What is X-ray fluorescence analysis?

XRF (X-ray fluorescence analysis) is a method of physical analysis that directly determines virtually all chemical elements in powdered, liquid and solid materials.

The use of the method

This method is universal, because it is based on fast and simple sample preparation. He received the method of wide dissemination in industry, in the field of scientific research. X-ray fluorescence method of analysis has great potential, useful for very complex analysis of different environmental objects, as well as for quality control of products and for 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 Berks. As a detector, they took a Geiger counter, which showed high sensitivity with respect to the atomic number of the element nucleus.

The helium or vacuum medium in the research method was used in 1960. They were used to determine the light elements. Also, lithium fluoride crystals were used. They were used for diffraction. Rhodium and chrome tubes were used to excite the wave band.

Si (Li) - drifting silicon lithium 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 device was worse.

Automated analytical part and process control passed machine with the advent of computers. The control was carried out from the panel on the device or the computer keyboard. Instruments for analysis became so popular that they were included in the missions "Apollo-15" and "Apollo-16".

At the moment, space stations and ships launched into space are equipped with these devices. This allows us to identify and analyze the chemical composition of the rocks of other planets.

The essence of the method

The essence of X-ray fluorescence analysis is the physical analysis. Analyze this way you can both solid bodies (glass, metal, ceramics, coal, rock, plastic), and liquids (oil, gasoline, solutions, paints, wine and blood). The method allows to determine very small concentrations, at the level of ppm (one part per million). Large, up to 100% of the sample, are also amenable to research.

This analysis is fast, safe and non-destructive for the environment. It has high reproducibility of results and accuracy of data. The method allows semiquantitative, qualitative and quantitative detection of all elements that are in the sample.

The essence of the X-ray fluorescent method of analysis is simple and understandable. If you leave the terminology aside and try to explain the method easier, it turns out. That the analysis is carried out on the basis of a comparison of the radiation that results from irradiation of the atom.

There is a set of standard data that is already known. Comparing the results with these data, scientists make a conclusion about the composition of the sample.

The simplicity and accessibility of modern apparatuses make it possible to apply them in the conditions of underwater research, space, various studies in the field of culture and the arts.

Principle of operation

This method is based on the analysis of the spectrum, which is obtained by the method of influencing the material, which is being investigated, 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. In this state, the atom is very little time, about 1 microsecond, and then returns to its basic state (quiet position). At this time, the electrons located on the outer shells or fill the vacant vacant places, and the excess energy is released in the form of photons, or transmit energy to other electrons located on the outer shells (they are called Auger electrons). At this time, each atom releases a photoelectron whose energy has a strict value. For example, iron irradiates X-rays emitting photons equal to Kα, or 6.4 keV. Accordingly, the number of quanta and energy can be used to judge the structure of matter.

Source of radiation

X-ray fluorescence method of metal analysis as a source for cure uses both isotopes of various elements and X-ray tubes. In each country different requirements are applied to the importation of emitting isotopes, respectively, in the manufacturing industry such techniques prefer to use an X-ray tube.

Such pipes are like with copper, silver, rhodium, molybdenum or other anode. In some situations, the anode is selected depending on the task.

The current and voltage for different elements are used different. It is enough to investigate light elements with a voltage of 10 kV, heavy ones - 40-50 kV, medium ones - 20-30 kV.

During the research of light elements, the surrounding atmosphere has a huge influence on the spectrum. To reduce this effect, a sample in a special chamber is placed in a vacuum or fills the space with helium. The excited spectrum is registered by a special device - a detector. On how high the spectral resolution of the detector depends on the accuracy of separating the photons of different elements from each other. Now the most accurate resolution is at the level of 123 eV. X-ray fluorescence analysis with this range conducts with an accuracy of 100%.

After the photoelectron has been converted into a voltage pulse, which is counted by special counting electronics, it is transmitted to the computer. On the spectrum peaks, which gave X-ray fluorescence analysis, it is easy to qualitatively determine which elements exactly belong to the sample under study. In order to accurately determine the quantitative content, it is necessary to study the obtained spectrum in a special calibration program. The program is created in advance. For this, prototypes are used whose composition is known in advance with high accuracy.

To put it simply, the obtained spectrum of the substance under study is compared elementarily with the known one. Thus, information on the composition of the substance is obtained.

Capabilities

X-ray fluorescence analysis allows to analyze:

• Samples whose size or mass is negligible (100-0.5 mg);
• A weighty reduction in the limits (lower by 1-2 orders of magnitude than the RFA);
• The analysis taking into account variations in the energy of quanta.

The thickness of the specimen to be examined should not be more than 1 mm.

