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Quantitative analysis of single aerosol particles using polycapillary X-ray optics
Spectrochimica Acta Part B 64 (2009) 1194–1197
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Quantitative analysis of single aerosol particles using polycapillary X-ray optics Tianxi Sun a,b,c,⁎, Zhiguo Liu a,b,c, Yude Li a,b,c, Guangfu Wang a,b,c, Guanghua Zhu a,b,c, Xunliang Ding a,b,c, Qing Xu d, Hui Liu a,b,c, Ping Luo a,b,c, Qiuli Pan a,b,c, Xiaoyan Lin a,b,c, Yuepeng Teng a,b,c a
The Key Laboratory of Beam Technology and Materials Modification of Ministry of Education, Beijing Normal University, Beijing, 100875, China College of Nuclear Science and Technology, Beijing Normal University, Beijing, 100875, China Beijing Radiation Center, Beijing, 100875, China d Institute of High Energy Physics, Chinese Academy of Science, Beijing, 100039, China b c
a r t i c l e
i n f o
Article history: Received 30 April 2009 Accepted 21 August 2009 Available online 31 August 2009 Keywords: Micro X-ray fluorescence Capillary X-ray optics Single aerosol particle analysis
a b s t r a c t A laboratory micro X-ray fluorescence spectrometer based on polycapillary X-ray optics (PXRO) was used to carry out the quantitative X-ray fluorescence analysis of single aerosol particles with smaller size than that of focal spot of PXRO. The minimum detection limits measured with the thin-film reference standards were in the range from 13.3 to 0.7 ng cm− 2 when the operating current and voltage were 70 mA and 35 kV, respectively. In order to reduce the effects of the inhomogeneous distributions of the X-ray intensity in the focal spot of the PXRO on the analysis results, the sensitivities were corrected by using a Gaussian function for the quantitative analysis of single aerosol particles. The accuracy of the analysis of single standard solution drops was on average 25% depending on the element and concentration. The precision of the analysis was better than 5%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Quantitative X-ray fluorescence (XRF) analysis of individual aerosol particles is a useful method for the studies on performances of aerosol particles [1,2]. There are many instruments which may be used to carry out the quantitative XRF analysis of individual aerosol particles, such as electron probe X-ray microanalysis (EPXMA), laser mass microanalysis (LAMMA), synchrotron micro X-ray fluorescence (SMXRF), micro proton-induced X-ray emission (Micro-PIXE) and laboratory micro X-ray fluorescence (MXRF) spectrometer [1–8]. The existing laboratory MXRF spectrometer is often based on polycapillary X-ray optics (PXRO) [6–8], and moreover, this optics is often used in the SMXRF facilities [2,9]. When the size of particle is larger than that of the focal spot of PXRO, the method of quantitative analysis of single aerosol particles using PXRO is mature [2,6]. However, when the size of particle is smaller than that of the focal spot of the PXRO, there are some difficulties in quantitatively analyzing such single aerosol particles using the PXRO with standard sample. The reasons of this are as follows. When the MXRF facilities based on the PXRO are calibrated with standard sample, the size of the focal spot of the PXRO is smaller than that of the standard sample. When concentrations of elements in a single aerosol particle with smaller size than that of the focal spot of the PXRO were obtained using the sensitivities determined from standard sample, the sensitivities must be corrected
according to the size and the shape of a single aerosol particle. And furthermore, it is well known that the distribution of the X-ray intensity in the focal spot of the PXRO is a Gaussian distribution. This is not helpful in accurately measuring the elemental XRF spectrum of a single aerosol particle with a smaller size than that of the focal spot of the PXRO. The element distributions even in such a small single particle are not uniform. Experimental results indicate that the elemental XRF spectrum of such a small single aerosol particle will vary when the aerosol particle is placed at different positions in the focal spot of the PXRO, and this variation of the elemental XRF spectrum increases with the increasing size of aerosol particles [10]. And therefore, in order to obtain the average concentration of elements in a single particle, the Gaussian distribution in the focal spot of the PXRO must be taken into account when the sensitivities are corrected with the size and the shape of a single aerosol particle. In the present paper, the performances of the PXRO for carrying out the quantitative XRF analysis of single aerosol particles with smaller size than that of the focal spot of the PXRO are studied. We used a Gaussian function to correct the sensitivities obtained from standard sample with larger size than that of the focal spot of the PXRO according to the shapes and sizes of individual aerosol particles under consideration. As an example of the application of the PXRO in quantitative analysis of single aerosol particles, the single particles of soil dust and dust from construction field in the micron size range are analyzed. 2. Experimental setup
⁎ Corresponding author. College of Nuclear Science and Technology, Beijing Normal University, Beijing, 100875, China. Tel.: +86 10 62207171; fax: +86 10 62208258. E-mail address: [email protected] (T. Sun). 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.08.005
Fig. 1 schematically shows the realization of the MXRF spectrometer based on the PXRO. The X-ray source is a Mo rotating anode X-ray
T. Sun et al. / Spectrochimica Acta Part B 64 (2009) 1194–1197
1195
Table 2 The MDLs of MXRF spectrometer based on PXRO for thin-film reference standards (35 kV, 70 mA, 500 s).
