X-Ray Fluorescence in Biological Sciences. Группа авторов

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СКАЧАТЬ Tea black classic “Bouquet” M4 Tea green classic “Bouquet” M5 Tea black long leaf “Kazachok” M6 Petiolar green tea M7 Tea black skull (petiolar) С1 Tea black long leaf classic Krasnodar since 1901, Sochi, Solokhaul village

      3.6.1 Instrumentation

      The main studies were carried out with the TXRF spectrometer S2 PICOFOX (Bruker, Germany) with a 50 W X‐ray tube and Mo‐anode, multilayer monochromator (Ni/C), and silicon‐drift detector (SDD) with energy resolution ~150 eV. Measurements of analytical line intensities for all elements were performed at 50 kV X‐ray tube voltage and 500 μA current. The material prepared for analysis was applied to quartz substrates. All measurements are made in air.

      A S4 Pioneer (Bruker, Germany) WD X‐ray spectrometer with a power of up to 4 kW with a Rh anode and a 75 μm thickness Be window was used. When the atoms P, S, Cl, and K were excited, the voltage on the X‐ray tube was 30 kV, the current was 60 mA, and the measurement time was twenty seconds. In time measurement the intensity of the analytical Kα lines of Ca, Ti, Mn, Fe, Ni, Cu, Zn, Rb, Sr, and Lα for Ba, a voltage of 50 kV and a current strength of 40 mA was used. The signal recording time for Ca, Mn, Fe, and Sr was twenty seconds, for Ba ‐ fourty seconds and for the remaining elements ‐ thirty seconds. The analyzing crystals used were PET for P, S, and Cl, and LiF 200 for the remaining elements. All measurements were made in a vacuum.

      3.6.2 Suspension Preparation

      To prepare the suspension, 2.0 ml of distilled water, an internal Ga standard with a final concentration in the dry residue of 100 mg/kg, was added to 20 mg of pre‐dried (at 85 °C for four hours) and overloaded tea leaves in the agate mortar, and mixed thoroughly. Next, 10 μl of suspension was applied to a quartz substrate, dried, and measured for five hundred seconds.

      3.6.3 Infusion Preparation

      The standard tea preparation procedure indicated on the package was reproduced. When brewed, 240 mg of tea leaves were placed in special bakes, poured with 30 ml of distilled water (temperature ~90 °C). After 5–7 minutes, 1 ml of solution was taken. Then, 100 μl of 100 mg/l internal Ga standard was added and thoroughly mixed before 10 μl was applied to a siliconized quartz glass carrier and dried. The measurement time of the sample was five hundred seconds.

      3.6.4 Acid Digestion

      10 mg of ground and dried tea leaves were placed in a special glass tube. 1 ml of concentrated nitric acid and 50 μl of hydrogen peroxide were added and heated in an oven until fully evaporated. At the end of the process, 200 μl of 10% HNO₃, 20 μl of 100 mg/l internal Ga standard were added and mixed thoroughly. 10 μl of the resulting solution was placed to a quartz carrier, dried by infrared radiation and measured for five hundred seconds.

      3.6.5 Preparation of Samples for WDXRF

      Samples were pressed as a tablet on a boric acid substrate using a semiautomatic hydraulic press. For this purpose, about 1 g of tea leaf powder was crushed in an agate mortar. Next, 0.5 g of the resulting material was pressed into tablets at a force of 18 tons.

      3.6.6 Results and Discussion

      The results show that particles of the samples prepared via acid decomposition were uniformly distributed on the quartz carrier. Non‐uniform distribution of large particles of different shapes over a carrier surface is registered for sample prepared from suspension. The authors found that analysis of the sample suspension is a fast and simple procedure for the targeted determination of elements such as Ca, Mn, Cu, Zn, Br, Sr. Possible explanations for the inaccurate information on elements with small atomic numbers (P, S, and K) are explained by the differences in effects of absorption by the tea matrix, confirmed by studies of samples of biological origin [13, 89]. In addition, for P, S, and K, the effect of inhomogeneity in the sample distribution on the carrier was more significant when the suspension was used. It should also be noted that the determination of Cl and Br using the acid decomposition method is difficult due to the volatility of these elements, and therefore the use of suspension is recommended for them. The best measurement results for most elements are obtained by analyzing solutions after acid decomposition of tea leaves, whereby this sample preparation method is used to analyze samples of Krasnodar tea. To determine Cl and Br, samples were prepared as suspensions. The average value of the relative standard deviation of the analysis results from the certified values of the CRM for the method used in the verification was not more than 6% for Mg, P, S, Mn, Ni, Cu, Zn, Br, Rb; for K, Ca, Sr, Ba, and Pb – not more than 16%. The largest discrepancies were obtained by determining Ti and Fe. The total uncertainty of TXRF results, considering all stages of analysis (sample preparation and measurement), for all elements except for Ti was on average no more than 16% (n = 3). The uncertainty of applying a sample onto the carrier averaged 7% (n = 7).

Table 3.5 shows the ranges of element concentrations, average concentrations, and standard deviations from the average (n = 3) obtained from the analysis of 19 samples of tea leaves of Krasnodar tea.

      The Student's test was used to evaluate the two quantitative data sets obtained by TXRF and wavelength dispersive X‐ray fluorescence (WDXRF). Good results convergence was obtained for Cl, K, Ca, Mn, Fe, Cu, Zn, Rb, Sr, and Ba. However, the concentrations of P, S, Ni observed in the data had relatively low convergence (74% on average), which is explained by the possible influence of unconsidered factors, which demands further research. Advantages of TXRF over WDXRF include using fewer CRM to quantify data, reducing detection limits for most elements by about an order of magnitude, and not having matrix effects when emitters meet the thin layer criterion.

3.7 Interelement Effects and Procedures of their Accounting

      The present level of development of the theory of X‐ray fluorescence excitation allows researchers to accurately calculate fluorescence intensities for homogeneous samples. In this case software enables us to take into account different matrix effects: the effect of enhancement of element atoms, primary and fluorescent radiation scattered from the sample atoms, enhancement by sample Auger‐ and photoelectrons, cascade transitions, etc. [7790–92].

Table 3.5 Range, mean (C_mean ) and standard deviation (S) of elements' concentrations in tea leaves for the set of Krasnodar tea samples.

СКАЧАТЬ

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Element	Concentrations in tea leaves, mg/kg
Range	Mean and standard deviation
P	2497–5083	3629 ± 616
S	3057–4822	3830 ± 496
K