XRF (X-ray fluorescence analysis) is a physical analysis method that directly determines almost all chemical elements in powder, liquid and solid materials.
The benefits of the method
This method is universal as it is based on quick and easy sample preparation. The method has been widely used in industry, in the field of scientific research. The X-ray fluorescence method of analysis has tremendous potential, useful in very complex analysis of various environmental objects, as well as in quality control of manufactured products and in the analysis of finished products and raw materials.
History
X-ray fluorescence analysis was first described in 1928 by two scientists - Glocker and Schreiber. The device itself was created only in 1948 by scientists Friedman and Burks. As a detector, they took a Geiger counter, which showed high sensitivity with respect to the atomic number of the element's nucleus.
The helium or vacuum medium in the research method began to be used in 1960. They were used to determine light elements. Also started using fluoride crystalslithium. They were used for diffraction. Rhodium and chromium tubes were used to excite the waveband.
Si(Li) - silicon lithium drift detector was invented in 1970. It provided high data sensitivity and did not require the use of a crystallizer. However, the energy resolution of this instrument was worse.
Automated analytical part and process control transferred to the machine with the advent of computers. The control was carried out from the panel on the instrument or the computer keyboard. Analyzers became so popular that they were included in the Apollo 15 and Apollo 16 missions.
At the moment, space stations and ships launched into space are equipped with these devices. This allows you to identify and analyze the chemical composition of the rocks of other planets.
Method Essence
The essence of X-ray fluorescence analysis is to conduct a physical analysis. It is possible to analyze in this way both solids (glass, metal, ceramics, coal, rock, plastic) and liquids (oil, gasoline, solutions, paints, wine and blood). The method allows you to determine very small concentrations, at the ppm level (one part per million). Large samples, up to 100%, are also amenable to research.
This analysis is fast, safe and non-destructive to the environment. It has high reproducibility of results and data accuracy. The method allows semi-quantitative, qualitative and quantitative detection of all elements that are in the sample.
The essence of the X-ray fluorescence method of analysissimple and understandable. If you leave the terminology aside and try to explain the method in a simpler way, then it turns out. That the analysis is carried out on the basis of a comparison of the radiation that results from the irradiation of an atom.
There is a set of standard data that is already known. By comparing the results with these data, scientists conclude what the composition of the sample is.
The simplicity and accessibility of modern devices allow them to be used in underwater research, space, various studies in the field of culture and the arts.
Working principle
This method is based on the analysis of the spectrum, which is obtained by exposing the material to be examined by X-rays.
During irradiation, the atom acquires an excited state, which is accompanied by the transition of electrons to quantum levels of a higher order. The atom stays in this state for a very short time, about 1 microsecond, and after that it returns to its ground state (quiet position). At this time, the electrons located on the outer shells either fill the vacant places, and release the excess energy in the form of photons, or transfer energy to other electrons located on the outer shells (they are called Auger electrons). At this time, each atom emits a photoelectron, the energy of which has a strict value. For example, iron, when exposed to X-rays, emits photons equal to Kα, or 6.4 keV. Accordingly, by the number of quanta and energy, one can judge the structure of matter.
Radiation source
The X-ray fluorescence method of metal analysis uses both isotopes of various elements and X-ray tubes as a source for healing. Each country has different requirements for the export and import of emitting isotopes, respectively, in the industry for the production of such equipment, they prefer to use an X-ray tube.
Such tubes come with copper, silver, rhodium, molybdenum or other anodes. In some situations, the anode is chosen depending on the task.
Current and voltage are different for different elements. It is enough to investigate light elements with a voltage of 10 kV, heavy - 40-50 kV, medium - 20-30 kV.
During the study of light elements, the surrounding atmosphere has a huge impact on the spectrum. To reduce this effect, the sample in a special chamber is placed in a vacuum or the space is filled with helium. The excited spectrum is recorded by a special device - a detector. The accuracy of separation of photons of different elements from each other depends on how high the spectral resolution of the detector is. Now the most accurate is the resolution at the level of 123 eV. An X-ray fluorescence analysis is carried out by a device with such a range with an accuracy of up to 100%.
After the photoelectron has been converted into a voltage pulse, which is counted by special counting electronics, it is transmitted to the computer. From the peaks of the spectrum, which gave X-ray fluorescence analysis, it is easy to qualitatively determine whichthere are elements in the studied sample. In order to accurately determine the quantitative content, it is necessary to study the resulting spectrum in a special calibration program. The program is pre-created. For this, prototypes are used, the composition of which is known in advance with high accuracy.
To put it simply, the obtained spectrum of the studied substance is simply compared with the known one. Thus, information about the composition of the substance is obtained.
Opportunities
X-ray fluorescence analysis method allows you to analyze:
- samples whose size or mass is negligible (100-0.5 mg);
- significant reduction in limits (lower by 1-2 orders of magnitude than XRF);
- analysis taking into account variations in quantum energy.
The thickness of the specimen to be examined should not exceed 1 mm.