Fig. 1. Scheme of MXRF spectrometer based on PXRO.
generator (RIGAKU RU-200, 60 kV–200 mA) whose spot size is 300 × 300 μm2. In order to obtain a monochromatic excitation, a Nb filter with a thickness of 77.4 μm was added between the PXRO and the sample. The detector system is an XFlash® Detector 2001 RÖNTEC and RÖNTECMAX Spectrometer. The energy resolution of this detector system at 5.9 keV is 140 eV. The working distance of the electronic video microscope is 95 mm. The maximum total magnification of this microscope is 486. This microscope provides the function of size measurement based on the method of staff gauge. The size of the particle is measured with this microscope. The focal spot size of the PXRO decreases with the increasing energies, and the energy dependence of the focal spot size for the PXRO can be measured by using the backscatter method [11]. In this experiment the monochromatic excitation of Mo–Kα was used, and the focal spot size of the PXRO for Mo–Kα is 30.5 μm. The gain of the power flux density in the focal spot of the PXRO can be calculated by the following: !2 2 L Din gain = ηðEÞ⋅ ⋅ fin Dspot
ð1Þ
where η(E) is the measured transmission efficiency of the PXRO at energy E, which can be measured by using a pinhole [11], L is the distance from the X-ray source to the focal spot of the PXRO, fin is the input focal distance of the PXRO, Din and Dspot are the diameter of the entrance and the focal spot of the PXRO, respectively. The gain of the PXRO for Mo–Kα is 2970. Other parameters of the PXRO are reported in Table 1. 3. Results and discussions 3.1. Minimum detection limits of MXRF spectrometer based on PXRO The minimum detection limits (MDLs) can be calculated by the following formula [12]: qffiffiffiffiffiffi Ii;B pffiffi MDL = 3⋅Ci ⋅ Ii;N ⋅ t
ð2Þ
where Ci is the concentration of element i in the standard sample, Ii,B and Ii,N are the measured background and characteristic X-ray intensity of element i in counts per second, respectively, t is the acquisition time in live seconds. The MDLs of the MXRF spectrometer based on the PXRO are reported in Table 2, which were measured using a series of thin-film reference standards (Micromatter, Seattle,
MDL (ng cm− 2)
RSD (%)
Element
MDL (ng cm− 2)
RSD (%)
Si P S Cl K Ca Ti V Cr Mn Fe Ni Cu Zn Ga As
13.3 11.7 10.6 7.4 3.8 2.3 2.6 2.2 1.5 1.3 1.2 0.9 0.8 0.75 0.71 0.73
4.2 2.8 2.3 2.2 1.6 2.2 1.7 2.6 1.5 1.4 2.1 1.9 2.1 2.3 1.5 1.1
Se Br Sr Cd Sn Te I Cs Ba Pr Eu Dy Tm Lu Au Pb
0.72 0.88 0.70 3.7 3.1 2.7 2.4 2.0 1.8 1.7 1.6 1.5 1.8 0.9 0.81 0.74
1.2 2.5 1.2 2.5 1.3 1.9 1.8 2.1 2.0 2.5 2.0 1.5 2.0 2.1 1.8 2.2
WA, USA). These thin-film reference standards are prepared by vacuum deposition resulting in a highly uniform deposit. They are deposited on the Mylar film with a thickness of 6 μm. In order to reduce the effects of the possible inhomogeneous element distributions of this thin-film reference standards on the analysis results, we repeated the measurements of the standard reference samples with the focal spot of the PXRO locating at 15 different positions on the thin-film reference standards, and the relative standard deviation (RSD) was calculated (Table 2). According to the RSD in Table 2, the thin-film reference standards may be used for the reference standard for the MXRF analysis. The possible method of lowering the MDLs of the MXRF spectrometer based on a PXRO is to place a specially designed polycapillary Xray collimator (PXRC) in the detection channel [9]. This will improve the signal-to-noise ratio because of the restriction of the detector field of view by the PXRC, and accordingly lower the MDLs of the laboratory spectrometer based on a PXRO. 3.2. Calibration for MXRF spectrometer based on PXRO For the quantification, we used backscatter fundamental parameter method of QXAS package [13]. In order to establish the calibration constants for scattered radiation, a series of metal foils is used, such as Al, V, Fe, Ni, Cu, Y, Sn and Ta. In order to calculate the calibration constant for fluorescence radiation, the thin-film reference standards mentioned above were used. The measurements were performed in air. Fig. 2 shows an example of the XRF spectrum of reference standard of Fe. The resulting sensitivity coefficients as a function of element are shown in Fig. 3. The calibration procedure was evaluated by analyzing the standard samples prepared by pipetting standard solutions on the Mylar film with a thickness of 6 μm with ordinary pipette equipment. The standard solutions are a series of commercial single element reference standards. The accuracy of our measurement is on average 16% depending on the element and concentration. The precision of our analysis is better than 1.5% for most of the elements, with exception of Si, for which it is about 3.7%. 3.3. Application of the PXRO in quantitative XRF analysis of single aerosol particles
Table 1 The parameters of the PXRO. Length (mm) Input diameter (mm) Output diameter (mm) Output focal distance at 17.4 keV (mm) Input focal distance at 17.4 keV (mm)
Element
83.3 4.7 3.2 14.3 72.7
The integral sensitivity coefficient (ISC) is defined here as the sensitivity coefficient obtained by using the full excitation X-rays in the focal spot of the PXRO. The differential sensitivity coefficient (DSC) is defined here as the sensitivity coefficient corresponding to the fractional part of the excitation X-rays in the focal spot of the PXRO.
