matter by the OES method was made in the work of the physicist Kirchhoff and the chemist Bunsen in 1859. By introducing salts of various elements into the burner flame, the scientists found bright lines in the spectrum, the number and wavelength of which were different for different elements.

Now such experiments are demonstrated in school chemistry lessons. In particular, by placing a flame in the table salt of the burner, we will see the famous doublet of 588,9950 and 589,5924 nm of the emission spectrum "for the sodium D-line", due to the high intensity of which the bright yellow color of the flame occurs.

Using spectral analysis, helium was detected in the spectrum of the Sun a quarter of a century earlier than it was obtained on Earth. And this is not the only example of the discovery of a previously unknown element using spectral analysis.

For a long time, the OES method was considered high-quality. After the pioneering work of Niels Bohr in 1913, it was suggested that the intensity of the lines of an element is related to the number of its atoms in the sample.

The world is approaching the centenary of the use of the OES in practice. In 1923, the method was introduced in steel mills in England as a qualitative method of sorting metal. It was only after the publication of Gerlach's seminal work in 1925 that the principles he formulated led to the development of quantitative analysis techniques.

After the implementation of the program outlined by the Lansberg group (Moscow State University, 1932), the first domestic styloscopes and stylometers were developed and manufactured.

The first domestic spectroscopes SL-1 and SL-3 were mass-produced in Leningrad in 1935 (developed by Rozhdestvensky and Prokofiev).

The most important advantages of OES are expressiveness, high accuracy, low detection limits, low cost, and ease of sample preparation. And finally, after all the improvements, it is easy to perform the analysis with the help of modern spectrometers.

The main areas of application are analysis of the composition of metals and alloys, powder analysis (powder metallurgy and 3D printing), study of geological samples and mineral raw materials, analysis of water and soil, analysis of liquids and motor oils

The optical emission spectrometer consists of

1. The source of excitation of the spectrum and the tripod in which the analyzed sample is installed. These devices serve to excite the glow of the sample atoms.

2. A polychromator designed to separate the spectral lines of the elements.

3. Radiation receivers with a recording system. They convert light into an electrical signal, which is digitized and transmitted to a computer.

4. A processing and control system consisting of a computer and software. The computer controls all the nodes of the device, and the software allows you to calculate the concentrations of elements in the sample.

What is needed

The intensity of the spectral line of the analyzed element, in addition to its concentration, depends on a large number of different factors. Theoretically, it is impossible to calculate the relationship between the intensity of the line and the concentration of the corresponding element. Therefore, the analysis requires standard samples that are similar in composition to the analyzed sample.

Pre-standard samples are burned on the device. Based on the results of these burns, a calibration graph is constructed for each analyzed element, i.e., the dependence of the intensity of the spectral line of the element on its concentration. Subsequently, during the analysis of the samples, these calibration graphs are used to recalculate the measured intensities in the concentration.

Preparation of samples for analysis

It should be borne in mind that several milligrams of the sample from its surface are actually analyzed. Therefore, to obtain the correct results, the sample must be homogeneous in composition and structure. An example of a non-uniform material is unbleached cast iron.

When analyzing metal in the foundry or smelting industry, it is recommended to use special coquilles for casting samples.

An important role is played by the preparation of the sample surface. We offer machines for the preparation of samples from hard steels and soft metals. Cutting machines, scissors, etc. can be used for sampling during the input control of sampling materials.

Special adapters are used to analyze small samples, such as bars or wires.

For steels, cast irons and other strong materials for the preparation of the analyzed surface, treatment with abrasive paper (sandpaper) or abrasive stone of medium size, 40 or 60 according to GOST 3647-80 is used. At the same time, it should be borne in mind that many abrasive materials during grinding introduce silicon, aluminum and phosphorus into the surface of the sample with abrasive particles, which can affect the results of the analysis.