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Improved methods for elemental analysis of atmospheric aerosols for evaluating human health impacts of aerosols in East Asia
Atmospheric Environment xxx (2014) 1e4
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Improved methods for elemental analysis of atmospheric aerosols for evaluating human health impacts of aerosols in East Asia Tomoaki Okuda a, *, James J. Schauer b, Martin M. Shafer b a b
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Environmental Chemistry and Technology Program, University of Wisconsin-Madison, 660 North Park Street, Madison, WI 53706-1413, USA
h i g h l i g h t s Comprehensive trace element analysis of PM was carried out using SF-ICP-MS. A rapid and simple element analysis was carried out using EDXRF. The analytic results obtained by these two methods agreed well. The concentrations of 44 elements in PM were obtained by SF-ICP-MS. EDXRF can analyze each sample as fast as 900 s (15 min) for 16 elements.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2013 Received in revised form 23 January 2014 Accepted 27 January 2014
This paper provides improved elemental analysis methods for the characterization of atmospheric particulate matter (PM) samples. With the aim of developing an approach for comprehensive trace element analysis of small mass of PM samples, we coupled an enhanced microwave-assisted acid digestion method with high-resolution magnetic sector inductively coupled plasma-mass spectrometry (SF-ICP-MS). We also propose a rapid and simple method using energy dispersive X-ray fluorescence spectrometry (EDXRF) that has secondary targets and three-dimensional polarization optics for screening elemental composition of PM. We obtained the concentrations of 44 elements ranged from 103 to 105 mg/g by SF-ICP-MS, and 16 elements ranged from 101 to 105 mg/g by EDXRF. The analytic results obtained by these two methods agreed well. Comprehensive analysis for a large set of elements was demonstrated by using the improved SF-ICP-MS method, while EDXRF coupled with fundamental parameter (FP) quantification method can analyze several selected elements as fast as 900 s (15 min) per sample with only minimal sample pretreatment. We provide two possible choices of analysis methods for elucidating elemental composition of PM according to the number of samples, target elements, sample amounts, time and cost for analysis required. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: EDXRF Microvolume microwave-assisted acid digestion Particulate matter SF-ICP-MS Trace elements
1. Introduction A large number of health studies are currently being conducted, or are planned for the future, that are directed toward understanding the effects on human health of exposure to atmospheric particulate matter (PM). Many of these efforts would be greatly enhanced if cost-effective analytic methods were available that could provide detailed elemental composition of PM. There is still much to learn about the role of chemical composition of PM in environmental health perspectives (Schauer et al., 2010), and improved robust cost-effective methods for analyzing elements in
* Corresponding author. E-mail address: [email protected] (T. Okuda).
PM are needed more than ever. Recently, Schauer et al. (2010) have reported that the optimization of sampling and inductively coupled plasma-mass spectrometry (ICP-MS) analysis techniques for the measurement of trace metals and selected trace element-isotope signatures in PM samples collected with personal exposure samplers. The proposed methods will enable advanced characterization of PM from low-volume personal exposure samples, and thereby provide data for improved exposure assessments and source apportionment of elemental components of atmospheric PM. However, ICP-MS analysis essentially requires destructive acid digestion that needs a time-consuming, complex pretreatment. Alternatively, X-ray fluorescence spectrometry (XRF) has been used for measuring major elements in aerosols with only minimal sample pretreatment. Traditionally, it is thought that the bench-top type energy-dispersive XRF (EDXRF) is unsuitable for trace element
1352-2310/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2014.01.043
Please cite this article in press as: Okuda, T., et al., Improved methods for elemental analysis of atmospheric aerosols for evaluating human health impacts of aerosols in East Asia, Atmospheric Environment (2014), http://dx.doi.org/10.1016/j.atmosenv.2014.01.043
2
T. Okuda et al. / Atmospheric Environment xxx (2014) 1e4
analysis because of its relatively low sensitivity; however, it has increasingly been applied to aerosol multi-element measurements because of the improvements in sensitivity associated with the utilization of many new techniques such as secondary targets and three-dimensional polarization optics (Okuda et al., 2013a, b, c, d, e; Okuda and Hatoya, 2013; Spolnik et al., 2005; Yatkin et al., 2012; Zhang et al., 2012). Okuda et al. (2014) have even tried to analyze sub-milligrams of PM collected with personal exposure samplers using EDXRF, and they have successfully shown that EDXRF would be suitable for applying to epidemiological studies since it could analyze enormous number of samples for obtaining chemical compositions of aerosols. Note that XRF is generally described as nondestructive, and it’s true for nonvolatile elements; however, some lighter elements and semivolatile compounds have the potential to decomposition or volatilize by X-ray radiation and/or from being in a vacuum generated to prevent air from attenuating X-rays (Baron and Willeke, 2001). For example, XRF analysis under vacuum (20 Pa) for 12 h caused 12.3e23.5% reduction of several methoxyphenols in ambient PM samples collected on PTFE filters (Simpson et al., 2005). There have been a few published studies that address the comparability of EDXRF and ICP-MS data in the context of PM analysis. For example, Yatkin et al. (2012) have concluded that EDXRF could be considered as an alternative method to ICP-MS for measurements of selected elements in PM. Reasonably good agreement is observed between the methods when PM filter loading is adequate for EDXRF quantification (Herner et al., 2006; Niu et al., 2010; Steinhoff et al., 2000). Consequently, ICP-MS and EDXRF techniques have their own merits when considering elemental analysis for aerosol samples; however, there is still room for improvement in the methods, such as analytical time and the level of detection limits. This paper provides a comparison of the improved elemental analysis methods for the characterization of atmospheric PM samples. With the aim of developing an approach for comprehensive trace element analysis of a few milligrams of PM samples, we coupled an enhanced microwave-assisted acid digestion method with high-resolution magnetic sector inductively coupled plasmamass spectrometry (SF-ICP-MS). We also propose a rapid and simple method using energy dispersive X-ray fluorescence spectrometry (EDXRF) that has secondary targets and three-dimensional polarization optics for screening elemental composition of PM. 2. Experimental 2.1. Improved SF-ICP-MS method The basic analytical method for elemental analysis used in this study has been previously reported (Hamad et al., 2012; Schauer et al., 2010; Zhang et al., 2008). In this study, several significant analytical improvements are presented. We have listed a few important considerations/improvements to standard ICP-MS methods below: a. Very few published reports/methods incorporate magnetic sector ICP-MS, instead relying on quadrupole instrumentation. Dramatic improvements in both background noise and sensitivity are characteristic of magnetic sector ICP-MS, resulting in orders-of-magnitude gains in signal to noise. b. The magnetic-sector ICP-MS also very importantly effectively isolates and removes from consideration (by high mass resolution) many spectral interferences that still plague the quantification of many elements by quadrupole ICP-MS. This allows the magnetic-sector method to quantify not only at much lower concentrations, but also a much larger number of elements.