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T. Sun et al. / Spectrochimica Acta Part B 64 (2009) 1194–1197
Fig. 2. XRF spectrum of the thin-film reference standard of Fe.
Fig. 4. XRF spectrum of a single aerosol particle of soil dust.
The DSC will be modulated by the distribution of the excitation X-ray intensity in the focal spot of the PXRO. When the laboratory MXRF spectrometer based on the PXRO is used to carry out the quantitative XRF analysis of single aerosol particles with smaller size than that of the focal spot of the PXRO, the DSC should be used. However, there were difficulties in obtaining the DSC through direct measurement. The ISC may be used here instead of DSC to carry out the quantitative XRF analysis of single aerosol particles with smaller size than that of the focal spot of the PXRO, but it must be corrected by the distribution of the excitation X-ray intensity in the focal spot of the PXRO, the position of particle under consideration in the focal spot of the PXRO, and the shape and size of the analyzed particle. Considering that the distribution of the excitation X-ray intensity in the focal spot of the PXRO is a Gaussian distribution, the ISC was corrected via the twodimensional Gaussian function according to shapes and sizes of individual particle whose center is placed at the center of focal spot of the PXRO: 2 2 1 −x + z DSC = ISC⋅∬ pffiffiffiffiffiffi ⋅e 2σ 2 dxdz 2π⋅σ D
ð3Þ
where σ could be obtained by using a wire scan or the derivation of a knife edge scan, and D is dependent on the shape and the size of the individual particle under consideration. In our study, the shape of the analyzed particle was assumed to be spherical or ellipsoidal, and the corresponding equations of D are the following Eqs. (4) and (5), respectively: 2
2
x +z =
2 D 2
Fig. 3. Integral sensitivity coefficients as a function of element.
ð4Þ
x2 z2 + 2 = 1: A2 B
ð5Þ
The diameter D of the particle-sphere, the large half-axis A and small half-axis B of the particle-ellipsoid were estimated using the electronic video microscope. In order to evaluate the accuracy and the precision of our method, a series of standard solution drops with a smaller size than that of the focal spot of the PXRO was analyzed, respectively. Such small standard solution drops were collected on the Mylar film with a thickness of 6 μm daubed with the mixture of pure petroleum jelly (manufactured in USA) and C6H5CH3 with special pipette equipment made in our laboratory with taper glass monocapillary whose output diameter is 2 μm, which is different from pipette equipment used in Section 3.2. The accuracy of the analysis of these single standard solution drops was on average 25% depending on the element and concentration. The precision of the analysis was better than 5%. For example, when a standard solution drop with a diameter 3.8 μm is analyzed, the accuracies of the analysis of Cr and Ni were 13.7% and 11.3%, respectively. Their precisions were 2.5% and 1.9%, respectively. For sampling, a Battelle-type cascade impactor which has eight size-fractionated stages with the size range of <0.25, 0.25–0.5, 0.5–1, 1–2, 2–4, 4–8, 8–16 and >16 μm, respectively was used. In every range, the aerosol particles are collected on the Mylar film with a thickness of 6 μm daubed with the mixture of pure petroleum jelly (manufactured in USA) and C6H5CH3. Because this sample will be analyzed in a single particle respectively, the sample time must be controlled so that the aerosol particles were well separated with
Fig. 5. XRF spectrum of a single aerosol particle of dust from construction field.
