X-ray spectral analysis occupies an important place among all methods of research materials. It is widely used in various fields of technology due to the possibility of express control without destroying the test sample. The time it takes to determine one chemical element can be only a few seconds, there are practically no restrictions on the type of test substances. The analysis is carried out both qualitatively and quantitatively.
The essence of x-ray analysis
X-ray spectral analysis is one of the physical methods of research and control of materials. It is based on an idea common to all spectroscopy methods.
The essence of x-ray analysis is the ability of a substance to emit characteristic x-ray radiation during the bombardment of atoms by fast electrons or quanta. At the same time, their energy should be greater than the energy that is needed to tear an electron out of the shell of an atom. Such an effect leads not only to the appearance of a characteristic emission spectrum, consisting of a small number of spectral lines, but also to a continuous one. Evaluation of the energy composition of the detected particles makes it possible to draw conclusions about the physical and chemical properties of the investigated object.
Depending on the method of exposure to the substance, either particles of the same kind 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
X-ray analysis methods allow us to study the chemical composition of the substance. This method can also be used as an express method of non-destructive testing. The following types of substances can participate in the study:
- metals and alloys;
- rocks;
- glass and ceramics;
- liquids;
- abrasive materials;
- 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;
- state diagrams of alloys;
- atomic structure and dislocation of the crystal lattice;
- internal stresses;
- coefficient of thermal expansion and other characteristics.
Based on this method, X-ray flaw detection is used in production, which allows one to detect various types of inhomogeneities in materials:
- sinks;
- foreign inclusions;
- pores
- cracks;
- lack of fusion of 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 fluorescence. Excitation of atoms is produced by primary x-ray radiation (high-energy photons). This lasts about a microsecond, after which they move into a calm, basic position. In this case, excess energy is emitted in the form of a photon. Each substance emits these particles with a certain level of energy, so that it can be accurately identified.
- X-ray radiometric. Excitation of atoms of a substance is carried out by gamma radiation from a radioactive isotope.
- Electron probe. Activation is carried out by a focused electron beam with an energy of several tens of keV.
- Analysis with ion excitation (protons or heavy ions).
The most common X-ray analysis method is fluorescence. X-ray excitation during the bombardment of a sample by 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 is widely used 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 a compound). Changing the brightness of the lines makes it possible to quantify its concentration.
An x-ray tube is a cylinder 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 electric current to temperatures that ensure 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 acceleration of electrons occurs.
Charged particles moving at high speed fall on the anode and excite bremsstrahlung. In the wall of the tube there is a transparent window (most often it is made of beryllium), through which x-rays exit. The anode in X-ray analysis instruments is made of several types of metal: tungsten, molybdenum, copper, chromium, palladium, gold, rhenium.
Decomposition of radiation into the spectrum and its registration
There are 2 types of dispersion of x-rays into the spectrum - wave and energy. The first type is the most common. X-ray spectrometers operating on the principle of wave dispersion have crystal analyzers that scatter waves at a certain angle.
For the decomposition of x-ray radiation into a spectrum, single crystals are used:
- lithium fluoride;
- quartz;
- carbon;
- potassium or thallium acid phthalate;
- silicon.
They play the role of diffraction gratings. For mass multi-element analysis, the instruments use a set of such crystals that almost completely cover the entire range of chemical elements.
To obtain an x-ray, or diffraction pattern recorded on a film, x-ray cameras are used. Since this method is laborious and less accurate, it is currently used only for flaw detection in X-ray spectral analysis of metals and other materials.
As detectors of emitted particles, proportional and scintillation counters are used. The latter type has high sensitivity in the field of hard radiation. Photons incident on the photocathode of the detector are converted into an electrical voltage pulse. The signal is first fed to the amplifier, and then to the input of the computer.
Application area
X-ray fluorescence analysis is used for the following purposes:
- determination of harmful impurities in oil and oil products (gasoline, lubricants and others); heavy metals and other hazardous compounds in soil, air, water, food;
- analysis of catalysts in the chemical industry;
- precision determination of the period of the crystal lattice;
- revealing the thickness of protective coatings by non-destructive method;
- determination of the sources of raw materials from which the item is made;
- calculation of microvolumes of a substance;
- determination of the main and impurity components of rocks in geology and metallurgy;
- the study of objects of cultural and historical value (icons, paintings, murals, jewelry, dishes, jewelry and other objects from various materials), their dating;
- determination of the composition for forensic analysis.
Sample preparation
For the study, sample preparation is first required. They must meet the following conditions for x-ray analysis:
- Homogeneity. The easiest way is to provide this condition for liquid samples. When the solution is stratified, immediately before the study, it is mixed. For chemical elements in the short-wavelength region of radiation, homogeneity is achieved by abrasion into powder, and in the long-wavelength region, by fusion with flux.
- Resistance to external influences.
- Compliance with the sizes of the loading device.
- Optimum roughness of solid samples.
Since liquid samples have a number of drawbacks (evaporation, a change in their volume when heated, precipitation due to 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 through the adapter is installed in a clip.
For quantitative analysis, powder samples are recommended to be compressed into a tablet form. To do this, the substance is abraded 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 cells using a pipette, while checking for the absence of bubbles.
Sample preparation, selection of the analysis methodology and the optimal mode, selection of standards and the construction of analytical graphs from them is carried out by an X-ray spectral analysis technician who must know the basics of physics, chemistry, the structure of spectrometers and the research methodology.
Qualitative Analysis
Determination of the qualitative composition of the samples is carried out to identify certain chemical elements in them. Quantification is not carried out. The study is carried out in the following order:
- sample preparation;
- spectrometer preparation (warming up, setting the goniometer, setting the wavelength range, scanning step and exposure time in the program);
- quick scan of the sample, recording the obtained spectra in the computer's memory;
- decoding of the obtained spectral decomposition.
The radiation intensity at each moment of scanning is displayed on the computer monitor in the form of a graph, the wavelength is plotted along the horizontal axis, and the radiation intensity along the vertical axis. The software of modern spectrometers allows you to automatically decrypt the received data. The result of a high-quality X-ray spectral analysis is a list of lines of chemicals that were found in the sample.
Inaccuracies
False identified chemical elements can often occur. This is due to the following reasons:
- random deviations of scattered bremsstrahlung;
- scattering lines from the anode material, background radiation;
- instrument errors.
The greatest inaccuracy is revealed in the study of samples, in which light elements of organic origin predominate. When conducting x-ray spectral analysis of metals, the fraction of scattered radiation is less.
Quantitative analysis
Before quantitative analysis, a special setup of the spectrometer is required - its calibration using standard samples. The spectrum of the test sample is compared with the spectrum obtained from the irradiation of calibration samples.
The accuracy of the determination of 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 the constants determined in advance experimentally or theoretically.
Advantages
The advantages of the X-ray spectral method include:
- the possibility of non-destructive research;
- high sensitivity and accuracy (determination of impurity content up to 10 -3 %);
- wide range of analyzed chemical elements;
- ease of sample preparation;
- universality;
- Possibility of automatic interpretation and high performance of the method.
disadvantages
Among the disadvantages of x-ray analysis, the following are distinguished:
- increased safety requirements;
- the need for individual graduation;
- a difficult interpretation of the chemical composition with a close arrangement of the characteristic lines of some elements;
- the need to manufacture anodes from rare materials to reduce the background characteristic radiation, which affects the reliability of the results.