X-Ray Fluorescence in Biological Sciences. Группа авторов

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СКАЧАТЬ as the resulting ash or digestion products are usually easily soluble in water or when exposed to acids. At present, wet decomposition in closed vessels is facilitated by microwave radiation.

In several papers, it is recommended to use the shortest possible time between tablet briquetting and measurement to avoid deformation of flat tablet surfaces [52, 53,55–57]. In the work Gunicheva and Chuparina [55] the question of the effect of aging tablets of standard samples (Certified Reference Material, CRM) of plant materials and its effect on the accuracy of X‐ray fluorescent determination of elemental content from Na to Fe is justified. It has been found that regardless of the biological origin of the plant material for all analytes, the primary reasons for aging the specimens are mechanical destruction and contamination of tablet surfaces during measurement. It is shown that the response of each element to its aging is individual. It has been revealed that the change in intensity that corresponds with the age of CRM specimens is significantly lessened when used as a reference on a CRM adequate in nature to the analyzed materials.

      Chuparina et al. [58] studied the distribution of chemical elements in different parts of the girasol, flowers, leaves, and upper parts of the stem. Lower parts of the stem and tubers were selected separately. Each selected part of the plant was air dried and ground to a powder state. From a mixture of 7.2 g of powder and 0.8 g of boric acid (binder) thoroughly mixed in the agate mortar, two tablets with a diameter of 40 mm were pressed at a force of 16 tons. In this case, the plant material undergoes minimal changes (chemical or thermal effects are excluded). Analytical line intensities were measured by X‐ray spectrometers VRA‐30 (GDR) and SRM‐25 (USSR). The elemental content (Na to Sr) was determined by an α‐correction method using theoretical coefficients. According to the results obtained in [58], the authors divided the investigated elements into two groups. The first group includes K, P, S, and Zn, and the second group includes Na, Mg, Al, Si, Cl, Ca, Mn, Fe, and Sr.

      In the considered papers preliminary ashing (insulation) or fusion of samples with flux is used significantly less often. It is noted that to prepare saturated tablets it is necessary to use a large mass of material compared to that required for rocks [42, 59, 60]. The amount of material required to provide a saturated layer is determined by the characteristics of the short‐wave radiation of the analytical lines used and the scattered characteristic radiation of the anode of the X‐ray tube (for example, Rh Kα), if it is used as a standard, for example in the method of the background standard [60].

      Anawar et al. [61] investigated the effect of different drying procedures on changes in metal and metalloid contents (Sc, Fe, Zn, As, Sb, La) in dried plant materials. The authors examined the effects of freeze drying, air drying, and oven drying processes on the contents of the test elements in plant biomass. Seven varieties of native plant species collected near the mine have been analyzed by instrumental neutron activation analysis. In quantitative analysis, plant samples for freeze and furnace drying procedures show higher levels of biomass than after the air‐drying procedure. Particularly significant losses have been identified for Hg and As. It is noted that this is typical of all plant species studied. The authors concluded that the freeze‐drying process can be recommended as a more controlled, faster, and reliable procedure for determining the contents of the studied elements in some plant materials.

      Kuehner and Pella [62] proposed a procedure for preparing glass discs for determining K, Ca, Mn, Fe, Zn, Rb, and Pb using the example of a CRM analysis of fruit tree leaves. The contents were determined by an energy dispersive spectrometer equipped with a W‐anode X‐ray tube (35 kV, 20 mA) and a secondary Mo target. To remove the organic component, HNO₃ was added to the pre‐dried CRM leaf material, then boiled at 100 °C and H₂SO₄ was added, followed by heating to remove the HNO₃. The dried residue was fused to 6.5 g of Li₂B₄O₇, and the resulting glass disc was polished. The authors obtained satisfactory results, and the relative errors ranged from 2 to 10%.

      At the end of this section it should be noted that the pre‐preparation procedure of material should be simple and cheap. It is desirable that this procedure be conducive to automation. These requirements stem from the need to investigate many food samples.