T. Sun et al. / Spectrochimica Acta Part B 64 (2009) 1194–1197 Table 3 Element concentrations of single particles of soil dust and dust from construction field with the size range of 0.5–1 and 2–4 μm, respectively. Element
Dust from construction field
0.5–1 μm
2–4 μm
0.5–1 μm
2–4 μm
25.8
24.1
16.5 1.4 1.5 2.0 32.5 0.6 0.09 6.9 0.09 0.05
15.8 1.1 0.9 1.8 35.9 0.7 0.1 7.6 0.08 0.07
1.1 2.2 8.9 0.9 0.08 5.3 0.07 0.09
estimating the diameter of the aerosol particle with the electronic video microscope. 4. Conclusions
Concentration (%) Soil dust
Si S Cl K Ca Ti Mn Fe Cu Zn
1197
0.7 2.5 9.3 0.5 0.09 6.5 0.06 0.08
The order of magnitude of the gain in power density of the PXRO with a relative small focal spot was 103. This is helpful for the analysis of small sample. But the Gaussian distribution of X-ray intensity in the focal spot of the PXRO is not helpful for the quantitative analysis of a single particle with a smaller size than that of the focal spot of the PXRO. When the MXRF spectrometer based on the PXRO is used to quantitatively analyze such single particles, in order to reduce the effects of the inhomogeneous distributions of the X-ray intensity in the focal spot of the PXRO on the analysis results, the sensitivities may be corrected by using a Gaussian function according to the shape and size of a single particle under consideration. Acknowledgements
minimum separation distance greater than the excitation source beam diameter. As an example of the application of the MXRF spectrometer based on PXRO in the quantitative XRF analysis of single aerosol particles, the particles of soil dust and dust from construction field with the size range of 0.5–1 μm and 2–4 μm respectively are analyzed. The soil dust samples were collected from the open lands from where windblown soil dust could be transported. The dusts in the construction field were mainly from the cement. They were collected from the construction field. All samples of soil dust and dust from construction field were air dried and ground and then placed respectively in a sealed container with an exit and an inlet. When the wind is insufflated into the container through its inlet by a blower, the single particles can be collected on the Mylar film with a thickness of 6 μm by the Battelle-type cascade impactor placed at the exit the container. Figs. 4 and 5 show the XRF spectra of a single aerosol particle of soil dust and dust from construction field, respectively. In the case of soil dust and dust from construction field, a density of 1.5 g/cm3 and 1.7 g/cm3 was assumed, respectively. As shown in Table 3, the concentrations of particles from various sources are different. These elemental compositions may be considered as the chemical fingerprint of the single aerosol particles for air pollution sources, and the size-resolved fingerprint database for the air pollution sources may be accordingly established based on the quantitative XRF results for single aerosol particles. The origin of single aerosol particles can be identified by the comparison between the elemental compositions of unknown single aerosol particles and those of single aerosol particles from air pollution sources. And therefore, the size-resolved aerosol apportionment may be carried out based on the quantitative XRF results for single aerosol particles. In our work, there are some factors that will affect the accuracy of our method of quantitative XRF analysis of single aerosol particles. In order to obtain a monochromatic excitation, the metal filter is used. The monochromaticity of the monochromatic excitation obtained with this filter method is lower than that of the monochromatic excitation obtained with the secondary target and crystal, respectively. And therefore, for higher accuracy, the method of obtaining the monochromatic excitation with secondary target or crystal may be used. In our study, the shape of the analyzed particle was assumed to be spherical or ellipsoidal. In fact, there are few aerosol particles with a right spherical or ellipsoidal shape. In addition, the accuracy of our method of quantitative XRF analysis also relies on the method of
The authors acknowledge Dr. R. Padilla from XRF group of IAEA laboratories of AUSTRIA for all his help with this research. This research was supported by the Natural Science Foundation of Beijing, China (1092013), the Specialized Research Fund for the Doctoral Program of Higher Education of China (200800271021), the Key Project of Chinese Ministry of Education (108125) and the Beijing Key Laboratory of Applied Optics (JD100270543). References [1] F. Zimmermann, M. Ebert, A. Worringen, L. Schütz, S. Weinbruch, Environmental scanning electron microscopy (ESEM) as a new technique to determine the ice nucleation capability of individual atmospheric aerosol particles, Atmos. Environ. 