Copyright JCPDS-International Centre for Diffraction Data 2014 ISSN 1097-0002 219 AEROSOL FILTER ANALYSIS USING POLARIZED OPTICS EDXRF WITH THIN FILM FP METHOD Takao Moriyama 1), Atsushi Morikawa 1), Makoto Doi 1) and Scott Fess 2) 1) Rigaku Corporation, Osaka 569-1146, Japan 2) Applied Rigaku Technologies, Inc., Austin, TX 78717 ABSTRACT Interest in atmospheric aerosol issue has been increasing worldwide. XRF is an important atmospheric aerosol monitoring tool for inorganic component analysis, because XRF is rapid and easy analysis method. In particular EDXRF has drawn attention for aerosol analysis. However EDXRF has difficulties that (1) many overlapping peaks exist for aerosol filter analysis and (2) many thin film standard samples are required when empirical calibration method is employed. Accurate analysis method of aerosol filter samples without the need for large sets of standards by semi-quantitative analysis software RPF-SQX, which includes the exact profile fitting and thin film FP method, is described by using the EDXRF spectrometer equipped with secondary targets and polarized optics. INTRODUCTION The component analysis of atmospheric aerosol such as SPM (Suspended Particle Matters) is important for the estimation of the hazardous pollutant source, as well as for the evaluation of the influence on human health and climate change on a global scale. The inorganic component of aerosol collected on filters can be analyzed easily and non-destructively using XRF. XRF method is effective when a large number of filter samples needs to be analyzed rapidly, such as for trend analysis. Especially EDXRF has attracted attention due to handling of the samples and compact size compared to WDXRF, and has been often used (Okuda, 2013). The EDXRF spectrometer equipped with secondary targets and polarized optics used in this study has advantages of high PB (Peak to Background) ratio and low LLD (Lower Limit of Detection) compared to EDXRF spectrometers with conventional direct excitation. This spectrometer also has advantage of reduced sample damage due to small amount of X-ray radiation. The quantification software is based on the semi-quantitative analysis method. This software uses full profile fitting of measured spectra combining thin film FP method with the iteration procedure.
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Copyright JCPDS-International Centre for Diffraction Data 2014 ISSN 1097-0002 220 Since this method utilizes the peak profiles of each element which are calculated theoretically in the fitting process, accurate quantitative results can be obtained even if there are overlapping peaks. Even if bulk samples are used as standards, it can be applied to the analysis of thin film such as filter samples by using the fitting process with the combination of thin film FP method. In the experiment, accurate quantitative results for standard aerosol filters with this method were obtained. In this paper, polarized optics EDXRF, thin film FP calculation and profiling fitting procedure and the analysis results are described. INSTRUMENT The Rigaku EDXRF spectrometer NEX CG with polarized optics was used. It has an air cooled end-window 50W X-ray tube with Pd target and polarized optics with up to five secondary targets for high peak to background ratio (Moriyama, 2011). The detector is a Silicon Drift Detector. The measuring atmosphere of filter samples is in vacuum and measuring diameter is 20mm. SEMI-QUANTITATIVE ANALYSIS SOFTWARE The NEX CG includes RPF-SQX (Rigaku Profile Fitting - Spectra Quant X) software (Hara et al., 2010) for the standardless analysis based on a fundamental parameter method (Kataoka, 1989) combined with full profile fitting method. The sensitivities are pre-calibrated using pure materials to cover the analyses of all elements from Na to U. The RPF-SQX software can cover different types of samples including multi-layered films and aerosol collected filters. The computational scheme of RPF-SQX is summarized in Figure 1. The calculation procedure follows that (1)first, a sample is measured to obtain a spectrum. (2)Next the initial values of the thickness and contents in the sample are set. (3)Then, a profile is constructed for each individual element using the intensity obtained by the FP method and response function. The total spectrum is then calculated by summing up each individual FP quantification procedure Measured spectra Set thickness and conc. Construct profiles using FP method and response function Measured spectrum = Calculated spectrum yes Result no Figure 1. The computational scheme of RPF-SQX.