c. We employed a unique microwave assisted acid digestion method that uses less than 2 mL of a mix of ultra-high purity acids. Not only does this enable the quantitative recovery of all elements from the PM, but significantly lowers the acid matrix blank. d. The low-volume, high sensitivity methods enable the quantification of nearly 50 elements in PM masses as low as 50 mg and at a reasonable throughput of 25e30 samples per day. e. All analyses were carried-out in a clean-room, specifically built and dedicated to trace element research. Detailed analytical procedure has been described below. Elemental analysis by mass spectrometry was performed in the trace metals clean room at the University of WisconsineMadison’s Wisconsin State Laboratory of Hygiene (Madison, WI). A ThermoFinnigan, Element 2, double-focusing, magnetic sector (high-resolution) ICP-MS (SF-ICP-MS) was used to acquire elemental data for over 50 elements in the PM samples. The sample introduction system consisted of an ESI FAST (SC-E2-DXS) nebulizer/autosampler (with enclosure) with a PEEK switching valve and 2 mL sample loop. The complete analytical system is located within a dedicated trace metal clean room. This approach enables accurate and precise quantification of low levels of elements in complex PM sample digests and extracts. The signal-to-noise of SF-ICP-MS is far superior to that of quadrupole ICP-MS for most elements, and when operated in medium (R ¼ 4000) or high (R ¼ 10,000) resolution modes, spectral interferences (e.g. polyatomic ions or elemental isobars) that compromise quantification of many elements by traditional quadrupole ICP-MS, are eliminated. Samples are typically diluted with 2% (v/v) ultra-high purity 16 M nitric acid to a final volume of 5 mL and introduced into the plasma at a flow rate of 0.4 mL/min with a PFA ST nebulizer, quartz cyclonic spray chamber, and quartz torch with 1.5 mm injector. Standard nickel sampler and skimmer cones are used. Fifty-four isotopes are acquired in Low resolution (R ¼ 300); 32 isotopes in Medium resolution, and 9 isotopes in High resolution. A sample acquisition consists of 4 runs 4 passes, with 10 samples per peak, for a total analysis time of 4 min per sample. With the ESI autosampler system, carry-over between samples can virtually be eliminated with quite short equilibration/rinse times (18 s each for sample uptake and rinsing). Plasma conditions (16.0 L min1 cool gas, 0.90 L min1 aux gas, 1.00 L min1 nebulizer gas, 1350 W Rf power) are optimized for our acid digest matrix. The analytical sequence is based on EPA 200.8, with an initial calibration, 2nd source check standards and continuing calibration check blanks and standards at 12 sample intervals. The typical SF-ICPMS batch includes 25 participant samples, 2 sample matrix spikes, 2 blank spikes, 4 certified reference materials (CRMs), 4-5 matrix blanks, 2 method blanks, and 2 sample duplicates in addition to the check blanks and calibration verification checks. CRMs included are: NIST SRMs San Joaquin Soil (2709), Urban Particulate Matter (1648a), Used Autocatalyst (2556), and Marine Sediment (2702). Instrumental detection limits (3-sigma) were in the range of 0.01e2 ng/L for most trace elements and in the range of 5e50 ng/L for major elements. Extraction spike recoveries at the 40 and 80 ng level (equivalent to 2 and 4 mg/L) were, with only minor exceptions, all within our acceptance window (85e115%). Sample spike recoveries were also well within our acceptance window (85e115%). The mean analytical instrument precision for total trace elements at typical environmental levels was 4.8% and for the major metals 1.6%. 2.2. EDXRF method Filter samples were analyzed by EDXRF without any pretreatment using an EDXL300 spectrometer manufactured by Rigaku
Please cite this article in press as: Okuda, T., et al., Improved methods for elemental analysis of atmospheric aerosols for evaluating human health impacts of aerosols in East Asia, Atmospheric Environment (2014), http://dx.doi.org/10.1016/j.atmosenv.2014.01.043
T. Okuda et al. / Atmospheric Environment xxx (2014) 1e4
Corp., Japan. For emitting primary radiation, an X-ray tube (Imax ¼ 2 mA, Vmax ¼ 50 kV) with a 50 W Pd anode was used. EDXL300 has three-dimensional (Cartesian geometry) polarization optics. The primary beam from the X-ray tube first irradiates a polarizing target (secondary target) placed along the first axis, and then the 90 -scattered X-rays go along the second axis to the sample. After scattering at 90 at the sample surface again, the Xrays reach the semiconductor detector, which is placed along the third axis. By scattering at 90 twice, the primary beam is eliminated by polarization, thereby reducing the spectral background considerably. The secondary targets allow the researcher to optimize the excitation source for the elements of interest. In this study, we used three secondary targets. The secondary targets, voltage, and duration time were Mo: 50 kV, and 400 s; Cu: 50 kV, and 400 s; and RX9 (graphite crystal): 25 kV, and 100 s. Electric current was automatically optimized (at the maximum of 50 W) based on the fluorescent X-ray intensity. The analysis time (15 min/sample) has been optimized by a previous study (Okuda and Hatoya, 2013). Prior to the analysis, the sample chamber was pumped out to reach a vacuum of 1 Pa in order to avoid the X-ray interferences caused by the air. The quantification of each element in PM samples was performed using the fundamental parameter (FP) method. The general concept for the FP quantification is described elsewhere (Beckhoff et al., 2006), but briefly, the FP quantification is based on the theoretical relation between the measured X-ray intensities and the concentrations of the elements in the sample. The FP quantification considers many parameters in the calculations such as the interaction and absorption between photons and atoms, the thickness of atomic layer, the elemental compositions. In certain cases, FP quantification can be challenging if the sample thickness is sub-optimal (e.g. less than centimeters in thickness). This could potentially be the case with the filter samples analyzed in this study. EDXL300 has a powerful FP algorithm called as Rigaku Profile Fitting-Spectra Quant X (RPF-SQX) that allows the users to achieve standardless analysis whereby all parameters are based upon theoretical equations, the fundamental parameter database, and precise modeling of the detector, X-ray tube, and instrumental geometry; however, the details of the actual algorithm is secret for users since RPF-SQX is proprietary. One of the advantages of using FP standardless analysis is that we do not need to prepare standard materials of each target element. Yatkin et al. (2012) have concluded that the standardless EDXRF analysis was found to be more efficient than linear calibration for the quantification of most of the studied elements. Hence, we decided to use the standardless analysis for our study. For the routine analysis, instrument calibration was performed daily using a Herzog glass pellet with