41 (2007) 8219–8227. [2] L. Vincze, A. Somogyi, J. Osán, B. Vekemans, S. Török, K. Janssens, F. Adams, Quantitative trace element analysis of individual fly ash particles by means of Xray microfluorescence, Anal. Chem. 74 (2002) 1128–1135. [3] F. Chiminello, D. Ceccato, P. Mittner, Micro-PIXE study of tropospheric aerosols in an Antarctic coastal environment, Nucl. Instrum. Methods Phys. Res. B 219–220 (2004) 171–175. [4] V.M. Dekov, A. Van Put, D. Eisma, R. Van Grieken, Single particle analysis of suspended matter in the Makasar Strait and Flores Sea with particular reference to tin-bearing particles, J. Sea Res. 41 (1999) 35–53. [5] M. Toyoda, K. Kaibuchi, M. Nagasono, Y. Terada, T. Tanabe, S. Hayakawa, J. Kawai, X-ray analysis of a single aerosol particle with combination of scanning electron microscope and synchrotron radiation X-ray microscope, Spectrochim. Acta Part B 59 (2004) 1311–1315. [6] R. Padilla, P. Van Espen, A. Abrahantes, K. Janssens, Semiempirical approach for standardless calibration in M-XRF spectrometry using capillary lenses, X-ray Spectrom. 34 (2005) 19–27. [7] A. Alsecz, J. Osán, S. Kurunczi, B. Alföldy, A. Várhegyi, S. Török, Analytical performance of different X-ray spectroscopic techniques for the environmental monitoring of the recultivated uranium mine site, Spectrochim. Acta Part B 62 (2007) 769–776. [8] K. Nakano, K. Tanaka, X. Ding, K. Tsuji, Development of a new total reflection X-ray fluorescence instrument using polycapillary X-ray lens, Spectrochim. Acta Part B 61 (2006) 1105–1109. [9] T. Sun, X. Ding, Z. Liu, X. Wei, D. Chen, Q. Xu, Y. Huang, X. Lin, H. Sun, Characterization of a confocal 3D micro X-ray fluorescence facility based on polycapillary X-ray optics and Kirkpatrick-Baez mirrors, Spectrochim. Acta Part B 63 (2008) 76–80. [10] T. Sun, Z. Liu, G. Zhu, H. Liu, Q. Xu, Y. Li, G. Wang, H. Sun, P. Luo, Q. Pan, X. Ding, Identification of origin of single aerosol particles using polycapillary X-ray lens, Nucl. Instrum.Methods Phys. Res. B 267 (2009) 171–174. [11] T. Sun, X. Ding, Measurements of energy dependence of properties of polycapillary X-ray lens by using organic glass as a scatterer, J. Appl. Phys. 97 (2005) 124904 (1)–124904(7). [12] A. Somogyi, M. Drakopoulos, L. Vincze, B. Vekemans, C. Camerani, K. Janssens, A. Snigirev, F. Adams, ID18F: a new micro-X-ray fluorescence end-station at the European Synchrotron Radiation Facility (ESRF): preliminary results, X-ray Spectrom. 30 (2001) 242–252. [13] www.iaea.org/OurWork/ST/NA/NAAL/pci/ins/xrf/pciXRFdown.php.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Quantitative analysis of single aerosol particles using polycapillary X-ray optics Tianxi Sun a,b,c,⁎, Zhiguo Liu a,b,c, Yude Li a,b,c, Guangfu Wang a,b,c, Guanghua Zhu a,b,c, Xunliang Ding a,b,c, Qing Xu d, Hui Liu a,b,c, Ping Luo a,b,c, Qiuli Pan a,b,c, Xiaoyan Lin a,b,c, Yuepeng Teng a,b,c a
The Key Laboratory of Beam Technology and Materials Modification of Ministry of Education, Beijing Normal University, Beijing, 100875, China College of Nuclear Science and Technology, Beijing Normal University, Beijing, 100875, China Beijing Radiation Center, Beijing, 100875, China d Institute of High Energy Physics, Chinese Academy of Science, Beijing, 100039, China b c
a r t i c l e
i n f o
Article history: Received 30 April 2009 Accepted 21 August 2009 Available online 31 August 2009 Keywords: Micro X-ray fluorescence Capillary X-ray optics Single aerosol particle analysis
a b s t r a c t A laboratory micro X-ray fluorescence spectrometer based on polycapillary X-ray optics (PXRO) was used to carry out the quantitative X-ray fluorescence analysis of single aerosol particles with smaller size than that of focal spot of PXRO. The minimum detection limits measured with the thin-film reference standards were in the range from 13.3 to 0.7 ng cm− 2 when the operating current and voltage were 70 mA and 35 kV, respectively. In order to reduce the effects of the inhomogeneous distributions of the X-ray intensity in the focal spot of the PXRO on the analysis results, the sensitivities were corrected by using a Gaussian function for the quantitative analysis of single aerosol particles. The accuracy of the analysis of single standard solution drops was on average 25% depending on the element and concentration. The precision of the analysis was better than 5%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Quantitative X-ray fluorescence (XRF) analysis of individual aerosol particles is a useful method for the studies on performances of aerosol particles [1,2]. There are many instruments which may be used to carry out the quantitative XRF analysis of individual aerosol particles, such as electron probe X-ray microanalysis (EPXMA), laser mass microanalysis (LAMMA), synchrotron micro X-ray fluorescence (SMXRF), micro proton-induced X-ray emission (Micro-PIXE) and laboratory micro X-ray fluorescence (MXRF) spectrometer [1–8]. The existing laboratory MXRF spectrometer is often based on polycapillary X-ray optics (PXRO) [6–8], and moreover, this optics is often used in the SMXRF facilities [2,9]. When the size of particle is larger than that of the focal spot of PXRO, the method of quantitative analysis of single aerosol particles using PXRO is mature [2,6]. However, when the size of particle is smaller than that of the focal spot of the PXRO, there are some difficulties in quantitatively analyzing such single aerosol particles using the PXRO with standard sample. The reasons of this are as follows. When the MXRF facilities based on the PXRO are calibrated with standard sample, the size of the focal spot of the PXRO is smaller than that of the standard sample. When concentrations of elements in a single aerosol particle with smaller size than that of the focal spot of the PXRO were obtained using the sensitivities determined from standard sample, the sensitivities must be corrected