Copyright JCPDS-International Centre for Diffraction Data 2014 ISSN 1097-0002 221 profile. (4)The calculated total spectrum is then compared with the measured spectrum. The contents are adjusted until both spectra become equal in the figure. The final results are obtained by fitting of the entire spectrum, not peak intensity. The profile of each individual element is calculated using the FP method and the response function of the detector to match the measured spectrum. The response function of the detector is required for RPF-SQX. When the semiconductor detector receives monochromatic x-rays, the output signal does not only have a peak but also additional artifacts. The response function consists of a main peak, tail, shelf, escape peak and sum peak at double the energy of the main peak. The hypermet function was adopted as the equation of response function (Campbell, 1992). Figure 2 shows the profiles of the Mn-K and K lines using the response function obtained from a Manganese metal sample. In this method, the sum peak created by the combination of the K and K main peaks have also been considered. The parameters expressing the response function depend on the energy of the main peak. These parameters have been pre-determined during system calibration and are stored in the software data base. Figure 3 illustrates an example of spectral fitting using the response functions. This figure shows the measured spectrum and fitting profile of a soil sample which contains a small amount of hazardous elements As, Se, Mn-K Escape (4.155keV) Mn-K Escape (4.752keV) Mn-K (5.895keV) Mn-K (6.492keV) 1Tail Shelf Escape peak 1.74keV 5.895+5.895 kev Sum peak lines using the response function 2468Energy(keV) Figure 2. The profiles of the Mn-K and K Figure 3. Results of the measured spectrum and fitting profile of a soil sample (JSAC0466). 10Mn-K Sum peak (11.79keV) Mn-K and Mn-K Sum peak (12.39keV) 124X-ray Intensity (Log scale) Mn-K Sum peak (12.98keV)
Copyright JCPDS-International Centre for Diffraction Data 2014 ISSN 1097-0002 222 Pb and Hg. Their peaks overlap with each other. The profile of each K and L line is fitted using the response function and the intensity obtained by the FP method for accurate peak overlap correction. There is a good match between measured and fitted spectrum even though many peaks overlap. It is necessary to use the thin film FP method for analysis of filter samples. The theoretical XRF intensity equation of the thin film FP method was used. (Shiraiwa, 1969), (Kataoka, 1990). Figure 4(a) shows the spectra of Pb-L and L lines obtained by measuring bulk and thin film lead samples. The intensities of spectra are normalized with peak top intensities of the Pb-L line. As shown in the figure, the intensity of L line of the thin film is clearly smaller than that of the bulk metal. Figure 4(b) shows the relationship between intensity ratios of Pb-L to Pb-L and sample thickness. The intensity ratio of L to L ratio decreases as sample becomes thinner due to self-absorption of Pb. Therefore, in order to obtain accurate full profile fitting, it is necessary to perform profile fitting using the thin film FP method to reflect the self-absorption. Table 1 compares the analysis results of the thin film samples using both α and β analytical lines. Bulk pure materials were used for the sensitivities of indi- (a) (b) Figure 4. (a)peak profiles of Pb-L and L lines of bulk and think film (50.5 m/cm 2 ) lead samples. (b) Relationship between intensity ratios of Pb-L to Pb-L and sample thickness. Table 1 Analysis results of the thin film standard samples using and analytical lines.