3
known elemental composition. In order to check the instrument condition, SRM2783 (Air Particulate on Filter Media, provided by NIST) was analyzed daily. When EDXRF results were compared with ICP-MS, ICP-AES, or the certified/reference values of reference materials, the ratio of EDXRF results to those obtained by the other methods were, with only minor exceptions, all within our acceptance window (85e115%) (Okuda et al., 2013a; Okuda and Hatoya, 2013). The mean analytical instrument precision for total elements at typical environmental levels was 5%. 2.3. PM samples Diesel exhaust PM samples were collected on PTFE filters (Whatman 2 mm PTFE 46.2 mm Filter, PP Ring Supported for PM2.5). PTFE-coated aluminum cyclones were used upstream of the sampling media to obtain PM2.5 (particulate matter which passes through a size-selective inlet with a 50% efficiency cut-off at an aerodynamic diameter of 2.5 mm) in the exhaust gases. A 24 L per minute flow rate through each of the cyclones was maintained using calibrated critical orifices. A dilution source sampler was used to dilute hot tailpipe emissions with clean dilution air. The engine used for the present study was a 1996 model year, 3116 Caterpillar, 6.6 L, medium duty diesel engine without an exhaust gas recirculation system or diesel particulate filter after-treatment system. The engine speed and load were controlled using an engine dynamometer. A platinum/cerium fuel additive was added to the diesel fuel. Similar sampling settings have been reported elsewhere (Magara-Gomez et al., 2012; Okuda et al., 2009). Seven sets of experiments were carried out in this study. Mass loadings of the samples ranged from 1.4 to 3.7 mg/filter. Duplicate PM samples were collected at each experiment, and each filter of each set of the samples was subjected to EDXRF and acid-digestion/SF-ICP-MS analysis separately. 3. Results and discussion Fig. 1 shows the concentrations of elements in diesel exhaust PM samples analyzed by SF-ICP-MS and EDXRF. We obtained the concentrations of 44 elements ranged from 103 to 105 mg/g by SF-ICPMS. This result shows that our improved SF-ICP-MS method can provide comprehensive elemental analysis for characterizing PM. Note that the concentrations of cerium and platinum were very high because the bimetallic fuel additive was added to the diesel fuel in this study. We also obtained the concentrations of 16 elements ranged from 101 to 105 mg/g by EDXRF. The number of elements analyzed by EDXRF was less than that by SF-ICP-MS due to relative inferiority of sensitivity for EDXRF. However, EDXRF
Fig. 1. The concentrations of elements in diesel exhaust PM samples analyzed by SF-ICP-MS and EDXRF.
Please cite this article in press as: Okuda, T., et al., Improved methods for elemental analysis of atmospheric aerosols for evaluating human health impacts of aerosols in East Asia, Atmospheric Environment (2014), http://dx.doi.org/10.1016/j.atmosenv.2014.01.043
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T. Okuda et al. / Atmospheric Environment xxx (2014) 1e4
provided the analytical values of silicon and chloride that were not analyzed by SF-ICP-MS. Okuda and Hatoya (2013) have reported that the analytical data for silicon by EDXRF was reliable. Consequently, SF-ICP-MS and EDXRF have their own advantages for the characterization of PM samples. Fig. 2 shows the ratio of the concentrations of elements analyzed by EDXRF to those analyzed by SF-ICP-MS. In this figure, we included 10 elements that we obtained at least five sets of the data by SF-ICP-MS and EDXRF. Each concentration ratio of EDXRF to SF-ICP-MS was as follows. Mg: 1.2 0.3; P: 0.7 0.2; S: 1.2 0.3; Ca: 0.8 0.2; Fe: 1.1 0.4; Ni: 1.0 0.3; Cu: 0.9 0.2; Zn: 1.2 0.2; Ce: 1.1 0.0; and Pt: 1.1 0.2. Generally, the results obtained by these two methods agreed well. Differences in the concentrations of those elements between EDXRF and SF-ICP-MS were not significant statistically (simple t-test) except for phosphorus (p < 0.01). These results show that EDXRF can provide reliable data for the concentrations of selected elements in PM samples. Comprehensive analysis for a large set of elements was demonstrated by using the improved SF-ICP-MS method, while EDXRF coupled with fundamental parameter (FP) quantification method can analyze several selected elements as fast as 900 s (15 min) per sample with only minimal sample pretreatment. This analytical time of our EDXRF method is much faster than previous studies that had taken usually 30e90 min per sample for analysis (Niu et al., 2010; Spolnik et al., 2005; Steinhoff et al., 2000; Yatkin et al., 2012; Zhang et al., 2012). EDXRF can also measure several elements such as Si and Cl that were not analyzed by SF-ICP-MS. We have analyzed NIST SRM2783, which was PM deposited onto a polycarbonate filter, and diesel PM collected on a PTFE filter using EDXRF in this study. Furthermore, in previous studies conducted by Okuda et al. we demonstrated that the same EDXRF analysis method could be successfully applied to the determination of elements in ambient PM samples of many cities in East Asian countries collected on PTFE, quartz fiber, and cellulose nitrate filters (Okuda et al., 2013a, b, 2014, c, d; Okuda and Hatoya, 2013). According to these studies, we offer an easy and rapid analytical method using EDXRF for screening the concentrations of elements in various PM samples, and also a highly sensitive comprehensive analytical method using SF-ICP-MS for obtaining more detailed elemental compositions. As for the costs for analysis, additionally, typical costs for SF-ICP-MS (including sample pretreatments) run from $75 to $100 on a per sample basis, whereas those for EDXRF run less than $1/sample in most cases because the running cost for EDXRF is only due to the electricity for that instrument (50 W). We provide two possible choices of analysis methods for elucidating elemental composition of PM according to the number of samples, target elements, sample amounts, time and cost for analysis required.