according to the size and the shape of a single aerosol particle. And furthermore, it is well known that the distribution of the X-ray intensity in the focal spot of the PXRO is a Gaussian distribution. This is not helpful in accurately measuring the elemental XRF spectrum of a single aerosol particle with a smaller size than that of the focal spot of the PXRO. The element distributions even in such a small single particle are not uniform. Experimental results indicate that the elemental XRF spectrum of such a small single aerosol particle will vary when the aerosol particle is placed at different positions in the focal spot of the PXRO, and this variation of the elemental XRF spectrum increases with the increasing size of aerosol particles [10]. And therefore, in order to obtain the average concentration of elements in a single particle, the Gaussian distribution in the focal spot of the PXRO must be taken into account when the sensitivities are corrected with the size and the shape of a single aerosol particle. In the present paper, the performances of the PXRO for carrying out the quantitative XRF analysis of single aerosol particles with smaller size than that of the focal spot of the PXRO are studied. We used a Gaussian function to correct the sensitivities obtained from standard sample with larger size than that of the focal spot of the PXRO according to the shapes and sizes of individual aerosol particles under consideration. As an example of the application of the PXRO in quantitative analysis of single aerosol particles, the single particles of soil dust and dust from construction field in the micron size range are analyzed. 2. Experimental setup
⁎ Corresponding author. College of Nuclear Science and Technology, Beijing Normal University, Beijing, 100875, China. Tel.: +86 10 62207171; fax: +86 10 62208258. E-mail address: [email protected] (T. Sun). 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.08.005
Fig. 1 schematically shows the realization of the MXRF spectrometer based on the PXRO. The X-ray source is a Mo rotating anode X-ray
T. Sun et al. / Spectrochimica Acta Part B 64 (2009) 1194–1197
1195
Table 2 The MDLs of MXRF spectrometer based on PXRO for thin-film reference standards (35 kV, 70 mA, 500 s).
Fig. 1. Scheme of MXRF spectrometer based on PXRO.
generator (RIGAKU RU-200, 60 kV–200 mA) whose spot size is 300 × 300 μm2. In order to obtain a monochromatic excitation, a Nb filter with a thickness of 77.4 μm was added between the PXRO and the sample. The detector system is an XFlash® Detector 2001 RÖNTEC and RÖNTECMAX Spectrometer. The energy resolution of this detector system at 5.9 keV is 140 eV. The working distance of the electronic video microscope is 95 mm. The maximum total magnification of this microscope is 486. This microscope provides the function of size measurement based on the method of staff gauge. The size of the particle is measured with this microscope. The focal spot size of the PXRO decreases with the increasing energies, and the energy dependence of the focal spot size for the PXRO can be measured by using the backscatter method [11]. In this experiment the monochromatic excitation of Mo–Kα was used, and the focal spot size of the PXRO for Mo–Kα is 30.5 μm. The gain of the power flux density in the focal spot of the PXRO can be calculated by the following: !2 2 L Din gain = ηðEÞ⋅ ⋅ fin Dspot
ð1Þ
where η(E) is the measured transmission efficiency of the PXRO at energy E, which can be measured by using a pinhole [11], L is the distance from the X-ray source to the focal spot of the PXRO, fin is the input focal distance of the PXRO, Din and Dspot are the diameter of the entrance and the focal spot of the PXRO, respectively. The gain of the PXRO for Mo–Kα is 2970. Other parameters of the PXRO are reported in Table 1. 3. Results and discussions 3.1. Minimum detection limits of MXRF spectrometer based on PXRO The minimum detection limits (MDLs) can be calculated by the following formula [12]: qffiffiffiffiffiffi Ii;B pffiffi MDL = 3⋅Ci ⋅ Ii;N ⋅ t
ð2Þ
where Ci is the concentration of element i in the standard sample, Ii,B and Ii,N are the measured background and characteristic X-ray intensity of element i in counts per second, respectively, t is the acquisition time in live seconds. The MDLs of the MXRF spectrometer based on the PXRO are reported in Table 2, which were measured using a series of thin-film reference standards (Micromatter, Seattle,
MDL (ng cm− 2)
RSD (%)
Element
MDL (ng cm− 2)
RSD (%)
Si P S Cl K Ca Ti V Cr Mn Fe Ni Cu Zn Ga As
13.3 11.7 10.6 7.4 3.8 2.3 2.6 2.2 1.5 1.3 1.2 0.9 0.8 0.75 0.71 0.73
4.2 2.8 2.3 2.2 1.6 2.2 1.7 2.6 1.5 1.4 2.1 1.9 2.1 2.3 1.5 1.1
Se Br Sr Cd Sn Te I Cs Ba Pr Eu Dy Tm Lu Au Pb
0.72 0.88 0.70 3.7 3.1 2.7 2.4 2.0 1.8 1.7 1.6 1.5 1.8 0.9 0.81 0.74
1.2 2.5 1.2 2.5 1.3 1.9 1.8 2.1 2.0 2.5 2.0 1.5 2.0 2.1 1.8 2.2
WA, USA). These thin-film reference standards are prepared by vacuum deposition resulting in a highly uniform deposit. They are deposited on the Mylar film with a thickness of 6 μm. In order to reduce the effects of the possible inhomogeneous element distributions of this thin-film reference standards on the analysis results, we repeated the measurements of the standard reference samples with the focal spot of the PXRO locating at 15 different positions on the thin-film reference standards, and the relative standard deviation (RSD) was calculated (Table 2). According to the RSD in Table 2, the thin-film reference standards may be used for the reference standard for the MXRF analysis. The possible method of lowering the MDLs of the MXRF spectrometer based on a PXRO is to place a specially designed polycapillary Xray collimator (PXRC) in the detection channel [9]. This will improve the signal-to-noise ratio because of the restriction of the detector field of view by the PXRC, and accordingly lower the MDLs of the laboratory spectrometer based on a PXRO. 3.2. Calibration for MXRF spectrometer based on PXRO For the quantification, we used backscatter fundamental parameter method of QXAS package [13]. In order to establish the calibration constants for scattered radiation, a series of metal foils is used, such as Al, V, Fe, Ni, Cu, Y, Sn and Ta. In order to calculate the calibration constant for fluorescence radiation, the thin-film reference standards mentioned above were used. The measurements were performed in air. Fig. 2 shows an example of the XRF spectrum of reference standard of Fe. The resulting sensitivity coefficients as a function of element are shown in Fig. 3. The calibration procedure was evaluated by analyzing the standard samples prepared by pipetting standard solutions on the Mylar film with a thickness of 6 μm with ordinary pipette equipment. The standard solutions are a series of commercial single element reference standards. The accuracy of our measurement is on average 16% depending on the element and concentration. The precision of our analysis is better than 1.5% for most of the elements, with exception of Si, for which it is about 3.7%. 3.3. Application of the PXRO in quantitative XRF analysis of single aerosol particles
Table 1 The parameters of the PXRO. Length (mm) Input diameter (mm) Output diameter (mm) Output focal distance at 17.4 keV (mm) Input focal distance at 17.4 keV (mm)
Element
83.3 4.7 3.2 14.3 72.7
The integral sensitivity coefficient (ISC) is defined here as the sensitivity coefficient obtained by using the full excitation X-rays in the focal spot of the PXRO. The differential sensitivity coefficient (DSC) is defined here as the sensitivity coefficient corresponding to the fractional part of the excitation X-rays in the focal spot of the PXRO.
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T. Sun et al. / Spectrochimica Acta Part B 64 (2009) 1194–1197
Fig. 2. XRF spectrum of the thin-film reference standard of Fe.
Fig. 4. XRF spectrum of a single aerosol particle of soil dust.
The DSC will be modulated by the distribution of the excitation X-ray intensity in the focal spot of the PXRO. When the laboratory MXRF spectrometer based on the PXRO is used to carry out the quantitative XRF analysis of single aerosol particles with smaller size than that of the focal spot of the PXRO, the DSC should be used. However, there were difficulties in obtaining the DSC through direct measurement. The ISC may be used here instead of DSC to carry out the quantitative XRF analysis of single aerosol particles with smaller size than that of the focal spot of the PXRO, but it must be corrected by the distribution of the excitation X-ray intensity in the focal spot of the PXRO, the position of particle under consideration in the focal spot of the PXRO, and the shape and size of the analyzed particle. Considering that the distribution of the excitation X-ray intensity in the focal spot of the PXRO is a Gaussian distribution, the ISC was corrected via the twodimensional Gaussian function according to shapes and sizes of individual particle whose center is placed at the center of focal spot of the PXRO: 2 2 1 −x + z DSC = ISC⋅∬ pffiffiffiffiffiffi ⋅e 2σ 2 dxdz 2π⋅σ D
ð3Þ
where σ could be obtained by using a wire scan or the derivation of a knife edge scan, and D is dependent on the shape and the size of the individual particle under consideration. In our study, the shape of the analyzed particle was assumed to be spherical or ellipsoidal, and the corresponding equations of D are the following Eqs. (4) and (5), respectively: 2
2
x +z =
2 D 2
Fig. 3. Integral sensitivity coefficients as a function of element.
