The application of traditional chemical methods to determine the concentrations of individual elements usually results in labor‐intensive and long‐term chemical methods. Atomic absorption, neutron activation, electrochemical methods and titration have been the most frequently used for this purpose in recent decades. However, the need to pretreat the test materials (preconcentration, acid digestion, dry ashing, extraction, dilution) is associated with large errors and significant time‐consumption. Pereira et al. [29] noted that the cost of studies using energy dispersive X‐ray fluorescence (EDXRF) spectrometer methodology was 10 times lower than that of studies using standard methodology based on sample mineralization in the determination of metals using atomic spectrometry in the absorption or emission variant. Karak et al. [30, 31] noted that even though tea is grown in more than 40 countries, published papers on the study of its chemical composition are limited to China, India, Japan, Sri Lanka, and Turkey. As a result, more research is needed on this question to correctly understand the mechanisms of microelement storage by tea plants, the specification of microelement uptake, and their impact on human health as a result of regular consumption of tea grown in different countries.
3.2 The Equipment Used
The past decade has been characterized by the rapid development of individual XRF variants. Advances in capillary optics and micro‐XRF have been noted. A few new models of XRF spectrometers have been constructed which use polycapillary lenses and half lenses as collimating systems [32–36]. This is especially important in the case of the use of X‐ray fluorescence for detection of certain elements in vivo in bones, tissues, and individual organs. Dynamic development is typical for thermoelectrically‐cooled detectors [33, 36], TXRF [37–40] and spectrometers with polarized radiation [41–44]. Convenient portable spectrometers are widely used for analysis of various samples, including plant materials [45–50].
The publications reviewed set out versions for the use of the following models XRF spectrometers: the multichannel spectrometer of SRM‐25 (USSR), scanning X‐ray spectrometer of VRA‐30 (Germany, GDR), SPARK‐1‐2М (OAO NPP Burevestnik, S.‐Petersburg, Russia), Rigaku ZSX‐100e and 3270E (Japan), S4 Explorer (Bruker AXS, Germany), Spectro‐X‐LAB2000 (Germany, ED), X‐ray TXRF spectrometers − EXTRA II (Germany) and S2 Picofox (Bruker AXS, Germany), Shimadzu EDX 700 (Japan), Niton XL3t900s portable ED spectrometer, and the Epsilon 5 ED spectrometer with polarizer from PANanalytical, ElvaX Industrial (Elvatech Ltd., Ukraine, ED). In most works, the authors do not discuss the reasons for choosing a specific XRF version. It can be assumed that the main parameters for selecting an X‐ray spectrometer are the cost of equipment and published data on metrological characteristics of commercially available spectrometers.
3.3 Preparation of Samples for Analysis
Requirements for the preparation procedure required by specimens to be used in XRF, as well as factors affecting the value of sample preparation errors, are considered in the monograph of Revenko [42]. Specific information about this important procedure can be found in the papers dealing with application of XRF to the analysis of plant materials [12,50–53]. As analytical chemistry develops, sample preparation becomes an increasingly important stage of analysis, taking up to 80% of the total analysis time in some cases [52].
In preparation for XRF plant samples, either the directly dried material (thorough grinding followed by tablet compression) or one of the lyophilization variants is used. Typical strategies for tea sample preparation include a dry treatment or wet decomposition (in open and closed systems) [54]. Both options lead to the decomposition and the destruction of the complex organic matrix of tea and facilitate the extraction of elements into the solution, 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