EDXRF / SF-ICP-MS
3.0
2.0
1.0
0.0
Mg
P
S
Ca
Fe
Ni
Cu
Zn
Ce
Pt
Fig. 2. The ratio of the concentrations of elements analyzed by EDXRF to those analyzed by SF-ICP-MS. The dashed lines show the arithmetic mean of a set of data for each element.
Acknowledgments This research was supported partly by JSPS/MEXT KAKENHI Grant Numbers 22710016 and 23120707, the Environment Research and Technology Development Fund (B-0904) of the Ministry of the Environment, Japan, Steel Industry Foundation for the Advancement of Environmental Protection Technology, and Mizuho Foundation for the Promotion of Sciences.
References Baron, P.A., Willeke, K. (Eds.), 2001. Aerosol Measurement: Principles, Techniques, and Applications, second ed. John Wiley & Sons, New Jersey. Beckhoff, B., Kanngiesser, B., Langhoff, N., Wedell, R., Wolff, H. (Eds.), 2006. Handbook of Practical X-ray Fluorescence Analysis. Springer-Verlag, Berlin and Heidelberg. Hamad, S.H., Schauer, J.J., Shafer, M.M., Al-Raheem, E.A., Satar, H., 2012. The distribution between the dissolved and the particulate forms of 49 metals across the Tigris River, Baghdad, Iraq. The Scientific World Journal. Article ID 246059, 13. Herner, J.D., Green, P.G., Kleeman, M.J., 2006. Measuring the trace elemental composition of size-resolved airborne particles. Environmental Science & Technology 40, 1925e1933. Magara-Gomez, K.T., Olson, M.R., Okuda, T., Walz, K.A., Schauer, J.J., 2012. Sensitivity of hazardous air pollutant emissions to the combustion of blends of petroleum diesel and biodiesel fuel. Atmospheric Environment 50, 307e313. Niu, J., Rasmussen, P.E., Wheeler, A., Williams, R., Chénier, M., 2010. Evaluation of airborne particulate matter and metals data in personal, indoor and outdoor environments using ED-XRF and ICP-MS and co-located duplicate samples. Atmospheric Environment 44, 235e245. Okuda, T., Fujimori, E., Hatoya, K., Takada, H., Kumata, H., Nakajima, F., Hatakeyama, S., Uchida, M., Tanaka, S., He, K., Ma, Y., Haraguchi, H., 2013a. Rapid and simple determination of multi-elements in aerosol samples collected on quartz fiber filters by using EDXRF coupled with fundamental parameter quantification technique. Aerosol and Air Quality Research 13, 1864e1876. Okuda, T., Hatoya, K., 2013. Development of non-destructive simultaneous analytical method for multi-elements in PM2.5 using energy dispersive X-ray fluorescence spectrometry with a fundamental parameter quantification technique. Earozoru Kenkyu 28, 214e221 (in Japanese with English abstract). Okuda, T., Takada, H., Kumata, H., Nakajima, F., Hatakeyama, S., Uchida, M., Tanaka, S., He, K., Ma, Y., 2013b. Inorganic chemical characterization of aerosols in four Asian mega-cities. Aerosol and Air Quality Research 13, 436e449. Okuda, T., Hatoya, K., Nakai, S., Tamura, K., 2014. Non-destructive simultaneous multi-elemental analysis of PM2.5 using energy dispersive X-ray fluorescence spectrometry with a fundamental parameter quantification technique and its application to an epidemiological study. Earozoru Kenkyu (in press), (in Japanese with English abstract). Okuda, T., Gunji, Y., He, K., Ma, Y., 2013c. Persulfate wet oxidation method for the determination of total phosphorus in atmospheric aerosols and its application for a year-round observation in Beijing. Asian Journal of Atmospheric Environment 7, 169e175. Okuda, T., Kojima, R., Watanobe, K., Matsuura, S., Okubo, K., Hatoya, K., Gunji, Y., 2013d. Development of the analytical method for chromium in air adapted to the revision of substances requiring priority action. Journal of Japan Society for Atmospheric Environment 48, 268e273 (in Japanese with English abstract). Okuda, T., Schauer, J.J., Olson, M.R., Shafer, M.M., Rutter, A.P., Walz, K.A., Morschauser, P.A., 2009. Effects of a platinum-cerium bimetallic fuel additive on the chemical composition of diesel engine exhaust particles. Energy & Fuels 23, 4974e4980. Schauer, J.J., Majestic, B.J., Sheesley, R.J., Shafer, M.M., DeMinter, J.T., Mieritz, M., 2010. Improved Source Apportionment and Speciation of Low-volume Particulate Matter Samples. Research Report 153. Health Effects Institute, Boston, MA. Simpson, C.D., Paulsen, M., Dills, R.L., Liu, L.-J.S., Kalman, D.A., 2005. Determination of methoxyphenols in ambient particulate matter: tracers for wood combustion. Environmental Science & Technology 39, 631e637. Spolnik, Z., Belikov, K., Van Meel, K., Adriaenssens, E., De Roeck, F., Van Grieken, R., 2005. Optimization of measurement conditions of an energy dispersive X-ray fluorescence spectrometer with high-energy polarized beam excitation for analysis of aerosol filters. Applied Spectroscopy 59, 1465e1469. Steinhoff, G., Haupt, O., Dannecker, W., 2000. Fast determination of trace elements on aerosol-loaded filters by X-ray fluorescence analysis considering the inhomogeneous elemental distribution. Fresenius Journal of Analytical Chemistry 366, 174e177. Yatkin, S., Gerboles, M., Borowiak, A., 2012. Evaluation of standardless EDXRF analysis for the determination of elements on PM10 loaded filters. Atmospheric Environment 54, 568e582. Zhang, N.N., Cao, J.J., Ho, K.F., He, Y.Q., 2012. Chemical characterization of aerosol collected at Mt. Yulong in wintertime on the southeastern Tibetan Plateau. Atmospheric Research 107, 76e85. Zhang, Y.X., Schauer, J.J., Shafer, M.M., Hannigan, M.P., Dutton, S.J., 2008. Source apportionment of in vitro reactive oxygen species bioassay activity from atmospheric particulate matter. Environmental Science & Technology 42, 7502e 7509.
