- Email: [email protected]
Development of an X-ray fluorescence spectrometric method for the analysis of atmospheric aerosol samples
Microchemical Journal 79 (2005) 37 – 41 www.elsevier.com/locate/microc
Development of an X-ray fluorescence spectrometric method for the analysis of atmospheric aerosol samples Vira´g Szila´gyi*, Zsuzsanna Hartya´ni Agricultural Research Institute of the Hungarian Academy of Science, Martonva´sa´r Department of Earth and Environmental Science, University of Veszpre´m, 8201 Veszpre´m, P.O.B. 158, Hungary
Abstract A simple, rapid method was developed for the analysis of aerosol samples by X-ray fluorescence spectrometry. Aerosol measurements were made using various sample supports (Whatman and Teflon filters, ProleneR foil). The calibration procedure was carried out by dripping 500 Al of a gradually diluted multi-elemental standard solution (CertiPURR 11355) onto the top of the sample supports, which were then dried at ambient temperature. Thirteen elements, namely Na, Al, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr and Pb were calibrated and quantified. The optimal measurement parameters (excitation conditions, measuring times for each element) were determined on the basis of blank values and the amplitude of the signals. The filters were covered with Ta or Re plates to ensure infinite thickness for the penetration depth of the primary X-ray beam. It was also demonstrated that these plates served as a secondary target. The accuracy, precision and detection limits (0.01–0.18 mg/kg) were calculated. All the analytical parameters were better when Teflon filters and ProleneR foil were used than in the case of Whatman quartz fibre filters. D 2004 Elsevier B.V. All rights reserved. Keywords: Aerosol; Inorganic components; XRF; Method development
1. Introduction The most appropriate methods for the direct determination of inorganic species in atmospheric aerosols are proton-induced X-ray emission analysis (PIXE) [1–3], total reflection X-ray fluorescence spectrometry (TXRF) [1,4], X-ray fluorescence spectrometry (XRF) [1–6] and instrumental neutron activation analysis (INNA) [7]. The morphology of airborne particulates could be determined by Xray diffraction analysis (XRD) [8,9], analytical scanning electron microscope (ASEM) or analytical transmission electron microscope (ATEM) [10]. Generally, Nuclepore filters are used for PIXE analysis [7,11] and cellulose nitrate filters for TXRF [12]. For other analyses, Whatman [5,13] and Teflon [4] filters are used. The inductively coupled plasma emission spectroscopy (ICP) [14] and atomic
* Corresponding author. Tel./fax: +36 88423023. E-mail addresses: [email protected], [email protected] (V. Szila´gyi). 0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.09.002
absorption spectroscopy (AAS) techniques [13,15] can also be used for determining the elemental composition of airborne particulates but these methods involve a sample preparation step. The greatest advantage of the direct XRFS determination of aerosol samples is that the sample preparation procedures can be avoided in the case of trace amounts of sample. Therefore, errors arising from the sample preparation procedures are eliminated. In addition, depending on the concentration, a wide range of elements (O–U) can be determined with these methods. Nowadays, the XRFS technique, as an analytical tool, is capable of measuring samples even of small quantity and thickness. Due to the development of spectrometers, the distance between the X-ray tube window and the sample target has decreased, resulting in increased sensitivity. In addition, the available energy has also increased. It is well known that in the X-ray fluorescence technique differences in the thickness of the sample and the sampling conditions severely detract from the accuracy of the analysis. Samples can be classified as: (a) samples of infinite thickness for X-ray penetration; (b) samples of finite
38
V. Szila´gyi, Z. Hartya´ni / Microchemical Journal 79 (2005) 37–41
thickness below the critical value; (c) samples of infinite thickness placed over a sample of finite thickness [16]. A simple thin-layer sample preparation method was described for XRFS analysis [17]. A portion of vehicle exhaust particulate or atmospheric dust certificate reference material was suspended in a small volume of a fluid on a hydrophilized polytetrafluoroethylene (PTFE) membrane filter, dispersed by air blown from a pump and dried by IR irradiation. A knowledge of the X-ray penetration depth of the measured element, which depends on the density of the matrix [18], is also very important. The main objectives of the research were the comparison of different aerosol supports (Whatman filter, Teflon filter and ProleneR foil) and the development of a quantitative method for the analysis of soil-derived elements in aerosol samples.
2. Materials and methods 2.1. Instrument, measurement parameters A PHILIPS PW 2404 X-ray spectrometer equipped with a 4-kW Rh anode tube was used in the present work. Duplex, scintillation and proportional counters were used as detectors, while the analysing crystals were LiF (200), PE 002-C and PX1. The target elements were Na, Al, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr and Pb. In general, the Ka lines of the elements were measured except for Pb, when the La line was chosen for quantification. In all cases, the intensity of the background was determined. Unfortunately, the determination of Zn was not possible using a Re plate, because the Zn Ka line interfered with the Re Lh line. Similarly, the determination of Cu was not possible with Ta plates. In the case of ProleneR foil, which was developed by Philips, the sample holder set consisted of a C-shaped sample holder and a steel cup. The sample supports had a 32-mm outside diameter and a thickness of 0.4 mm.
Sample supports are designed to hold up to 500 Al of liquid, which is subsequently dried in the central dimpled area. The patented dimpling process creates a 2-mm dimple inside a 5-mm outer dimple. These dimples allow gravitational forces to pull the liquid and residue to the lowest point in the support (the central dimple) while drying. For the Whatman and Teflon filters, a liquid sample holder with Mylar film was placed in a steel cup with an inner diameter of 27 mm. Since the filters are so thin that the X-rays can penetrate through the filter material, the radiation is able to excite the elements in the sample holder and in various parts of the instrument. The characteristic wavelengths of these elements adulterate the results, being measured together with the elements in the sample. This stray radiation can be eliminated by covering the sample with an binfinite thicknessQ material, for example a Ag plate [19], or by painting the platform and the sample holder with high purity silver paint [5]. In the present case, either a tantalum or a rhenium plate was chosen to eliminate the back-scattered radiation. Ta and Re are heavy elements, so plates of these metals are capable of absorbing the primary radiation passing through the aerosol filters. In addition, the characteristic lines of Ta and Re are very close to the absorption edges of some of the elements analysed (Fig. 1). The consequences are that these lines excite elements having K absorption edges directly on the long wavelength side of the Re and Ta lines (namely Fe, Co, Ni, Cu and Zn). This means that Ta and Re are not only absorbers for backscattered background radiation but also secondary targets. Applying Ta or Re plates results in the increased sensitivity of Ka lines for Fe, Co, Ni, Cu and Zn and a lower detection limit for these elements.
3. Results Three different filters (Whatman, Teflon and ProleneR) were tested as blank supports for the samples. Weak Ca,
Fig. 1. Absorption edges of elements and characteristic lines of Ta, Re and Rh.
V. Szila´gyi, Z. Hartya´ni / Microchemical Journal 79 (2005) 37–41
Fe and Ni signals were identified, all of which originated from the instrument. The highest signals for these elements, and also for K and Zn, were obtained using Whatman filters. These elements are contaminants of the Whatman filter. Two problems were encountered when testing these supports. The Whatman filter is made of quartz fibres and has a thickness of only about 300 Am. Since the aerosol particles become embedded in the filter material, it is necessary to calculate the X-ray penetration depth for the Whatman filter. Calculations indicate that the penetration depths of the measured elements are b100 Am (Fig. 2). Information is obtained for a depth of 100 Am or less, depending on the atomic numbers of the elements. Therefore, as the concentration of the solution on the filters increases, the signals will not increase linearly. The standard solution dripped onto the filter soaked into the filter material. The secondary X-rays from elements at a depth of more than 100 Am were not detected, because they were absorbed by the filter material. The penetration depth of the X-ray beam (x) was calculated as follows: x¼
2:69 l¯ q
ð1Þ
As shown in Eq. (1), x depends on the matrix, the density of the sample (q) (g cm3) and a constant characteristic of the instrument (2.69). The matrix is characterised by the average mass absorption coefficient (l¯ ), presented in Eq. (2). X l¯ ¼ Wj l j ð2Þ where W j and l j are the mass percentage and mass absorption coefficient of the element. l j depends on the element as well as on the wavelength (units: cm2 g1).The second problem encountered was the heat sensitivity of the Teflon filter, which became brittle and cracked after irradiation. Consequently, the X-ray beam was capable of destroying the Teflon filter depending on the intensity of
39
primary excitation and the measurement time. For this reason, the total measuring time had to be decreased. This was achieved by measuring fewer elements at a time, rather than reducing the measurement time for each element. The use of a high performance X-ray tube (4 kW) is advantageous for the excitation of the elements, but destroys the Teflon filter. 3.1. Calibration The calibration procedures can be evaluated in several manners. For example, a solution prepared from the soluble salts of the target metal can be dripped onto the filter materials [1,6,20,21]. Another possibility is to use an aerosol generator [6,21] for the preparation of standard filter materials. The preparation of glass disc bequivalent filtersQ is a further possible calibration method [19]. In the present project, the calibration procedure used for all three (Whatman, Teflon and ProleneR) filters involved a series of home-made standards based on a certified standard solution. Seven gradually diluted model solutions were prepared, with a concentration range of 100–700 Ag/l for each element. This concentration range was chosen after preliminary measurements on the intensity of real samples and standard samples. The solutions were composed of diluted ICP multi-element standard (CertiPURR 11355) dripped onto the supports and dried in air before using them as calibration standards. The measurements were performed in a vacuum. The analytical curves were estimated with the least squares method. The R values of the analytical curves for each element are presented in Table 1, while some typical analytical curves can be seen in Fig. 3. The repeatability of the measurements was evaluated by measuring each sample three times and calculating RSD% values. The RSD% values were lower than 10% in the case of ProleneR (except for Al, Fe) and Teflon (except for Al, Pb) and b5% for Whatman filters (except for Cr, Fe and Ni). For all three supports, the difference between the Ta and Re plates was not significant. The repeatability of the sample
Fig. 2. X-ray penetration depths of elements for Whatman filter.
