Scientists from the Faculty of Chemistry of the Lomonosov Moscow State University have performed computations and derived new equations, allowing to conduct X-ray fluorescence analysis with higher accuracy in comparison to algorithms, existing nowadays. At the same time this method doesn't require a large number of reference materials and provides with the possibility to operate with complex composition samples. The chemists have represented their research in the journal Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms.
X-ray fluorescence analysis (XRF analysis) is a tool used for studying substances which allows to detect their chemical (elemental) composition. This technique is based on measurement and analysis of spectrum, emerging as a result of substance X-ray irradiation. When interacting with radiation photons, atoms of the studied reference material move to excited state, which doesn't last for a long time. And after that an atom returns to its ground state. Meanwhile, every atom emits a photon with definite energy, which provides chemists with an opportunity to get the idea of the substance structure.
X-ray tubes are often used as a radiation source. Reference materials, whose composition is known, allow to calculate detected element content from measured radiation intensity.
One of the X-ray fluorescence analysis problems, which have not been solved yet, is the presence of a substantial amount of light elements (II-III periods of Mendeleev periodic system) in many real samples. Very often radiation of these light elements can't be registered. X-ray fluorescence radiation of light elements is referred to as soft (long-wave) radiation, so you can't use salt crystals as an analyzer of radiation wavelength since distances between planes where atoms of these crystals lie are too small - smaller than a wave length of light elements' radiation.
At the same time ordinary diffraction gratings, namely optical devices, composed of a set of regularly situated slits, are also unsuitable. The reason is that they are aimed at radiation with the wavelength of about tens or hundreds of nanometers, while you need radiation with the wavelength of several nanometers. So, the only one solution is to use expensive synthetic multilayer mirrors which are not available in every spectrometer.
There is also a fundamental problem - namely, low fluorescence yield (ration of the number of emergent photons to the absorbed ones) of light elements. This means that very powerful X-ray tubes are necessary, that again leads to increase of device cost and, consequently, analysis cost. Moreover, processes of X-ray fluorescence excitation of light elements are more complicated than processes of excitation of heavy elements and aren't studied as well, so traditional X-ray fluorescence analysis techniques don't guarantee good results all the time.
Andrey Garmay, a doctoral student at the Analytical Chemistry Department at the Faculty of Chemistry of the Lomonosov Moscow State University and one the project authors comments: "So, there are three difficulties with oxygen, carbon and other light elements: one technical and two fundamental ones. You need expensive devices for solving the first and second problems and fundamental physical researches to solve the third one. Nowadays, indirect methods of determination of light elements' content in most case turn out to be much cheaper and more accurate even when good equipment is available. That's why we are also proceeding in this very direction".
Also difficulties emerge in case of different nonstandard objects, for instance, technological products of complex shape, if it's not easy to find appropriate reference materials for them. At the same time the most accurate analytical techniques work in narrow ranges of samples compositions and often require dozens of reference materials.
Andrey Garmay shares: "Taking into consideration the experience of XRF analysis development, we've decided to use for analysis not absolute intensities of elements' radiation, but their ratios and also the ratio of intensities of X-ray tube characteristic radiation, coherently (without wavelength change) to incoherently (energy of a part of scattered photons is less than energy of initial beam quanta) scattered by a sample. We've managed to derive new equations, allowing to conduct analysis with equal or even higher accuracy than existing algorithms. At the same time, these equations require no more than one or two reference materials and could operate in wide ranges of sample compositions."
The scientists began to use an internal standard method in order to neutralize the impact of experimental factors, changing from one measurement to another, on analytical response. Thus, these factors, influencing two close signals in spectrum approximately identically, compensate each other and measurement error becomes lower when not absolute values but ratios of these signals are used. The chemists have decided to try to replace a part of experimental measurements by computations in order to become less dependent on expensive standard samples and operate in wider ranges of sample compositions.
Andrey Garmay says: "Intensity ratio of characteristic radiation of an X-ray tube, coherently to incoherently scattered by a sample, has been used since the 60-s. But we've managed to derive an approximate formula for its description and combined with our equations in one tool, providing with an opportunity to conduct analysis in those cases, when there are no appropriate reference materials. Besides that we don't need to use many standard samples in order to get good results which means that time, spent for spectra measurement and analytical curves' plotting is reduced. Consequently the total analysis time is reduced, too'.
Moreover, the method elaborated by the chemists has turned out to be the only one suitable for analysis of nonstandard objects with high content of undetected light elements in the absence of adequate reference materials.
The research author tells: "Initially we were looking for some tools able to improve accuracy of steel sample analysis but later on faced a problem of oxide material analysis. And since our spectrometer couldn't register oxygen radiation, we had to look for other problem solving means, starting out from existing techniques. We've studied fundamental equations, connecting intensities of characteristic and scatter radiation with composition of reference materials and managed to derive new simplified formulas, necessary for our analysis".
In the course of the works the scientists have measured spectra of high-alloy steel samples, iron-ore material samples and a powder blend of metal oxides with known composition. Using the new approach along with other well-proven XRF analysis techniques, the chemists have conducted analysis and assured themselves of the fact that the elaborated tool allows to get more precise results, especially in the absence of adequate reference materials.
The scientists still need to prove experimentally that their method is applicable for determination not only of IV period elements, but also of heavier elements. Besides that, the researchers are going to optimize the analysis procedure and make it easier without loss of accuracy.
Andrey Garmay sums up: "In the long term perspective we are going to check, if it's possible to estimate the qualitative composition of undetected light elements, judging by the wavelength distribution of bremsstrahlung radiation of an X-ray tube, scattered by a sample. This could make our method more universal".
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Journal
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms