Читать книгу X-Ray Fluorescence in Biological Sciences - Группа авторов - Страница 51

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.

Element	Concentrations in tea leaves, mg/kg
Range	Mean and standard deviation
P	2497–5083	3629 ± 616
S	3057–4822	3830 ± 496
K	13 202–29 710	19 984 ± 4385
Ca	4173–6922	5371 ± 787
Mn	480–2007	1222 ± 451
Fe	96.1–327	177 ± 71
Ni	2.96–12.6	7.90 ± 2.45
Cu	10.2–33.8	19.8 ± 6.28
Zna	20–40.7	33 ± 7.0
Brb	1.8–6.23	3.24 ± 1.24
Rb	15.2–166	67.1 ± 46.6
Sr	10.6–51.3	20.8 ± 9.58
Ba	8.89–63.2	40.5 ± 14.2
Pb	n/dc–1.23	0.305 ± 0.316

^a ‐ In sample M6 Zn with concentration of 1223 mg/l is found, it was not considered at C _mean estimation.

^b ‐ Analysis of suspension

^c ‐ n/d – it is not determined.

In the 1970s the contribution the secondary electrons, scattered and fluorescent radiation, and the influence of the divergence of the primary beam to the intensities of X‐ray fluorescence were estimated by Irkutsk X‐ray physicists [77, 93]. The influence of inaccuracies in the fundamental parameters, inaccuracies in the description of the distribution of the energy of the primary radiation, etc. were evaluated [42, 75, 76, 94].

We must list some Russian researchers who have contributed to the solution to these problems: N.F. Losev, G.V. Pavlinsky, V.P. Afonin, A.G. Revenko, Yu.I. Velichko, B.I. Kitov, V.Ya. Borkhodoev, A.L. Finkel'shtein, et al. At that time, many investigations were conducted by our foreign colleagues. First, one has to mention J. Sherman, H. Ebel, T. Shiraiwa, N. Fujino, L.S. Birks, M. Mantler, J. Criss, J. Gilfrich, B. Vrebos, K. Nielson, and others. These researches are being successfully continued by B. Kanngiesser, B. Beckhoff, W. Malzer, R. Sitko, and others.

The possibility of the application of theoretical intensities was used in the Analytical Center at Institute of the Earth's Crust SB RAS (Irkutsk) to select specific CRMs suitable for calibration to convert measured intensities of analytical lines into concentrations of analyzed elements for different types of geological samples [3495–99].

The estimates of inter‐elemental effects on the intensity of analytical lines for tea, coffee, and some plants which were used in calibration and validation of XRF were presented by Revenko and Sharykina [15]. Table 3.6 show the estimates of the change of the relative specific intensities (I_rel) calculated by the authors of this chapter for the analytical lines of some elements. Variations of the intensity of coherently and non‐coherently scattered radiation of the Rh Kα‐line anode of the X‐ray tube are also given. The minimum and maximum I_rel values are set in bold. Calculations of the intensities were carried out using the program developed by Finkelstein and Afonin [100]. The program algorithm includes the contributions of secondary and tertiary excitation effects as well as the contribution of the radiation scattered by the sample. CRM of the gabbro SGD‐2 [101], diluted in the ratio of 19 : 1 water was taken to calculate the relative intensities of all analytical lines as the reference sample. The calculated intensities were normalized to the ratio of the concentrations of C_samp/C_ref, where, C_samp and C_ref are the concentrations of the element in the CRM or the sample plants and in a reference sample, respectively. For all analytical lines, I_rel for a reference sample is 1.000.

Table 3.6 The relative specific intensities of I_rel for the analytical lines of some of elements, as well as the intensities of coherently (coh) and incoherently (nc) scattered radiation of the Rh Kα line for a group of samples of plant materials. The minimum and maximum Irel values in Table 3.6 are shown in bold.