Please cite this article in press as: Okuda, T., et al., Improved methods for elemental analysis of atmospheric aerosols for evaluating human health impacts of aerosols in East Asia, Atmospheric Environment (2014), http://dx.doi.org/10.1016/j.atmosenv.2014.01.043
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Improved methods for elemental analysis of atmospheric aerosols for evaluating human health impacts of aerosols in East Asia Tomoaki Okuda a, *, James J. Schauer b, Martin M. Shafer b a b
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Environmental Chemistry and Technology Program, University of Wisconsin-Madison, 660 North Park Street, Madison, WI 53706-1413, USA
h i g h l i g h t s Comprehensive trace element analysis of PM was carried out using SF-ICP-MS. A rapid and simple element analysis was carried out using EDXRF. The analytic results obtained by these two methods agreed well. The concentrations of 44 elements in PM were obtained by SF-ICP-MS. EDXRF can analyze each sample as fast as 900 s (15 min) for 16 elements.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2013 Received in revised form 23 January 2014 Accepted 27 January 2014
This paper provides improved elemental analysis methods for the characterization of atmospheric particulate matter (PM) samples. With the aim of developing an approach for comprehensive trace element analysis of small mass of PM samples, we coupled an enhanced microwave-assisted acid digestion method with high-resolution magnetic sector inductively coupled plasma-mass spectrometry (SF-ICP-MS). We also propose a rapid and simple method using energy dispersive X-ray fluorescence spectrometry (EDXRF) that has secondary targets and three-dimensional polarization optics for screening elemental composition of PM. We obtained the concentrations of 44 elements ranged from 103 to 105 mg/g by SF-ICP-MS, and 16 elements ranged from 101 to 105 mg/g by EDXRF. The analytic results obtained by these two methods agreed well. Comprehensive analysis for a large set of elements was demonstrated by using the improved SF-ICP-MS method, while EDXRF coupled with fundamental parameter (FP) quantification method can analyze several selected elements as fast as 900 s (15 min) per sample with only minimal sample pretreatment. We provide two possible choices of analysis methods for elucidating elemental composition of PM according to the number of samples, target elements, sample amounts, time and cost for analysis required. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: EDXRF Microvolume microwave-assisted acid digestion Particulate matter SF-ICP-MS Trace elements
1. Introduction A large number of health studies are currently being conducted, or are planned for the future, that are directed toward understanding the effects on human health of exposure to atmospheric particulate matter (PM). Many of these efforts would be greatly enhanced if cost-effective analytic methods were available that could provide detailed elemental composition of PM. There is still much to learn about the role of chemical composition of PM in environmental health perspectives (Schauer et al., 2010), and improved robust cost-effective methods for analyzing elements in
* Corresponding author. E-mail address: [email protected] (T. Okuda).
PM are needed more than ever. Recently, Schauer et al. (2010) have reported that the optimization of sampling and inductively coupled plasma-mass spectrometry (ICP-MS) analysis techniques for the measurement of trace metals and selected trace element-isotope signatures in PM samples collected with personal exposure samplers. The proposed methods will enable advanced characterization of PM from low-volume personal exposure samples, and thereby provide data for improved exposure assessments and source apportionment of elemental components of atmospheric PM. However, ICP-MS analysis essentially requires destructive acid digestion that needs a time-consuming, complex pretreatment. Alternatively, X-ray fluorescence spectrometry (XRF) has been used for measuring major elements in aerosols with only minimal sample pretreatment. Traditionally, it is thought that the bench-top type energy-dispersive XRF (EDXRF) is unsuitable for trace element
1352-2310/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2014.01.043
Please cite this article in press as: Okuda, T., et al., Improved methods for elemental analysis of atmospheric aerosols for evaluating human health impacts of aerosols in East Asia, Atmospheric Environment (2014), http://dx.doi.org/10.1016/j.atmosenv.2014.01.043
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T. Okuda et al. / Atmospheric Environment xxx (2014) 1e4
analysis because of its relatively low sensitivity; however, it has increasingly been applied to aerosol multi-element measurements because of the improvements in sensitivity associated with the utilization of many new techniques such as secondary targets and three-dimensional polarization optics (Okuda et al., 2013a, b, c, d, e; Okuda and Hatoya, 2013; Spolnik et al., 2005; Yatkin et al., 2012; Zhang et al., 2012). Okuda et al. (2014) have even tried to analyze sub-milligrams of PM collected with personal exposure samplers using EDXRF, and they have successfully shown that EDXRF would be suitable for applying to epidemiological studies since it could analyze enormous number of samples for obtaining chemical compositions of aerosols. Note that XRF is generally described as nondestructive, and it’s true for nonvolatile elements; however, some lighter elements and semivolatile compounds have the potential to decomposition or volatilize by X-ray radiation and/or from being in a vacuum generated to prevent air from attenuating X-rays (Baron and Willeke, 2001). For example, XRF analysis under vacuum (20 Pa) for 12 h caused 12.3e23.5% reduction of several methoxyphenols in ambient PM samples collected on PTFE filters (Simpson et al., 2005). There have been a few published studies that address the comparability of EDXRF and ICP-MS data in the context of PM analysis. For example, Yatkin et al. (2012) have concluded that EDXRF could be considered as an alternative method to ICP-MS for measurements of selected elements in PM. Reasonably good agreement is observed between the methods when PM filter loading is adequate for EDXRF quantification (Herner et al., 2006; Niu et al., 2010; Steinhoff et al., 2000). Consequently, ICP-MS and EDXRF techniques have their own merits when considering elemental analysis for aerosol samples; however, there is still room for improvement in the methods, such as analytical time and the level of detection limits. This paper provides a comparison of the improved elemental analysis methods for the characterization of atmospheric PM samples. With the aim of developing an approach for comprehensive trace element analysis of a few milligrams of PM samples, we coupled an enhanced microwave-assisted acid digestion method with high-resolution magnetic sector inductively coupled plasmamass spectrometry (SF-ICP-MS). We also propose a rapid and simple method using energy dispersive X-ray fluorescence spectrometry (EDXRF) that has secondary targets and three-dimensional polarization optics for screening elemental composition of PM. 2. Experimental 2.1. Improved SF-ICP-MS method The basic analytical method for elemental analysis used in this study has been previously reported (Hamad et al., 2012; Schauer et al., 2010; Zhang et al., 2008). In this study, several significant analytical improvements are presented. We have listed a few important considerations/improvements to standard ICP-MS methods below: a. Very few published reports/methods incorporate magnetic sector ICP-MS, instead relying on quadrupole instrumentation. Dramatic improvements in both background noise and sensitivity are characteristic of magnetic sector ICP-MS, resulting in orders-of-magnitude gains in signal to noise. b. The magnetic-sector ICP-MS also very importantly effectively isolates and removes from consideration (by high mass resolution) many spectral interferences that still plague the quantification of many elements by quadrupole ICP-MS. This allows the magnetic-sector method to quantify not only at much lower concentrations, but also a much larger number of elements.