In the case of such a sample size, it is possible to suppress secondary processes in the sample, among which:
- multiple Compton scattering, which significantly broadens the peak in light matrices;
- bremsstrahlung of photoelectrons (contributes to the background plateau);
- inter-element excitation as well as fluorescence absorption which requires inter-element correction during spectrum processing.
Disadvantages of the method
One of the significant drawbacks is the complexity that accompanies the preparation of thin samples, as well as strict requirements for the structure of the material. For research, the sample must be very finely dispersed and highly uniform.
Another disadvantage is that the method is heavily tied to standards (reference samples). This feature is inherent in all non-destructive methods.
Application of method
X-ray fluorescence analysis has become widespread in many areas. It is used not only in science or industry, but also in the field of culture and arts.
Used in:
- environmental protection and ecology for the determination of heavy metals in soils, as well as for their detection in water, precipitation, various aerosols;
- mineralogy and geology carry out quantitative and qualitative analysis of minerals, soils, rocks;
- chemical industry and metallurgy - control the quality of raw materials, finished products and the production process;
- paint industry - analyze lead paint;
- jewelry industry - measure the concentration of precious metals;
- oil industry - determine the degree of contamination of oil and fuel;
- food industry - identify toxic metals in foods and ingredients;
- agriculture - analyze trace elements in various soils, as well as in agricultural products;
- archaeology - carry out elemental analysis, as well as dating of finds;
- art - they study sculptures, paintings, examine objects and analyze them.
Ghost settlement
X-ray fluorescence analysis GOST 28033 - 89 has been regulating since 1989. Documentall questions regarding the procedure are registered. Although many steps have been taken over the years to improve the method, the document is still relevant.
According to GOST, the proportions of the studied materials are established. The data is displayed in a table.
Table 1. Ratio of mass fractions
| Defined element | Mass fraction, % |
| Sulfur | From 0.002 to 0.20 |
| Silicon | "0.05 " 5.0 |
| Molybdenum | "0.05 " 10.0 |
| Titanium | "0, 01 " 5, 0 |
| Cob alt | "0.05 " 20.0 |
| Chrome | "0.05 " 35.0 |
| Niobium | "0, 01 " 2, 0 |
| Manganese | "0.05 " 20.0 |
| Vanadium | "0, 01 " 5, 0 |
| Tungsten | "0.05 " 20.0 |
| Phosphorus | "0.002 " 0.20 |
Applied equipment
X-ray fluorescence spectral analysis is carried out usingspecial equipment, methods and means. Among the equipment and materials used in GOST are listed:
- multichannel and scanning spectrometers;
- grinding and emery machine (grinding and grinding, type 3B634);
- surface grinder (Model 3E711B);
- screw-cutting lathe (model 16P16).
- cutting wheels (GOST 21963);
- electrocorundum abrasive wheels (ceramic bond, grain size 50, hardness St2, GOST 2424);
- sanding paper (paper base, 2nd type, brand BSh-140 (P6), BSh-240 (P8), BSh200 (P7), electrocorundum - normal, grain size 50-12, GOST 6456);
- technical ethyl alcohol (rectified, GOST 18300);
- argon-methane mixture.
GOST admits that other materials and apparatus may be used to provide accurate analysis.
Preparation and sampling according to GOST
X-ray fluorescence analysis of metals before analysis involves special preparation of the sample for further research.
Preparation is carried out in the appropriate order:
- The surface to be irradiated is sharpened. If necessary, wipe with alcohol.
- The sample is pressed tightly against the opening of the receiver. If the sample surface is not enough, then special limiters are used.
- The spectrometer is prepared for operation according to the instructions for use.
- The X-ray spectrometer is calibrated using a standard sample that complies with GOST 8.315. Homogeneous samples can also be used for calibration.
- Primary graduation is carried out at least five times. In this case, this is done during the operation of the spectrometer on different days.
- When carrying out repeated calibrations, it is possible to use two series of calibrations.
Result analysis and processing
The method of X-ray fluorescence analysis according to GOST involves the performance of two series of parallel measurements to obtain an analytical signal of each element under control.
It is allowed to use the expression of the value of the analytical result and the difference of parallel measurements. In units of measurement, the scales express the data obtained using the calibration characteristics.
If the allowable discrepancy exceeds parallel measurements, then the analysis must be repeated.
One measurement is also possible. In this case, two measurements are carried out in parallel with respect to one sample from the analyzed lot.
The final result is the arithmetic mean of two measurements taken in parallel, or the result of one measurement alone.
Dependence of results on sample quality
For X-ray fluorescence analysis, the limit applies only to the substance in which the element is detected. For different substances, the limits of quantitative detection of elements are different.
The atomic number that an element has can play a big role. Other things being equal, it is more difficult to determine light elements, and heavy elements are easier. Also, the same element is easier to identify in a light matrix than in a heavy one.
Accordingly, the method depends on the quality of the sample only to the extent that the element can be contained in it.