3.4 Examples of Practical Applications of XRF for Tea Research

Desideri et al. [63, 64] determined Mg, Al, P, S, K, Cl, Ca, Cr, Mn, Fe, Co, Cu, Ni, Zn, As, Br, Rb, Sr, Cd, Sn, I, Hg, Pb in samples of several kinds of tea and chamomile from Italy. The sample material was pre‐dried and then ground and mixed with wax. The study was carried out using Spectro‐X‐LAB2000 energy dispersion spectrometer with a polarizer. The authors determined the contents of essential, non‐essential (micro and trace) and toxic elements Pb, Cd, Hg, and As, according to the results of the analysis of toxic element content, were lower than is deemed permissible by the World Health Organization, so the use of these tea and herbal beverages is safe for human health.

      In a study by Mbaye et al. [65], a portable Niton XL3t900s spectrometer with Ag‐anode was applied for determination the contents of Mg, Al, P, S, K, Ca, Mn, Fe, Cu, Zn, As,Rb, Sr and Pb in tea samples for classification purposes for classification purposes. Commercially available tea samples purchased in China, Cameroon, and Luxembourg were investigated. Dried, milled tea was mixed with wax at a ratio of 10 : 1 and the tablets were pressed. The measurement time of the tablet was two hundred seconds. The precision of the method was tested by CRM INCT‐Tea Leaves‐1. To improve spectrum of contrast in the range of Cd Lα (3.317 keV) and K Kα lines (3.314 keV), the authors proposed the use of a combination of three filters: Al, Ti and Mo. Revenko et al. [66] provided a brief overview of the applications of filters installed between the window of the X‐ray tube and the radiator, and a theoretical and experimental assessment of the possibility of using Al‐filters of different thickness to increase the contrast of the detected Cs Lα₁ radiation has been made. Mbaye et al. [65] has found that K, Ca, Mn, and Fe concentrations are indicators of the geographical origin of tea. For tea from Luxembourg, K and Zn (<300 ppm) were two times higher than tea from China. Mn content in Chinese tea ranged from 0.7 to 1.2%, compared to Cameroonian tea and Luxembourg tea – 0.14 and 0.18%, respectively. The Fe content for Luxembourg tea is 1.2% compared to 0.16–0.45% for Chinese tea and 0.63% for Cameroonian tea. Note that the content of Pb for Luxembourg tea (100 ppm) is five times higher than for Chinese tea and two times higher for Cameroonian tea.

      De La Calle et al. [12] developed a procedure for the simultaneous determination of P, K, Ca, Mn, Fe, Cu, and Zn for plant and spice samples by a TXRF spectrometer S2 PICOFOX (Bruker AXS, Germany) equipped with a 50 W X‐ray tube with a Mo‐anode. Lyophilization followed by grinding and the addition of an internal standard was used to prepare the material for analysis. After the centrifugation procedure, the suspension was deposited on the reflector. The measurement time was five hundred seconds. The accuracy of the procedure, characterized by the repeatability of the determination results, was better than 9%. The correctness of the results of the determination, evaluated by t‐test, showed a good match between certified and experimental values for CRM plants. The proposed procedure was tested to analyze 19 plants. For this aim leaves (black, green and red tea, mate, birch, lime blossom, acacia, mint, thyme, cinnamon, oregon, basil, rosemary, sage), flowers (camomile), and fruits (black and white pepper, hot and sweet paprika) were used. It has been observed that Fe and Cu contents are an indicator in the analysis of different parts of the plant: leaves, flowers, and fruits. The lowest concentrations in the studied plants were obtained for Cu and Zn: from 7 ppm (birch), 10 ppm (mate) to 37 ppm (black tea) and from 15 ppm (white pepper) to 89 ppm (birch), respectively. For samples with tea, Fe content ranged from 210 ppm (matte) to 583 ppm (red tea). The maximum Fe content obtained for camomile colors is 3125 ppm. Tea mate showed a maximum Mn content of 2722 ppm compared to green tea of 1304 ppm, black and red of 631 and 787 ppm.

Ca and K are present in fresh green tea and treated black tea in significant amounts. They are extracted during brewing. K, Ca, and Mn, Fe in 138 samples of black and green tea from Turkey were determined by EDXRF with radioisotope sources ⁵⁵Fe and ²³⁸Pu [67, 68]. Tea was selected three times as the bush grew on plantations СКАЧАТЬ