40
V. Szila´gyi, Z. Hartya´ni / Microchemical Journal 79 (2005) 37–41
Table 1 R values of analytical curves Elements Na Al K Ca Cr Mn Fe Co Ni Cu Zn Sr Pb
ProleneR
filters and lower than 10% except for Cr, Fe and Ni on Whatman filters. Teflon
Whatman
Ta plate
Re plate
Ta plate
Re plate
Ta plate
Re plate
0.401 0.930 0.943 0.917 0.978 0.987 0.958 0.951 0.950 – 0.690 0.686 0.877
0.775 0.928 0.943 0.877 0.990 0.963 0.950 0.935 0.971 0.993 – 0.967 0.983
0.172 0.270 0.902 0.823 0.986 0.858 0.954 0.987 0.888 – 0.978 0.757 0.917
0.489 0.837 0.771 0.055 0.982 0.979 0.941 0.974 0.977 0.967 – 0.992 0.870
0.173 0.237 0.848 0.711 0.987 0.177 0.937 0.521 0.804 – 0.874 0.080 0.054
0.149 0.736 0.963 0.939 0.167 0.572 0.346 0.589 0.666 – 0.735 0.807 –
preparation was also investigated using three simultaneously prepared samples for each support and calculating RSD% values after the measurements. The RSD% values were lower than about 15% except for Fe and Pb on ProleneR foil, lower than 15% except for Na, Al, Cr and Pb on Teflon
3.2. Detection limit The lower limit of detection (LLD) was calculated using the SuperQ software, which takes into account the measuring time for each element (t), the background intensity (Rb) and the slope of the analytical curve (m). The following equation is applied: rffiffiffiffiffiffi 3 Rb LLD ¼ ð3Þ m t The LLD values obtained for the elements on the three different sample supports were compared (Fig. 4). The LLD values ranged from 0.01 to 0.13 mg/kg on ProleneR foil and from 0.03 to 0.18 mg/kg on Teflon filters except for Al. For both supports, the difference between the LLD values obtained using Ta and Re plates was not significant. The LLD values calculated for Whatman filters depended on the elements to be analysed and varied from 0.03 to 0.20 mg/kg.
Fig. 3. Analytical curves of Al, Cr and Ni.
V. Szila´gyi, Z. Hartya´ni / Microchemical Journal 79 (2005) 37–41
41
Acknowledgements The authors are indebted to Judit Szauer and La´szlo´ Mere´ nyi for their professional help in analysing the ´ gnes Molna´r atmospheric aerosol and soil samples, and to A and Andra´s Gelencse´r for their useful advice. This work was supported by the grants from the Higher Educational Scientific Research and Development (FKFP 0606/1997) and the Hungarian Scientific Research Fund (OTKA T026423 and T043574). Fig. 4. Lower limit of detection of measured elements using ProleneR foil.
For a few elements (Na, Al, Mn, Zn and Sr), however, the LLD values were not calculated, as the analytical curves were not acceptable due to line interferences.
4. Discussion Three different supports were measured as blank samples. Weak Ca, Fe, and Ni signals originating from the instrument were obtained for all of them. In the case of Whatman filters, higher blank signals were obtained for these elements and also for K and Zn. These elements are contaminants of the Whatman filter and must be taken into account in the calibration procedure. Comparative tests were performed on three different filters as possible substrates for atmospheric aerosol sampling. It could be seen during sample preparation that the standard solutions dried on the surface of ProleneR foil and Teflon filters, but in the case of Whatman filters the solution soaked into the filter material. Because the X-ray penetration depth of the measured elements is smaller than the thickness of the Whatman filter, this filter cannot be used for this sample preparation procedure. ProleneR foil and Teflon filters are equally applicable for XRF analysis, having appropriate precision and accuracy, though the Teflon filters may be damaged during the measurement, so they cannot be used for repeated measurements. It was thus concluded that the best results could be achieved using ProleneR foil or Teflon filter as sample supports. The technique developed for aerosol analysis using the XRFS method is novel because, to the best of our knowledge, Ta and Re plates have not been used before to eliminate the back-scattered radiation originating from various parts of the spectrometer. The emitted radiation of Ta or Re also causes the excitation of elements as a secondary target. The energies of the Ta and Re lines are slightly higher than the K absorption edges of some of the measured elements (Fe, Co, Ni, Cu, Zn).
