Electron-Induced X-Ray Intensity Ratios of Pb Lα/Lβ and As Kα/Kβ by 18-30 kev Applied Voltages Bolortuya DAMDINSUREN and Jun KAWAI Department of Materials Science and Engineering, Kyoto University Sakyo-ku, Kyoto 606-8501, Japan (Received 5 December 2016, Revised 10 January 2017, Accepted 11 January 2017) The principal goal of the present study is to evaluate intensity ratios of Pb and As by X-ray analysis. Lead and arsenic overlap and these ratios are important for toxic element analysis. Measurements of Pb-As mixtures were performed using a JEOL JSM-5610LVS scanning electron microscope (SEM-EDX) at 18-30 kv electron voltage and a silicon drift detector (SDD). From overlapped X-ray intensity at 10.5 kev, intensities of As Kα and Pb Lα in mixture were estimated by ratios of Pb Lα/Lβ and As Kα/Kβ in pure elements, and correlation between X-ray intensity and excitation voltage was evaluated. Calculated value was compared with measured intensity. [Key words] X-ray intensity ratio, Overlapping lines of As and Pb, SEM, EDX, EDS 1. Introduction Arsenic and lead are found naturally in the earth, but human activities, notably use of agricultural pesticides, industrial and mining processes, have been responsible for the more diffusion of these elements. Therefore, characterizing toxic elemental content is important for monitoring the human environment and in protecting public health. For the major toxic elements As, Pb, Cd, and Hg, approved destructive methods typically mean those based on Inductively Coupled Plasma Spectrometry (ICP- MS/OES), and Atomic Absorption Spectrometry (AAS) which require time-consuming and relatively expensive 1-4). The XRF-based approaches are generally non-destructive, rapid, inexpensive, and require limited sample preparation compared with destructive methods. Thus, XRF has the potential to facilitate rapid identification of potentially toxic products. However, in case of determination of As and Pb by X-ray analysis, As Kα emission is 10.54 kev, and Pb Lα emission is almost identical at 10.55 kev. Their spectral lines overlap, and it causes an interference issue, making it difficult to determine the content of Pb and As in a specimen. This work is aimed to determine X-ray intensity ratios of Pb and As measured by SEM- EDX, and this ratio is important as well as reducing the amount of error in the determined concentrations for toxic element analysis. 2. Experimental Prior to SEM-EDX analysis, several As-Pb mixtures with different contents were prepared from E-mail: damdinsuren.bolortuya.27z@st.kyoto-u.ac.jp 346 Adv. X-Ray. Chem. Anal., Japan 48, pp.346-351 (2017)
Fig.1 An appearance of the JSM-5610LVS SEM-EDX. 1000 ppm solution standards of the As and Pb. The substrates used were polished pure aluminum foil. Prepared mixtures were dropped on the substrates using a pipette, and then dried. Measurements have been performed using a JSM-5610LVS SEM-EDX and an SDD detector (Fig.1) attached in laboratory 5). The spectra were measured in vacuum with 18-30 kv electron excitation voltage. As the electron beam energy is changed, the number of edges that can be ionized is changed, and X-ray ranges are all less than the electron energy 6-8). In particular, overvoltage, U, of 2-3 is usually optimum for a given element but the maximum voltage of the SEM-EDX is 30 kv. Fig.2 shows comparison of spectra of As and Pb excited by 19 kev, 24 kev, and 30 kev electron voltages. From this figure, As Kα and Pb Lα lines in the spectrums are overlapped at 10.5 kev. In order to distinguish peaks of As and Pb from the overlapped peak at 10.5 kev, it is performed prior to any quantification of the elements based on the constant ratio maintained between the α and β lines of element in pure sample 9-11). We measured pure standard samples of Pb and As, and intensity ratios between the α and β lines of the pure element are correlated by applied voltage of SEM-EDX, and plotted in Fig.3. In the Fig.3, intensity ratios changed depending on electron excitation voltage. Intensity ratios of As Kα/Kβ in pure element are linearily correlated with applied voltages of 16 kev or above for As K lines because the edge energy is 12 kev. The intensity ratio of pure Pb Lα/Lβ is sharply decreased at 18-20 kv because of the excitation voltages were insufficient for Lβ emission of Pb (Pb Lβ with E c = 15.2 kev). Lβ intensity increases as the applied voltage becomes higher and intensity ratio is almost saturated at 26 kv as shown in Fig.3. It is able to estimate the intensity of the Pb Lα peak based on the ratio that exists between the Lα and Lβ. Otherwise, intensity of Pb Lα could be 347