Sample	Si Kα	K Kα	Ca Kα	Ti Kα	Fe Kα	Ni Kα	Sr Kα	Rh Kαcoh	Rh Kαnc
Ground coffee	1.015	0.955	0.740	0.649	0.687	0.720	0.705	0.734	0.812
Instant coffee	1.010	0.865	0.733	0.561	0.542	0.566	0.545	0.579	0.703
Tea1	1.034	0.985	0.908	0.880	0.885	0.930	0.932	0.942	0.964
Tea2	0.996	0.964	0.829	0.818	0.784	0.759	0.736	0.761	0.822
GBW 08505	1.041	1.106	0.971	0.973	0.995	1.075	1.088	1.088	1.054
Cinnamon	1.008	1.005	0.911	0.750	0.726	0.725	0.701	0.730	0.796
Turmeric	1.016	0.919	0.775	0.744	0.735	0.774	0.764	0.789	0.855
Black pepper	1.013	0.894	0.724	0.639	0.621	0.644	0.624	0.656	0.755
Pepper paprika	1.015	0.769	0.561	0.519	0.500	0.519	0.498	0.531	0.675
Rice flour	1.018	1.036	1.041	0.970	1.114	1.192	1.231	1.200	1.147
Wheat flour	1.018	1.009	0.999	1.023	1.050	1.116	1.142	1.125	1.094
Rye flour	1.018	0.995	0.943	0.970	0.964	1.164	1.037	1.035	1.030
Linen flour	1.006	0.851	0.748	0.701	0.691	0.721	0.708	0.737	0.823
Oatmeal	1.017	0.975	0.925	0.928	0.942	0.999	1.011	1.012	1.016
Haricot	1.015	0.916	0.763	0.731	0.724	0.761	0.749	0.776	0.847
SRM1573a	0.992	1.032	0.780	0.576	0.570	0.624	0.666	0.617	0.730
SRM1515	1.037	1.118	0.917	0.886	0.913	1.016	1.123	0.985	0.983
SBMT 02	1.034	1.012	0.847	0.819	0.841	0.934	1.030	0.913	0.942
LB1	1.011	1.028	0.984	0.923	0.929	0.972	0.979	0.984	0.989
Tr1	1.015	1.021	0.949	0.938	0.953	1.004	1.019	1.019	1.016

The CRM list includes: GBW 08505 – Tea leaves, SRM1573a – Tomato leaves, SRM1515 – Apple leaves, SBMT 02 – Herb mix, LB1 is CRM 8923‐2007 – Birch leaves, and Tr1 is CRM 8922‐2007 – Meadow herbal mix. The sample plants list includes ground coffee, instant coffee, two samples of tea leaves, cinnamon, turmeric, black pepper, pepper paprika, oatmeal, haricot, rice flour, wheat flour, rye and linen flour. The content ranges of some elements in CRMs and the sample plants are, %: Na (0.0024–1.96); Mg (0.095–1.2); Al (0.002–1.0); Si (0.009–1.1); P (0.083–0.6); S (0.097–0.96); Cl (0.023–1.92); K (0.37–4.44); Ca (0.054–5.05); Mn (0.0007–0.12); Fe (0.0056–0.26); Sr (0.0002–0.0345). The element content ranges in real plants are, %: Na (0.003–0.31); Mg (0.08–1.63); Al (0.004–2.37); Si (0.009–15.44); P (0.087–0.45); S (0.06–0.96); Cl (0.01–1.20); K (0.71–5.37); Ca (0.31–21.40); Mn (0.0020–1.2); Fe (0.0083–1.40); Sr (0.0025–0.091). For most elements, the concentration ranges are wider for real plants than comparable CRMs.

Our estimates showed that for analytical lines from Na Kα to Cl Kα, the I_rel values were close to the I_rel values for Si Kα. The weak dependence of I_rel on the chemical composition of the samples used in this case (0.996–1.041, see Table 3.6) confirms the possibility of using an external standard method to calculate concentrations. For Kα lines in the wavelength range Ni Kα to Sr Kα I_rel were close to the I_rel values for coherently scattered radiation of the Rh Kα ‐line. It follows from the obtained data that with the same contents of the analyzing elements the relative intensities of analytical lines from Fe Kα to Sr Kα can differ by 2.2–2.5 times (for K Kα in 1.34, for Ca Kα in 1.86 and for Ti Kα in 1.97 times). It is obvious that for elements with Z from ²⁰Ca to ³⁸Sr the proximity of the values I_rel to similar values for coherently scattered radiation of the Rh Kα‐line allows one to significantly improve accuracy of the analysis when using a method of the standard of a background. It should be noted that in publications of recent years have in some examples used the method of fundamental parameters. This method is included in most software modern X‐ray spectrometers, both large spectrometers and portable. In its application, it is important to ensure the homogeneity of the material of the samples to be analyzed. This problem is relatively easy to solve for coffee samples and more difficult for tea leaves.