Читать книгу X-Ray Fluorescence in Biological Sciences - Группа авторов - Страница 41

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 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.