c. We employed a unique microwave assisted acid digestion method that uses less than 2 mL of a mix of ultra-high purity acids. Not only does this enable the quantitative recovery of all elements from the PM, but significantly lowers the acid matrix blank. d. The low-volume, high sensitivity methods enable the quantification of nearly 50 elements in PM masses as low as 50 mg and at a reasonable throughput of 25e30 samples per day. e. All analyses were carried-out in a clean-room, specifically built and dedicated to trace element research. Detailed analytical procedure has been described below. Elemental analysis by mass spectrometry was performed in the trace metals clean room at the University of WisconsineMadison’s Wisconsin State Laboratory of Hygiene (Madison, WI). A ThermoFinnigan, Element 2, double-focusing, magnetic sector (high-resolution) ICP-MS (SF-ICP-MS) was used to acquire elemental data for over 50 elements in the PM samples. The sample introduction system consisted of an ESI FAST (SC-E2-DXS) nebulizer/autosampler (with enclosure) with a PEEK switching valve and 2 mL sample loop. The complete analytical system is located within a dedicated trace metal clean room. This approach enables accurate and precise quantification of low levels of elements in complex PM sample digests and extracts. The signal-to-noise of SF-ICP-MS is far superior to that of quadrupole ICP-MS for most elements, and when operated in medium (R ¼ 4000) or high (R ¼ 10,000) resolution modes, spectral interferences (e.g. polyatomic ions or elemental isobars) that compromise quantification of many elements by traditional quadrupole ICP-MS, are eliminated. Samples are typically diluted with 2% (v/v) ultra-high purity 16 M nitric acid to a final volume of 5 mL and introduced into the plasma at a flow rate of 0.4 mL/min with a PFA ST nebulizer, quartz cyclonic spray chamber, and quartz torch with 1.5 mm injector. Standard nickel sampler and skimmer cones are used. Fifty-four isotopes are acquired in Low resolution (R ¼ 300); 32 isotopes in Medium resolution, and 9 isotopes in High resolution. A sample acquisition consists of 4 runs 4 passes, with 10 samples per peak, for a total analysis time of 4 min per sample. With the ESI autosampler system, carry-over between samples can virtually be eliminated with quite short equilibration/rinse times (18 s each for sample uptake and rinsing). Plasma conditions (16.0 L min1 cool gas, 0.90 L min1 aux gas, 1.00 L min1 nebulizer gas, 1350 W Rf power) are optimized for our acid digest matrix. The analytical sequence is based on EPA 200.8, with an initial calibration, 2nd source check standards and continuing calibration check blanks and standards at 12 sample intervals. The typical SF-ICPMS batch includes 25 participant samples, 2 sample matrix spikes, 2 blank spikes, 4 certified reference materials (CRMs), 4-5 matrix blanks, 2 method blanks, and 2 sample duplicates in addition to the check blanks and calibration verification checks. CRMs included are: NIST SRMs San Joaquin Soil (2709), Urban Particulate Matter (1648a), Used Autocatalyst (2556), and Marine Sediment (2702). Instrumental detection limits (3-sigma) were in the range of 0.01e2 ng/L for most trace elements and in the range of 5e50 ng/L for major elements. Extraction spike recoveries at the 40 and 80 ng level (equivalent to 2 and 4 mg/L) were, with only minor exceptions, all within our acceptance window (85e115%). Sample spike recoveries were also well within our acceptance window (85e115%). The mean analytical instrument precision for total trace elements at typical environmental levels was 4.8% and for the major metals 1.6%. 2.2. EDXRF method Filter samples were analyzed by EDXRF without any pretreatment using an EDXL300 spectrometer manufactured by Rigaku
Please cite this article in press as: Okuda, T., et al., Improved methods for elemental analysis of atmospheric aerosols for evaluating human health impacts of aerosols in East Asia, Atmospheric Environment (2014), http://dx.doi.org/10.1016/j.atmosenv.2014.01.043
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Corp., Japan. For emitting primary radiation, an X-ray tube (Imax ¼ 2 mA, Vmax ¼ 50 kV) with a 50 W Pd anode was used. EDXL300 has three-dimensional (Cartesian geometry) polarization optics. The primary beam from the X-ray tube first irradiates a polarizing target (secondary target) placed along the first axis, and then the 90 -scattered X-rays go along the second axis to the sample. After scattering at 90 at the sample surface again, the Xrays reach the semiconductor detector, which is placed along the third axis. By scattering at 90 twice, the primary beam is eliminated by polarization, thereby reducing the spectral background considerably. The secondary targets allow the researcher to optimize the excitation source for the elements of interest. In this study, we used three secondary targets. The secondary targets, voltage, and duration time were Mo: 50 kV, and 400 s; Cu: 50 kV, and 400 s; and RX9 (graphite crystal): 25 kV, and 100 s. Electric current was automatically optimized (at the maximum of 50 W) based on the fluorescent X-ray intensity. The analysis time (15 min/sample) has been optimized by a previous study (Okuda and Hatoya, 2013). Prior to the analysis, the sample chamber was pumped out to reach a vacuum of 1 Pa in order to avoid the X-ray interferences caused by the air. The quantification of each element in PM samples was performed using the fundamental parameter (FP) method. The general concept for the FP quantification is described elsewhere (Beckhoff et al., 2006), but briefly, the FP quantification is based on the theoretical relation between the measured X-ray intensities and the concentrations of the elements in the sample. The FP quantification considers many parameters in the calculations such as the interaction and absorption between photons and atoms, the thickness of atomic layer, the elemental compositions. In certain cases, FP quantification can be challenging if the sample thickness is sub-optimal (e.g. less than centimeters in thickness). This could potentially be the case with the filter samples analyzed in this study. EDXL300 has a powerful FP algorithm called as Rigaku Profile Fitting-Spectra Quant X (RPF-SQX) that allows the users to achieve standardless analysis whereby all parameters are based upon theoretical equations, the fundamental parameter database, and precise modeling of the detector, X-ray tube, and instrumental geometry; however, the details of the actual algorithm is secret for users since RPF-SQX is proprietary. One of the advantages of using FP standardless analysis is that we do not need to prepare standard materials of each target element. Yatkin et al. (2012) have concluded that the standardless EDXRF analysis was found to be more efficient than linear calibration for the quantification of most of the studied elements. Hence, we decided to use the standardless analysis for our study. For the routine analysis, instrument calibration was performed daily using a Herzog glass pellet with
