What is x-ray fluorescence analysis?

X-ray powder diffraction (X-ray fluorescence analysis) is a physical analysis method that directly determines almost all the chemical elements in powdery, liquid, and solid materials.

The use of the method

This method is universal, as it is based on quick and easy sample preparation. The method was widely used in industry and in research. The X-ray fluorescence analysis method has great potential, useful in very complex analysis of various environmental objects, as well as in the 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 nucleus of an element.

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 lithium fluoride crystals. Used them for diffraction. Rhodium and chrome tubes were used to excite the wavelength range.

Si (Li) - drift 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.

The analytical part is automated and the process control is transferred to the machine with the advent of computers. Management was carried out from the panel on the device or the computer keyboard. Analytical instruments 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 rocks of other planets.

Method essence

The essence of x-ray fluorescence analysis is to conduct 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 low concentrations, at ppm (one part per million). Large, up to 100% samples are also amenable to research.

This analysis is quick, 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 analysis method is simple and straightforward. If we leave the terminology aside and try to explain the method easier, it turns out. That the analysis is based on a comparison of the radiation that results from the irradiation of an atom.

There is a set of standard data that is already known. Comparing the results with these data, scientists conclude what the composition of the test sample is.

The simplicity and accessibility of modern devices allows them to be used in 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 exposure to the material under study, by x-rays.

During irradiation, an atom acquires an excited state, which is accompanied by a transition of electrons to higher-order quantum levels. In this state, the atom is in very little time, about the 1st microsecond, and after that it returns to its ground state (calm position). At this time, the electrons located on the outer shells either fill the vacant vacant places, and the excess energy is released 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 whose energy has a strict meaning. For example, iron during X-ray irradiation emits photons equal to Kα, or 6.4 keV. Accordingly, the number of quanta and energy can be used to judge the structure of matter.

Radiation source

The X-ray fluorescence metal analysis method uses both isotopes of various elements and X-ray tubes as a source for healing . Each country uses different requirements for the export of emitting isotopes, respectively, in the industry of production of such equipment they prefer to use an x-ray tube.

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

The current strength 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 studies of light elements, the atmosphere has a huge effect on the spectrum. To reduce this effect, a sample located 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 the separation of the 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 123 eV. An X-ray fluorescence analysis device with this range carries out with an accuracy of 100%.

After the photoelectron is converted into a voltage pulse, which is calculated by special counting electronics, it is transmitted to a computer. From the peaks of the spectrum that X-ray fluorescence analysis gave, it is easy to qualitatively determine exactly which elements are present 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.

Simply put, the resulting spectrum of the studied substance is elementarily compared with the known one. Thus, information on the composition of the substance is obtained.

Opportunities

X-ray fluorescence analysis method allows you to analyze:

• samples whose size or weight are negligible (100-0.5 mg);
• a significant reduction in the limits (lower by 1-2 orders of magnitude than the RFA);
• analysis taking into account variations in the energy of quanta.

The thickness of the sample being tested should not be more than 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 in the light mantis significantly expands the peak;
• bremsstrahlung of photoelectrons (contributes to the background plateau);
• excitation between elements, as well as absorption of fluorescence, which requires interelement correction during processing of the spectra.

The disadvantages of the method

One of the significant drawbacks is the complexity that accompanies the preparation of thin samples, as well as the stringent requirements for the structure of the material. For research, the sample must be very fine dispersion and high uniformity.

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

Method application

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

It is applied in:

• environmental protection and ecology to determine heavy metals in soils, as well as to identify them in water, sediments, various aerosols;
• Mineralogy and geology conduct 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 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 - determine toxic metals in food products and ingredients;
• agriculture - trace elements are analyzed in various soils, as well as in agricultural products;
• archeology - conduct elemental analysis, as well as dating of finds;
• art - they study sculptures, paintings, carry out examination of objects and their analysis.

Gostovo settlement

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

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

Table 1. The ratio of mass fractions

Applied Equipment

X-ray fluorescence spectral analysis is carried out using special equipment, methods and tools. Among the used equipment and materials in GOST are listed:

• multichannel and scanning spectrometers;
• peeling-emery machine (grinding and grinding, type 3B634);
• surface grinding machine (model 3711);
• screw-cutting lathe (model 16P16).
• cutting discs (GOST 21963);
• electrocorundum abrasive wheels (ceramic bond with grain size 50, hardness St2, GOST 2424);
• grinding sandpaper (paper base, 2nd type, grade 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 equipment can be used that provide accurate analysis.

Preparation and sampling in accordance with GOST

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

Preparation is carried out in the following order:

1. The surface to be irradiated is sharpened. If necessary, wipe with alcohol.
2. The sample is tightly pressed to the hole of the receiver. If the sample surface is insufficient, then special limiters are used.
3. The spectrometer is prepared for operation according to the instructions for use.
4. An X-ray spectrometer is graduated using a standard sample that complies with GOST 8.315. Also for calibration can use homogeneous samples.
5. Initial calibration is carried out at least five times. This is done during the operation of the spectrometer on different days.
6. When carrying out repeated calibrations, it is possible to use two series of calibrations.

Analysis of results and processing

The method of X-ray fluorescence analysis according to GOST involves performing two series of parallel measurements to obtain an analytical signal of each element subjected to control.

It is allowed to use the expression of the value of the analytical result and the divergence of parallel measurements. In units of measure, scales express data obtained using calibration characteristics.

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

One measurement is also possible. In this case, two measurements are performed in parallel with respect to one sample from the analyzed batch.

The final result is the arithmetic mean of two measurements carried out in parallel, or the result of only one measurement.

The dependence of the results on the quality of the sample

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

The atomic number that an element has can play a large role. Other things being equal, it is more difficult to define light elements, and heavy ones are easier. In addition, the same element is easier to identify in a light matrix, and not in a heavy one.

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