X-ray spectral analysis occupies an important place among all methods of studying materials. It is widely used in various fields of technology due to the possibility of express control without destroying the test sample. The time for determining one chemical element can be only a few seconds; there are practically no restrictions on the type of substances under study. The analysis is carried out both in qualitative and quantitative terms.
The essence of X-ray spectral analysis
X-ray spectral analysis is one of the physical methods for the study and control of materials. It is based on an idea common to all methods of spectroscopy.
The essence of X-ray spectral analysis lies in the ability of a substance to emit characteristic X-ray radiation when atoms are bombarded by fast electrons or quanta. At the same time, their energy must be greater than the energy that is necessary to pull out an electron from the shell of an atom. Such an impact leads not only to the appearance of a characteristic radiation spectrum,consisting of a small number of spectral lines, but also continuous. Estimation of the energy composition of detected particles makes it possible to draw conclusions about the physical and chemical properties of the object under study.
Depending on the method of action on the substance, either particles of the same type or others are recorded. There is also X-ray absorption spectroscopy, but it most often serves as an auxiliary tool for understanding the key issues of traditional X-ray spectroscopy.
Types of Substances
Methods of X-ray spectral analysis allow us to study the chemical composition of a substance. This method can also be used as an express non-destructive testing method. The following types of substances may be included in the study:
- metals and alloys;
- rocks;
- glass and ceramics;
- fluid;
- abrasives;
- gases;
- amorphous substances;
- polymers and other organic compounds;
- proteins and nucleic acids.
X-ray spectral analysis also allows you to determine the following properties of materials:
- phase composition;
- orientation and size of single crystals, colloidal particles;
- alloy state diagrams;
- atomic structure and dislocation of the crystal lattice;
- internal stresses;
- thermal expansion coefficient and other characteristics.
Based on this method inproduction uses X-ray flaw detection, which allows you to detect various types of inhomogeneities in materials:
- shells;
- foreign inclusions;
- pores;
- cracks;
- Faulty welds and other defects.
Types of analysis
Depending on the method of generating X-rays, the following types of X-ray spectral analysis are distinguished:
- X-ray fluorescent. Atoms are excited by primary X-ray radiation (high-energy photons). This lasts for about a microsecond, after which they move into a calm, basic position. The excess energy is then emitted in the form of a photon. Each substance emits these particles with a certain level of energy, which makes it possible to accurately identify it.
- X-ray radiometric. Atoms of matter are excited by gamma radiation from a radioactive isotope.
- Electron probe. Activation is performed by a focused electron beam with an energy of several tens of keV.
- Assay with ion excitation (protons or heavy ions).
The most common method of X-ray spectral analysis is fluorescence. X-ray excitation when a sample is bombarded with electrons is called direct, and when irradiated with X-rays it is called secondary (fluorescent).
Fundamentals of X-ray Fluorescence Analysis
X-ray fluorescence method widelyused in industry and scientific research. The main element of the spectrometer is the source of primary radiation, which is most often used as X-ray tubes. Under the influence of this radiation, the sample begins to fluoresce, emitting x-rays of the line spectrum. One of the most important features of the method is that each chemical element has its own spectral characteristics, regardless of whether it is in a free or bound state (as part of any compound). Changing the brightness of the lines makes it possible to quantify its concentration.
An X-ray tube is a balloon inside which a vacuum is created. At one end of the tube there is a cathode in the form of a tungsten wire. It is heated by an electric current to temperatures that allow the emission of electrons. At the other end is an anode in the form of a massive metal target. A potential difference is created between the cathode and the anode, due to which the electrons are accelerated.
Charged particles moving at high speed hit the anode and excite bremsstrahlung. There is a transparent window in the wall of the tube (most often it is made of beryllium) through which the x-rays exit. The anode in X-ray spectral analysis devices is made of several types of metal: tungsten, molybdenum, copper, chromium, palladium, gold, rhenium.
Decomposition of radiation into a spectrum and its registration
There are 2 types of X-ray dispersion in the spectrum - wave and energy. The first type is the most common. X-ray spectrometers, operating on the principle of wave dispersion, have analyzer crystals that scatter waves at a certain angle.
Single crystals are used to decompose X-rays into a spectrum:
- lithium fluoride;
- quartz;
- carbon;
- acid potassium or thallium phthalate;
- silicon.
They play the role of diffraction gratings. For mass multi-element analysis, instruments use a set of such crystals that almost completely cover the entire range of chemical elements.