3
known elemental composition. In order to check the instrument condition, SRM2783 (Air Particulate on Filter Media, provided by NIST) was analyzed daily. When EDXRF results were compared with ICP-MS, ICP-AES, or the certified/reference values of reference materials, the ratio of EDXRF results to those obtained by the other methods were, with only minor exceptions, all within our acceptance window (85e115%) (Okuda et al., 2013a; Okuda and Hatoya, 2013). The mean analytical instrument precision for total elements at typical environmental levels was 5%. 2.3. PM samples Diesel exhaust PM samples were collected on PTFE filters (Whatman 2 mm PTFE 46.2 mm Filter, PP Ring Supported for PM2.5). PTFE-coated aluminum cyclones were used upstream of the sampling media to obtain PM2.5 (particulate matter which passes through a size-selective inlet with a 50% efficiency cut-off at an aerodynamic diameter of 2.5 mm) in the exhaust gases. A 24 L per minute flow rate through each of the cyclones was maintained using calibrated critical orifices. A dilution source sampler was used to dilute hot tailpipe emissions with clean dilution air. The engine used for the present study was a 1996 model year, 3116 Caterpillar, 6.6 L, medium duty diesel engine without an exhaust gas recirculation system or diesel particulate filter after-treatment system. The engine speed and load were controlled using an engine dynamometer. A platinum/cerium fuel additive was added to the diesel fuel. Similar sampling settings have been reported elsewhere (Magara-Gomez et al., 2012; Okuda et al., 2009). Seven sets of experiments were carried out in this study. Mass loadings of the samples ranged from 1.4 to 3.7 mg/filter. Duplicate PM samples were collected at each experiment, and each filter of each set of the samples was subjected to EDXRF and acid-digestion/SF-ICP-MS analysis separately. 3. Results and discussion Fig. 1 shows the concentrations of elements in diesel exhaust PM samples analyzed by SF-ICP-MS and EDXRF. We obtained the concentrations of 44 elements ranged from 103 to 105 mg/g by SF-ICPMS. This result shows that our improved SF-ICP-MS method can provide comprehensive elemental analysis for characterizing PM. Note that the concentrations of cerium and platinum were very high because the bimetallic fuel additive was added to the diesel fuel in this study. We also obtained the concentrations of 16 elements ranged from 101 to 105 mg/g by EDXRF. The number of elements analyzed by EDXRF was less than that by SF-ICP-MS due to relative inferiority of sensitivity for EDXRF. However, EDXRF
Fig. 1. The concentrations of elements in diesel exhaust PM samples analyzed by SF-ICP-MS and EDXRF.
Please cite this article in press as: Okuda, T., et al., Improved methods for elemental analysis of atmospheric aerosols for evaluating human health impacts of aerosols in East Asia, Atmospheric Environment (2014), http://dx.doi.org/10.1016/j.atmosenv.2014.01.043
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provided the analytical values of silicon and chloride that were not analyzed by SF-ICP-MS. Okuda and Hatoya (2013) have reported that the analytical data for silicon by EDXRF was reliable. Consequently, SF-ICP-MS and EDXRF have their own advantages for the characterization of PM samples. Fig. 2 shows the ratio of the concentrations of elements analyzed by EDXRF to those analyzed by SF-ICP-MS. In this figure, we included 10 elements that we obtained at least five sets of the data by SF-ICP-MS and EDXRF. Each concentration ratio of EDXRF to SF-ICP-MS was as follows. Mg: 1.2 0.3; P: 0.7 0.2; S: 1.2 0.3; Ca: 0.8 0.2; Fe: 1.1 0.4; Ni: 1.0 0.3; Cu: 0.9 0.2; Zn: 1.2 0.2; Ce: 1.1 0.0; and Pt: 1.1 0.2. Generally, the results obtained by these two methods agreed well. Differences in the concentrations of those elements between EDXRF and SF-ICP-MS were not significant statistically (simple t-test) except for phosphorus (p < 0.01). These results show that EDXRF can provide reliable data for the concentrations of selected elements in PM samples. Comprehensive analysis for a large set of elements was demonstrated by using the improved SF-ICP-MS method, while EDXRF coupled with fundamental parameter (FP) quantification method can analyze several selected elements as fast as 900 s (15 min) per sample with only minimal sample pretreatment. This analytical time of our EDXRF method is much faster than previous studies that had taken usually 30e90 min per sample for analysis (Niu et al., 2010; Spolnik et al., 2005; Steinhoff et al., 2000; Yatkin et al., 2012; Zhang et al., 2012). EDXRF can also measure several elements such as Si and Cl that were not analyzed by SF-ICP-MS. We have analyzed NIST SRM2783, which was PM deposited onto a polycarbonate filter, and diesel PM collected on a PTFE filter using EDXRF in this study. Furthermore, in previous studies conducted by Okuda et al. we demonstrated that the same EDXRF analysis method could be successfully applied to the determination of elements in ambient PM samples of many cities in East Asian countries collected on PTFE, quartz fiber, and cellulose nitrate filters (Okuda et al., 2013a, b, 2014, c, d; Okuda and Hatoya, 2013). According to these studies, we offer an easy and rapid analytical method using EDXRF for screening the concentrations of elements in various PM samples, and also a highly sensitive comprehensive analytical method using SF-ICP-MS for obtaining more detailed elemental compositions. As for the costs for analysis, additionally, typical costs for SF-ICP-MS (including sample pretreatments) run from $75 to $100 on a per sample basis, whereas those for EDXRF run less than $1/sample in most cases because the running cost for EDXRF is only due to the electricity for that instrument (50 W). We provide two possible choices of analysis methods for elucidating elemental composition of PM according to the number of samples, target elements, sample amounts, time and cost for analysis required.
