X-Ray Fluorescence in Biological Sciences. Группа авторов

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 located at different heights (0–240 m) above sea level. The tea material was subjected to known raw material treatment methods (twisting, crushing, grinding and pressing). For analysis, tea samples were ground in a vibration mill for 10 minutes to a particle size of 150 μm. Then, 200 mg tea was compressed into 24 mm diameter tablets and reinforced with Mylar film. The resulting thickness of the tablet for determining said elements in the plant material was 44.2 mg/cm². Since the peaks of the characteristic emission of the detected elements have overlays, the spectra obtained for specially prepared samples with known analyte content from the tea sample spectra were subtracted to determine K, Ca and Mn, Fe contents. Kα line intensities were normalized to the intensity of coherent and non‐coherent scattered radiation of sample atoms. In order to obtain a calibration function, the intensity of the Kα‐lines of the tea sample analytes was compared with the intensity of the lines of the samples prepared by the authors. The detection limits of K, Ca and Mn, Fe were 21, 40 and 116, and 157 ppm, respectively. The authors have found that tea collected for the first time from the bush contains the highest concentration of K (2.54%) and Fe (469 ppm) compared to late collection tea – K (2.38%) and Fe (358 ppm), respectively. For Ca and Mn, the reverse pattern is noticed: old leaves contain more Ca (0.47%) and Mn (0.24%) than young leaves – 0.39 and 0.2%, respectively. It is noted that the K and Ca contents in stems and leaves differ, so depending on the parts of the plant used, there is a difference in the contents for different tea versions througout the treatment. In the case of Fe, different contents have also been obtained depending on the tea treatment used; this may be due to contamination from the equipment.

      Xie et al. [69] used an EXTRA II TXRF spectrometer from Seifert, Germany (Mo‐anode, 50 kV, 10–38 mA) for simultaneous determination of 15 elements in 39 samples of tea grown in the main provinces of tea production in China. For analysis, dried tea material was digested in nitric and hydrochloric acids under high pressure, and an internal standard Ga was added to the solution after cooling. In addition, tea infusions were prepared according to a standard Chinese brew procedure, the resulting solution was filtered, cooled, adjusted to PH < 2, and then Ga was added. Suspensions were prepared by diluting the resulting solutions (1 : 5) with distilled water, aliquots were applied to quartz substrates and dried under an infrared lamp. The CRM tea GBW 08505 (China) was used to control the accuracy of the results. The range of P, S, K, Ca obtained was 0.15–3.1%, Mn, Fe – 50–1800 ppm, Ti, Ni, Cu, Zn, Rb, Sr, Ba, and Pb – 0.3–150 ppm. Solubility in tea was determined: for Ca, Ti, Fe, Ba, and Pb it was up to 17%, for other elements – from 7 to 86%. The influence of the origin, type and quality of tea samples has been studied. The content of elements in tea depends on the chemical and mineral composition of the soil in which it grows. For Oolong tea, P, Ni, Cu, and Zn contents were lower and Mn and Rb were higher compared to similar values in black and green tea. Oolong tea is a semi‐fermented tea that by Chinese classification occupies an intermediate position between green and black tea. Better quality black tea corresponds to higher P, Ni, and Zn content; the opposite trend is seen for K, Ca, Ti, Mn, Sr, and Ba. In addition, the solubility of K, Ca, Mn, Zn, Rb, and Sr increases with improved black tea quality. It is obtained that the solubility of all elements is directly proportional to the time of brewing. However, as the brew time was increased, the solubility for Ca and Fe changed slightly.

Salvador et al. [70] used a spectrometer EDXRF (Mo‐anode, Zr‐filter, 25 kV, 10 mА) for determination of content of elements (22 ≤ Z ≤ 30) in five types of tea (camomile, mint, with a melissa, apple, black) from six producers. Preparation of the samples for analysis was brewing of tea, lyophilization and leaching. The resulting solution was then filtered on a special membrane and analyzed in a spectrometer (spectrum measurement for three hundred seconds). In order to control the accuracy of the results, the procedure was applied with an exposition of one hundred seconds for similarly prepared standards, each containing one element, Fe, Co, Ni, Cu, or Zn. According to the analysis results, the presence of Fe, Ni and Cu was found in all tea samples analyzed. Ti, Cr, Co, and Zn are present in chamomile tea samples and in apple tea; Ti, Co, and Zn in mint tea and Ti, Mn, Co, and Zn in melissa tea. The maximum content of Ti and Fe