In the case of such a sample size, suppression of secondary processes in the sample is possible, among which:

• Multiple Compton scattering, which in the lungs of mastritis significantly expands the peak;
• Bremsstrahlung of photoelectrons (contributes to the background plateau);
• Excitation between elements, as well as absorption of fluorescence, which requires inter-element correction during the processing of spectra.

Disadvantages of the method

One of the weighty drawbacks is the complexity, which is accompanied by the preparation of thin samples, as well as stringent requirements for the structure of the material. For the study, the sample must be very finely dispersed and highly homogeneous.

Another drawback is that the method is strongly tied to standards (reference samples). Such a feature is inherent in all non-destructive methods.

Application of the method

X-ray fluorescence analysis has become widespread in many areas. It is used not only in science or production, but also in the field of culture and the arts.

Applies to:

• 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 conduct a quantitative and qualitative analysis of minerals, soils, and rocks;
• Chemical industry and metallurgy - control the quality of the raw materials, finished products and the production process;
• Paint and varnish industry - analyze lead paints;
• Jewelry industry - measure the concentration of valuable metals;
• Oil industry - determine the degree of contamination of oil and fuel;
• Food industry - identify toxic metals in food and ingredients;
• Agriculture - analyze microelements in various soils, as well as in agricultural products;
• Archeology - conduct elemental analysis, as well as the dating of findings;
• Art - conduct study of sculptures, pictures, carry out examination of subjects and their analysis.

The Gorost settlement

X-ray fluorescence analysis GOST 28033 - 89 regulates since 1989. The document spelled out all the questions concerning the procedure. Despite the fact that many steps have been taken over the years towards improving the method, the document is still relevant.

According to GOST, the ratios of the shares of the materials under study are established. The data is displayed in the table.

Table 1. Mass fraction ratio

Defined item	Mass fraction,%
Sulfur	0.002 to 0.20
Silicon	"0.05" 5.0
Molybdenum	"0.05" 10.0
Titanium	"0.01" 5.0
Cobalt	"0.05" 20.0
Chromium	"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 with the help of special equipment, methods and means. Among the used equipment and materials in GOST are listed:

• Multichannel and scanning spectrometers;
• Grinding machine (grinding-grinding, type 3B634);
• Surface grinding machine (model 3Е711В);
• Screw-cutting lathe (model 16P16).
• Cutting discs (GOST 21963);
• Electrocorundum abrasive wheels (ceramic bond with grain size 50, hardness St2, GOST 2424);
• Sandpaper, grinding (paper basis, type 2, grade БШ-140 (П6), БШ-240 (П8), БШ200 (П7), electrocorundum - normal, grain size 50-12, GOST 6456);
• Ethyl alcohol technical (rectified, GOST 18300);
• Argon-methane mixture.

GOST permits that other materials and equipment that can provide accurate analysis can be used.

Preparation and sampling according to GOST

X-ray fluorescence analysis of metals before the analysis assumes a special sample preparation for further investigation.

Preparation is carried out in the appropriate order:

1. The surface that will be irradiated is sharpened. If necessary, then wipe with alcohol.
2. The sample is pressed firmly against the opening of the receiver. If the surface of the sample is not enough, then special restrictors are used.
3. The spectrometer is prepared according to the instructions for use.
4. The X-ray spectrometer is graduated using a standard sample, which corresponds to GOST 8.315. Also, homogeneous samples can be used for calibration.
5. The primary graduation is carried out at least five times. This is done during the operation of the spectrometer on different days.
6. When carrying out repeated calibration, two series of calibrations are possible.

Analysis of results and processing

The method of X-ray fluorescence analysis according to GOST assumes the performance of two series of parallel measurements to obtain an analytical signal of each element being monitored.

It is possible to use the expression of the value of the analytical result and the discrepancy of the parallel measurements. In units of measurement of the scale, the data obtained with the help of the gradient characteristics are expressed.

If the allowable divergence exceeds the parallel measurements, then the analysis must be repeated.

It is also possible to perform one measurement. In this case, two measurements are taken in parallel with respect to one sample from the analyzed lot.

The final result is the arithmetic mean of two measurements conducted in parallel, or the result of a measurement alone.

Dependence of results on sample quality

For X-ray fluorescence analysis, the limit is relative only to the substance in which the element is detected. For different substances, the scope of quantitative detection of elements is different.

A big role can be played by the atomic number that the element has. Other things being equal, it's more difficult to determine the light elements, and the heavier ones are easier. In addition, the same element is easier to determine in a light matrix, rather than in a heavy matrix.

Accordingly, the method depends on the quality of the sample only to the extent that the element in its composition can be contained.