EDXRF / SF-ICP-MS
3.0
2.0
1.0
0.0
Mg
P
S
Ca
Fe
Ni
Cu
Zn
Ce
Pt
Fig. 2. The ratio of the concentrations of elements analyzed by EDXRF to those analyzed by SF-ICP-MS. The dashed lines show the arithmetic mean of a set of data for each element.
Acknowledgments This research was supported partly by JSPS/MEXT KAKENHI Grant Numbers 22710016 and 23120707, the Environment Research and Technology Development Fund (B-0904) of the Ministry of the Environment, Japan, Steel Industry Foundation for the Advancement of Environmental Protection Technology, and Mizuho Foundation for the Promotion of Sciences.
References Baron, P.A., Willeke, K. (Eds.), 2001. Aerosol Measurement: Principles, Techniques, and Applications, second ed. John Wiley & Sons, New Jersey. Beckhoff, B., Kanngiesser, B., Langhoff, N., Wedell, R., Wolff, H. (Eds.), 2006. Handbook of Practical X-ray Fluorescence Analysis. Springer-Verlag, Berlin and Heidelberg. Hamad, S.H., Schauer, J.J., Shafer, M.M., Al-Raheem, E.A., Satar, H., 2012. The distribution between the dissolved and the particulate forms of 49 metals across the Tigris River, Baghdad, Iraq. The Scientific World Journal. Article ID 246059, 13. Herner, J.D., Green, P.G., Kleeman, M.J., 2006. Measuring the trace elemental composition of size-resolved airborne particles. Environmental Science & Technology 40, 1925e1933. Magara-Gomez, K.T., Olson, M.R., Okuda, T., Walz, K.A., Schauer, J.J., 2012. Sensitivity of hazardous air pollutant emissions to the combustion of blends of petroleum diesel and biodiesel fuel. Atmospheric Environment 50, 307e313. Niu, J., Rasmussen, P.E., Wheeler, A., Williams, R., Chénier, M., 2010. Evaluation of airborne particulate matter and metals data in personal, indoor and outdoor environments using ED-XRF and ICP-MS and co-located duplicate samples. Atmospheric Environment 44, 235e245. Okuda, T., Fujimori, E., Hatoya, K., Takada, H., Kumata, H., Nakajima, F., Hatakeyama, S., Uchida, M., Tanaka, S., He, K., Ma, Y., Haraguchi, H., 2013a. Rapid and simple determination of multi-elements in aerosol samples collected on quartz fiber filters by using EDXRF coupled with fundamental parameter quantification technique. Aerosol and Air Quality Research 13, 1864e1876. Okuda, T., Hatoya, K., 2013. Development of non-destructive simultaneous analytical method for multi-elements in PM2.5 using energy dispersive X-ray fluorescence spectrometry with a fundamental parameter quantification technique. Earozoru Kenkyu 28, 214e221 (in Japanese with English abstract). Okuda, T., Takada, H., Kumata, H., Nakajima, F., Hatakeyama, S., Uchida, M., Tanaka, S., He, K., Ma, Y., 2013b. Inorganic chemical characterization of aerosols in four Asian mega-cities. Aerosol and Air Quality Research 13, 436e449. Okuda, T., Hatoya, K., Nakai, S., Tamura, K., 2014. Non-destructive simultaneous multi-elemental analysis of PM2.5 using energy dispersive X-ray fluorescence spectrometry with a fundamental parameter quantification technique and its application to an epidemiological study. Earozoru Kenkyu (in press), (in Japanese with English abstract). Okuda, T., Gunji, Y., He, K., Ma, Y., 2013c. Persulfate wet oxidation method for the determination of total phosphorus in atmospheric aerosols and its application for a year-round observation in Beijing. Asian Journal of Atmospheric Environment 7, 169e175. Okuda, T., Kojima, R., Watanobe, K., Matsuura, S., Okubo, K., Hatoya, K., Gunji, Y., 2013d. Development of the analytical method for chromium in air adapted to the revision of substances requiring priority action. Journal of Japan Society for Atmospheric Environment 48, 268e273 (in Japanese with English abstract). Okuda, T., Schauer, J.J., Olson, M.R., Shafer, M.M., Rutter, A.P., Walz, K.A., Morschauser, P.A., 2009. Effects of a platinum-cerium bimetallic fuel additive on the chemical composition of diesel engine exhaust particles. Energy & Fuels 23, 4974e4980. Schauer, J.J., Majestic, B.J., Sheesley, R.J., Shafer, M.M., DeMinter, J.T., Mieritz, M., 2010. Improved Source Apportionment and Speciation of Low-volume Particulate Matter Samples. Research Report 153. Health Effects Institute, Boston, MA. Simpson, C.D., Paulsen, M., Dills, R.L., Liu, L.-J.S., Kalman, D.A., 2005. Determination of methoxyphenols in ambient particulate matter: tracers for wood combustion. Environmental Science & Technology 39, 631e637. Spolnik, Z., Belikov, K., Van Meel, K., Adriaenssens, E., De Roeck, F., Van Grieken, R., 2005. Optimization of measurement conditions of an energy dispersive X-ray fluorescence spectrometer with high-energy polarized beam excitation for analysis of aerosol filters. Applied Spectroscopy 59, 1465e1469. Steinhoff, G., Haupt, O., Dannecker, W., 2000. Fast determination of trace elements on aerosol-loaded filters by X-ray fluorescence analysis considering the inhomogeneous elemental distribution. Fresenius Journal of Analytical Chemistry 366, 174e177. Yatkin, S., Gerboles, M., Borowiak, A., 2012. Evaluation of standardless EDXRF analysis for the determination of elements on PM10 loaded filters. Atmospheric Environment 54, 568e582. Zhang, N.N., Cao, J.J., Ho, K.F., He, Y.Q., 2012. Chemical characterization of aerosol collected at Mt. Yulong in wintertime on the southeastern Tibetan Plateau. Atmospheric Research 107, 76e85. Zhang, Y.X., Schauer, J.J., Shafer, M.M., Hannigan, M.P., Dutton, S.J., 2008. Source apportionment of in vitro reactive oxygen species bioassay activity from atmospheric particulate matter. Environmental Science & Technology 42, 7502e 7509.
Please cite this article in press as: Okuda, T., et al., Improved methods for elemental analysis of atmospheric aerosols for evaluating human health impacts of aerosols in East Asia, Atmospheric Environment (2014), http://dx.doi.org/10.1016/j.atmosenv.2014.01.043