223 Copyright JCPDS-International Centre for Diffraction Data 2014 ISSN 1097-0002 (a) As-K (b) Quantified Conc.(ppm) 11.0 11.5 11.5 12.5 L L L L K L L K L L X-ray intensity Pb-L Pb-L As-K fitting Thin film (160mg/cm2) 10.5 Quantified Conc.(ppm) Pb-L fitting Thick (1000mg/cm2) 13.0 Energy (kev) Figure 5. Spectra of polymer samples (BCR680 : As 30.6ppm Pb 107.6 ppm) of different thickness. (a) Measured spectra (b) Individual fitted spectra using RPF-SQX with quantified values. vidual elements. The result exhibits good agreement between standard and analysis values using any analytical line, even though the sensitivities have been registered using bulk samples and the agreement is essential requirement for the quantification process with full profile fitting. Figure 5 shows the measured and fitted spectra fitted using the thin film FP method of polymer samples with two different thicknesses. This polyethylene standard sample contains not only Pb but also As, which lines overlap with each other. Analytical value is about the same even when thickness of the sample is different and fitting profiles are matched to measured spectra. The results suggest that by using the thin film FP method and profile fitting, accurate values can be obtained even if spectra overlap and thickness of the sample is different. AEROSOL SAMPLE ANALYSIS The spectra of aerosol filter sample (NIST SRM 2783) are shown in Figure 6. It can be seen that it is necessary to consider the influence of the overlapping lines in order to obtain an accurate analysis result. The semi-quantitative analysis results of this sample using RPF-SQX are shown in Table 2. The analyzed results are in good agreement with standard values without the use of special thin film standard samples.
Copyright JCPDS-International Centre for Diffraction Data 2014 ISSN 1097-0002 224 Figure 6. Spectra of aerosol filter standard sample (NIST SRM 2783). Table 2 Analysis results of aerosol filter standard sample (NIST SRM 2783) obtained using RPF-SQX analysis #1806 and #1807 are serial numbers Figure 7 shows detection limits MDL (Method Detection Limits) of NEX CG. MDL is defined as 68% confidence level 1 rather than 3σ according to US EPA method I.O-3.3. The bars indicate MDLs obtained in this experiment by measuring the sample of MICROMATTER TM thin film standard samples. Measurement time is 300 sec. at each secondary target. The square plots show the typical MDLs by XRF listed in US EPA method IO-3.3. The experimental results of MDLs obtained in the study are superior for most of elements.
Copyright JCPDS-International Centre for Diffraction Data 2014 ISSN 1097-0002 225 Figure 7. Detection limits (MDL). The bars indicate MDLs obtained in this study. The square plots show the MDLs in US EPA method I.O. 3.3 CONCLUSION Aerosol filter samples contain many elements causing line overlaps in spectra. Combination of full profile fitting with detector response functions and thin film FP method gives accurate analysis results even for trace elements. Accurate theoretical intensity calculation for all lines for thin film enabled aerosol filter analysis using bulk standards. Good results could be obtained for a wide concentration range of inorganic elements in aerosol filters using semi-quantitative analysis without the need for a large number of reference standards. REFERENCES Campbell, J. L. and Wang, J.-X. (1992), Lorentzian contributions to x-ray lineshapes in Si(Li) spectroscopy, X-ray Spectrometry, 21, 223-227. Hara, S., Kawahara, N., Matsuo, T. and Doi, M. (2010), Development of Quantification Method using Fundamental Parameter Method for EDXRF, DXC2010, Denver. Kataoka, Y. (1989), Standardless x-ray fluorescence spectrometry (Fundamental parameter method using sensitivity library), The Rigaku Journal, 6, 33-39. Kataoka, Y. and Arai, T. (1990), Basic studies of multi-layer thin film analysis using fundamental parameter method, Advances in X-ray Analysis, 33, 213-223.
Copyright JCPDS-International Centre for Diffraction Data 2014 ISSN 1097-0002 226 Moriyama, T., Ikeda, S., Doi, M. and Fess, S. (2011) Trace element analysis using EDXRF with polarized optics Advances in X-ray Analysis, 54, 289-298. Okuda, T., Takada, H., Kumata, H., Nakajima, F., Hatakeyama, S., Uchida, M., Tanaka, S., He, K., Ma, Y. (2013), "Inorganic chemical characterization of aerosols in four Asian mega-cities" Aerosol and Air Quality Research, 13, No.2, 436-449 Shiraiwa, T., Fujino, N. (1969) Theoretical formulas for film thickness measurement by means of fluorescence X-rays, Advances in X-ray Analysis, 12, 446.