ð4Þ
x2 z2 + 2 = 1: A2 B
ð5Þ
The diameter D of the particle-sphere, the large half-axis A and small half-axis B of the particle-ellipsoid were estimated using the electronic video microscope. In order to evaluate the accuracy and the precision of our method, a series of standard solution drops with a smaller size than that of the focal spot of the PXRO was analyzed, respectively. Such small standard solution drops were collected on the Mylar film with a thickness of 6 μm daubed with the mixture of pure petroleum jelly (manufactured in USA) and C6H5CH3 with special pipette equipment made in our laboratory with taper glass monocapillary whose output diameter is 2 μm, which is different from pipette equipment used in Section 3.2. The accuracy of the analysis of these single standard solution drops was on average 25% depending on the element and concentration. The precision of the analysis was better than 5%. For example, when a standard solution drop with a diameter 3.8 μm is analyzed, the accuracies of the analysis of Cr and Ni were 13.7% and 11.3%, respectively. Their precisions were 2.5% and 1.9%, respectively. For sampling, a Battelle-type cascade impactor which has eight size-fractionated stages with the size range of <0.25, 0.25–0.5, 0.5–1, 1–2, 2–4, 4–8, 8–16 and >16 μm, respectively was used. In every range, the aerosol particles are collected on the Mylar film with a thickness of 6 μm daubed with the mixture of pure petroleum jelly (manufactured in USA) and C6H5CH3. Because this sample will be analyzed in a single particle respectively, the sample time must be controlled so that the aerosol particles were well separated with
Fig. 5. XRF spectrum of a single aerosol particle of dust from construction field.
T. Sun et al. / Spectrochimica Acta Part B 64 (2009) 1194–1197 Table 3 Element concentrations of single particles of soil dust and dust from construction field with the size range of 0.5–1 and 2–4 μm, respectively. Element
Dust from construction field
0.5–1 μm
2–4 μm
0.5–1 μm
2–4 μm
25.8
24.1
16.5 1.4 1.5 2.0 32.5 0.6 0.09 6.9 0.09 0.05
15.8 1.1 0.9 1.8 35.9 0.7 0.1 7.6 0.08 0.07
1.1 2.2 8.9 0.9 0.08 5.3 0.07 0.09
estimating the diameter of the aerosol particle with the electronic video microscope. 4. Conclusions
Concentration (%) Soil dust
Si S Cl K Ca Ti Mn Fe Cu Zn
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0.7 2.5 9.3 0.5 0.09 6.5 0.06 0.08
The order of magnitude of the gain in power density of the PXRO with a relative small focal spot was 103. This is helpful for the analysis of small sample. But the Gaussian distribution of X-ray intensity in the focal spot of the PXRO is not helpful for the quantitative analysis of a single particle with a smaller size than that of the focal spot of the PXRO. When the MXRF spectrometer based on the PXRO is used to quantitatively analyze such single particles, in order to reduce the effects of the inhomogeneous distributions of the X-ray intensity in the focal spot of the PXRO on the analysis results, the sensitivities may be corrected by using a Gaussian function according to the shape and size of a single particle under consideration. Acknowledgements
minimum separation distance greater than the excitation source beam diameter. As an example of the application of the MXRF spectrometer based on PXRO in the quantitative XRF analysis of single aerosol particles, the particles of soil dust and dust from construction field with the size range of 0.5–1 μm and 2–4 μm respectively are analyzed. The soil dust samples were collected from the open lands from where windblown soil dust could be transported. The dusts in the construction field were mainly from the cement. They were collected from the construction field. All samples of soil dust and dust from construction field were air dried and ground and then placed respectively in a sealed container with an exit and an inlet. When the wind is insufflated into the container through its inlet by a blower, the single particles can be collected on the Mylar film with a thickness of 6 μm by the Battelle-type cascade impactor placed at the exit the container. Figs. 4 and 5 show the XRF spectra of a single aerosol particle of soil dust and dust from construction field, respectively. In the case of soil dust and dust from construction field, a density of 1.5 g/cm3 and 1.7 g/cm3 was assumed, respectively. As shown in Table 3, the concentrations of particles from various sources are different. These elemental compositions may be considered as the chemical fingerprint of the single aerosol particles for air pollution sources, and the size-resolved fingerprint database for the air pollution sources may be accordingly established based on the quantitative XRF results for single aerosol particles. The origin of single aerosol particles can be identified by the comparison between the elemental compositions of unknown single aerosol particles and those of single aerosol particles from air pollution sources. And therefore, the size-resolved aerosol apportionment may be carried out based on the quantitative XRF results for single aerosol particles. In our work, there are some factors that will affect the accuracy of our method of quantitative XRF analysis of single aerosol particles. In order to obtain a monochromatic excitation, the metal filter is used. The monochromaticity of the monochromatic excitation obtained with this filter method is lower than that of the monochromatic excitation obtained with the secondary target and crystal, respectively. And therefore, for higher accuracy, the method of obtaining the monochromatic excitation with secondary target or crystal may be used. In our study, the shape of the analyzed particle was assumed to be spherical or ellipsoidal. In fact, there are few aerosol particles with a right spherical or ellipsoidal shape. In addition, the accuracy of our method of quantitative XRF analysis also relies on the method of
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