Fig.2 X-ray spectrum measured by 19 kv, 24 kv, and 30 kv of SEM-EDX electron excitation spectra for (A) As, (B) Pb, and (C) mixture of Pb and As. equalled to a multiple of Pb Lβ intensity and ratio of pure Pb Lα/Lβ in particular circumstance. When quantifying for As, the estimated Pb Lα peak is subtracted from the spectrum and As is determined in such a way that the leftover peak is As. Therefore, the overlapped X-ray intensity at 10.5 kev can be calculated by the following equation: (1) Fig.3 Dependence of intensity ratio in pure element and applied voltage. where: I p As Kα and I p As Kβ are intensities of pure As standards; I AsKβ is the intensity of As Kβ in an unknown sample; I p Pb Lα and I p Pb Lβ are intensities of pure Pb standard; I PbLβ is the intensity of Pb 348
Fig.4 Dependence of measured and calculated relative intensity at 10.5keV and electron acceleration voltage. Fig.5 Intensities of As Kα and Pb Lα were distinguished from overlapped peak. Lβ in an unknown sample. C is a correction factor corresponding to the measured intensity of As Kβ and Pb Lβ and their concentrations, and p means pure. 3. Results and discussion These elemental peaks excited by higher voltage of SEM-EDX are better described than lower voltage (Fig.2). If an overvoltage of at least U >1.3 is required to obtain a reasonably sensitive analysis, then for conventional analysis conditions E 0 (electron beam energy) must be 17 kev or above for Pb Lα with excitation edge energy E c = 13 kev; E 0 20 kev for Pb Lβ with E c = 15 kev; and E 0 16 kev for As K lines with E c = 12 kev. Otherwise, the 19 kv electron excited spectrum has weak peak because of the critical excitation energy insufficiently excited. Thus, X-ray line intensity increases as the beam energy increases and spectra of 24 and 30 kv have mostly enough peaks for elemental analysis. The intensities at 10.5 kev energy line were calculated by eq.(1) without C factor. The calculated and measured intensities are compared in Fig.4. Calculated value using eq.(1) is fitted to the measured Fig.6 Dependence of peak to background ratio and electron voltage. data. The intensity calculated by eq.(1) without C factor is lower than those measured data; the slope is smaller than that with C. From the equation, the intensities of As Kα and Pb Lα could be found by the ratios of Pb Lα/Lβ and As Kα/Kβ in pure standard samples. In the Fig.5, calculation results were compared with experimental sum intensity at 10.5 kev depending on SEM-EDX excitation voltage. Above results in Figs.4 and 5 indicate that we have possibility to determine the 349
Fig.7 Correlation between of X-ray intensity ratios and excitation voltage. intensities of As Kα and Pb Lα by the SEM-EDX method. For X-ray intensity determining, a peak-tobackground (P/B) ratio is the one of the important factors and it increases as the difference of electron and excitation edge energies increases. The P/B ratios of As Kα, As Kβ, Pb Lα and Pb Lβ were determined, and relations to the applied voltages are shown in Fig.6. From the curves, we can see that the optimal voltages of As and Pb measured by SEM-EDX- SDD are about 27-28 kv in each sample. Although the overvoltage is U background is increased. 350 2 at 29-30 kv, its continuum Fig.7 shows some ratios of Pb and As intensities such as Pb Lβ/Lα, As Kβ/Kα, (Pb Lβ)/(As Kα), (As Kβ)/(Pb Lα) using the calculated intensities of As Kα and Pb Lα. Solid curve indicates a ratio of As Kβ to overlapped peak at 10.5 kev and dotted curve is the ratio of Pb Lβ to the peak at 10.5 kev. Ratios of As Kβ/Kα, (Pb Lβ)/(As Kα) and As Kβ and Pb Lβ lines to overlapped peaks are almost saturated at 26 kv, however, Pb Lβ/Lα and (As Kβ)/ (Pb Lα) increased from 26 kv as shown in Fig.7. 4. Conclusions We measured the X-ray spectrums of As and Pb in mixture and pure solution standard samples using SEM-EDX, and the dependence between intensity ratios of Pb Lα/Lβ and As Kα/Kβ and applied voltages of the instrument was defined. From overlapped X-ray intensity at 10.5 kev, intensities of As Kα and Pb Lα in mixture were estimated by ratios of Pb Lα/Lβ and As Kα/Kβ in pure elements. Ratio of intensities can be used for improving the toxic elemental quantitative analysis. Acknowledgements Bolortuya Damdinsuren thanks the Higher Engineering Education Development joint project of Government of Mongolia (GM) and Japan International Cooperation Agency (JICA) for supporting her study. References 1 K. G. McIntosh, D. Guimarães, M. J. Cusack, A. Vershinin, Z. W. Chen, K. Yan, P. J. Parsons: Evaluation of portable XRF instrumentation for assessing potential environmental exposure to toxic elements, Environmental Analytical Chemistry, 96, 15-37 (2016). 2 L. Luo, B. Chu, Y. Li, T. Xu, X. Wang, J. Yuan, J.Sun, Y. Liu, Y. Bo, X. Zhan, S. Wang, L. Tang: Determination of Pb, As, Cd and trace elements in polluted soils near a lead-zinc mine using polarized X-ray fluorescence spectrometry and the characteristics of the elemental distribution in the area, X-Ray Spectrom., 41, 133-143 (2012). 3 J. R. Siebera, A. Mortensenb: Validation and traceability of XRF and SEM-EDS elemental analysis results for solder in high reliability applications, X-Ray Spectrom., 43, 259-268 (2014). 4 J. Kawai, H. Ishii: SEM-EDX-SR-XRF-XANES, J.
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