X-Ray Fluorescence in Biological Sciences. Группа авторов

Чтение книги онлайн.

СКАЧАТЬ all product components should be declared in the label; even those concentrations that do not pose a risk to human health or that do not pose a risk of exceeding permissible levels of toxic metals. Several countries have established databases of many product components. This information is used in the interests of the scientific community to support industry and government in the areas related to food and health.

Reviews of the current state of the problem of the application of X‐ray fluorescence analysis (XRF) in biology [2–4] and, in particular, in the study of the chemical composition of food products [3–7], showed that XRF can be successfully used to study such problems. The assortment of food products analyzed using XRF is very diverse: milk and dairy products, grain and bakery products, vegetables and fruits, spices, meat, fish and seafood, sugar, honey, alcoholic (alcohol, vodka, cognac, rum, wine, beer) and soft drinks (drinking and mineral water, juice, lemonade and cola, tea, and coffee). When examining food quality, they usually evaluate calorie content of carbohydrates, fats, fatty acids, sugar, cholesterol, proteins, carotene, vitamins, folic acid, as well as the content of individual elements Ca, Na, Fe, Mg, K, Zn, Cu, P, Se, etc. Consumed products are ranked by importance, depending on a few factors [8–10]. Several reviews discussed the use of XRF for studying the chemical composition of food products. Peculiarities of total reflection X‐ray fluorescence (TXRF) applications for wine and coffee research are considered in [11], food in [12, 13], natural waters in [14], tea and coffee in [15], drinks with sucrose matrix in [16]. Pashkova et al. [17, 18] presented reviews of the features of XRF application in determining elements in milk and dairy products. The climatic conditions for growing coffee beans, tea, fruits, and cereal products (wheat, rice, corn, etc.) are related to the expected quality and brand of the product. Measurement of trace elements is an effective way to verify the authenticity of product labeling [11–13,19–26]. It is a common practice to apply more than one classification method for solving this problem [25–28].

      There are a few reasons to study the chemical composition of food products:

       Study of biological processes occurring in plants during different periods of vegetation,

       Evaluation of the influence of natural factors on the accumulation of individual chemical elements by plants,

       Evaluation of food quality,

       Product brand verification,

       Checking of toxic elements content,

       Evaluation of changes in quality products during long‐term storage.

      It is obvious that XRF can make a significant contribution to the study of problems related to food and the health of the population. Unfortunately, information about XRF applications for these tasks is scattered in a large number of periodicals. Thus, publications from more than 35 journals were examined during the preparation of this chapter alone. Less than half of all articles on the application of XRF to investigate the chemical composition of tea and coffee were published in four journals. This is “X‐ray Spectrometry” (14), “Analytics and Control” (9), “Food Chemistry” (7), and “Journal of Analytical Atomic Spectrometry” (4). This chapter provides an overview of the current state of the problems troubling the use of XRF in the study of chemical composition of tea and coffee.

      Tea is one of the most popular beverages on Earth. Annual tea production exceeds 3.5 million tons. Tea destroys intestinal infections and is a good antibacterial and antimicrobial agent. The complex chemical composition of tea depends on many factors, including soil composition, growing conditions, tea variety, etc. Tea leaves contain most elements essential to human health. Tea is also one of the richest sources of antioxidants [8, 9]. The main chemical compounds present in tea are catechins, flavonoids, theaflavins, alkaloids, enzymes, vitamins, amino acids, aromatics, volatile oils, etc. They are typically determined by chromatographic methods [8, 9]. Because of its specific flavor and positive effect on human health, coffee along with tea is another one of the world's most popular drinks. The average European Union citizen consumes about 5 kg of coffee a year. Moderate consumption of coffee and tea has a positive effect on the human body due to its chemical composition. Coffee contains P and Ca, which contributes to bone strengthening.

      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 СКАЧАТЬ