X-ray cameras are used to obtain a radiograph, or a diffraction pattern fixed on photographic film. Since this method is time-consuming and less accurate, it is currently used only for flaw detection in X-ray spectral analysis of metals and other materials.
Proportional and scintillation counters are used as detectors of emitted particles. The latter type has a high sensitivity in the region of hard radiation. Photons falling on the photocathode of the detector are converted into an electrical voltage pulse. The signal first goes to the amplifier, and then to the input of the computer.
Scope of application
X-ray fluorescence analysis is used for the following purposes:
- determination of harmful impurities in oil andpetroleum products (gasoline, lubricants and others); heavy metals and other hazardous compounds in soil, air, water, food;
- analysis of catalysts in the chemical industry;
- precise determination of the period of the crystal lattice;
- detecting the thickness of protective coatings by a non-destructive method;
- determining the sources of raw materials from which the item is made;
- calculation of microvolumes of matter;
- determination of the main and impurity components of rocks in geology and metallurgy;
- study of objects of cultural and historical value (icons, paintings, frescoes, jewelry, dishes, ornaments and other items made of various materials), their dating;
- determination of composition for forensic analysis.
Sample preparation
For the study, sample preparation is preliminarily required. They must meet the following conditions for X-ray analysis:
- Uniformity. This condition can be met most simply for liquid samples. When stratifying the solution immediately before the study, it is mixed. For chemical elements in the short-wavelength region of radiation, homogeneity is achieved by grinding into powder, and in the long-wavelength region, by fusion with flux.
- Resistant to external influences.
- Fit with sample loader size.
- Optimal roughness of solid samples.
Since liquid samples have a number of disadvantages (evaporation, change in their volume when heated, precipitationprecipitate under the action of X-ray radiation), it is preferable to use dry matter for X-ray spectral analysis. Powder samples are poured into a cuvette and pressed. The cuvette is installed into the holder through the adapter.
For quantitative analysis, powder samples are recommended to be pressed into tablets. To do this, the substance is ground to a state of fine powder, and then tablets are made on the press. To fix friable substances, they are placed on a substrate of boric acid. Liquids are poured into the cuvettes using a pipette, while checking the absence of bubbles.
Preparation of samples, selection of an analysis technique and the optimal mode, selection of standards and construction of analytical graphs on them is carried out by an X-ray spectral analysis laboratory assistant who must know the basics of physics, chemistry, the design of spectrometers and the research methodology.
Qualitative analysis
Determination of the qualitative composition of samples is carried out to identify certain chemical elements in them. Quantification is not carried out. Research is carried out in the following order:
- preparing samples;
- preparing the spectrometer (warming it up, installing the goniometer, setting the wavelength range, scanning step and exposure time in the program);
- quick scanning of the sample, recording the obtained spectra in the computer's memory;
- deciphering the resulting spectral decomposition.
Intensity of radiation at each momentscanning is displayed on the computer monitor in the form of a graph, along the horizontal axis of which the wavelength is plotted, and along the vertical axis, the radiation intensity. The software of modern spectrometers makes it possible to automatically decode the obtained data. The result of a qualitative X-ray analysis is a list of lines of chemicals that were found in the sample.
Errors
Falsely identified chemical elements can often occur. This is due to the following reasons:
- random deviations of scattered bremsstrahlung;
- stray lines from the anode material, background radiation;
- instrument errors.
The greatest inaccuracy is revealed in the study of samples, which are dominated by light elements of organic origin. When conducting X-ray spectral analysis of metals, the proportion of scattered radiation is less.
Quantitative analysis
Before carrying out quantitative analysis, a special setting of the spectrometer is required - its calibration using standard samples. The spectrum of the test sample is compared with the spectrum obtained from irradiation of calibration samples.
The accuracy of determining chemical elements depends on many factors, such as:
- interelement excitation effect;
- background scattering spectrum;
- device resolution;
- linearity of the counting characteristic of the spectrometer;
- X-ray tube spectrum and others.
This method is more complicated and requires an analytical study, taking into account constants determined in advance experimentally or theoretically.
Dignity
The advantages of the X-ray method include:
- possibility of non-destructive testing;
- high sensitivity and accuracy (impurity determination up to 10-3%);
- wide range of analyzed chemical elements;
- easy sample preparation;
- versatility;
- possibility of automatic interpretation and high performance of the method.
Flaws
Among the disadvantages of X-ray spectral analysis are the following:
- increased safety requirements;
- need for individual graduation;
- difficult interpretation of the chemical composition when the characteristic lines of some elements are close;
- necessity to manufacture anodes from rare materials to reduce the background characteristic radiation that affects the reliability of the results.