References [1] U. W7tjen, H. Cave´, Nuclear Instruments & Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 109/110 (1996) 395. [2] J. Miranda, E. Andrade, A. Lo´pez-Sua´rez, R. Ledesma, T.A. Cahill, P.H. Wakabayashi, Atmospheric Environment 30 (20) (1996) 3471. [3] F. Aldape, J. Flores, M.R.V. Dı´az, B. Herna´ndez-Me´ndez, J.M.Z. Montoya, E.E. Blanco, A.F. Fuentes, L.M. Torres-Martı´nez, Nuclear Instruments & Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 150 (1999) 439. [4] K. Moloi, A. Viksna, E. Selin Lindgren, P. Standzenieks, X-ray Spectrometry 31 (2002) 27. [5] L. Samek, J. Injuk, P. Van Espen, R. Van Grieken, X-ray Spectrometry 21 (2002) 84. [6] O. Haupt, K. Linnow, R. Harmel, C. Schaefer, W. Dannecker, X-ray Spectrometry 26 (1997) 79. [7] I. Salma, W. Maenhaut, E´. Zemple´n-Papp, G. Za´ray, Atmospheric Environment 35 (2001) 4367. [8] B.L. Davis, Guo Jixiang, Atmospheric Environment 34 (2000) 2703. [9] V. Esteve, J. Rius, L.E. Ochando, J.M. Amigo´, Atmospheric Environment 31 (23) (1997) 3963. [10] A. Chabas, R.A. Lefe`vre, Atmospheric Environment 34 (1999) 225. [11] W. Maenhaut, F. Francois, J. Cafmeyer, O. Okunade, Nuclear Instruments & Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 109/110 (1996) 476. [12] M. Schmeling, R. Klockenk7mper, D. Klockow, Spectrochimica Acta, Part B: Atomic Spectroscopy 52 (1997) 985. [13] N. Kubilay, S. Yemenicioglu, A.C. Saydam, Marine Pollution Bulletin 30 (7) (1995) 475. [14] N.A.H. Janssen, D.F. Van Manson, K. Van Der Jagt, H. Harssema, G. Hoek, Atmospheric Environment 31 (8) (1997) 1185. [15] C. Bilos, J.C. Colombo, C.N. Skorupka, M.J. Rodriguez Presa, Environmental Pollution 111 (2001) 149. [16] P. Frigieri, R. Trucco, X-ray Spectrometry 3 (1) (1974) 40. [17] T. Kyotani, M. Iwatsuki, Analyst 123 (1998) 1813. [18] P. Frigieri, R. Trucco, E. Caretta, X-ray Spectrometry 4 (1) (1975) 28. [19] P. Quisefit, P. De Chateaubourg, S. Garivait, E. Steiner, X-ray Spectrometry 21 (2) (1994) 59. [20] O.V. Shuvayeva, P.K. Koutzenogii, V.B. Baryshev, V.I. Rezchihov, A.I. Smirnova, L.D. Ivanova, F.V. Sukchorukov, Nuclear Instruments & Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 405 (1998) 553. [21] O. Haupt, B. Klaue, C. Schaefer, W. Dannecker, X-ray Spectrometry 24 (5) (1995) 267.
Development of an X-ray fluorescence spectrometric method for the analysis of atmospheric aerosol samples Vira´g Szila´gyi*, Zsuzsanna Hartya´ni Agricultural Research Institute of the Hungarian Academy of Science, Martonva´sa´r Department of Earth and Environmental Science, University of Veszpre´m, 8201 Veszpre´m, P.O.B. 158, Hungary
Abstract A simple, rapid method was developed for the analysis of aerosol samples by X-ray fluorescence spectrometry. Aerosol measurements were made using various sample supports (Whatman and Teflon filters, ProleneR foil). The calibration procedure was carried out by dripping 500 Al of a gradually diluted multi-elemental standard solution (CertiPURR 11355) onto the top of the sample supports, which were then dried at ambient temperature. Thirteen elements, namely Na, Al, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr and Pb were calibrated and quantified. The optimal measurement parameters (excitation conditions, measuring times for each element) were determined on the basis of blank values and the amplitude of the signals. The filters were covered with Ta or Re plates to ensure infinite thickness for the penetration depth of the primary X-ray beam. It was also demonstrated that these plates served as a secondary target. The accuracy, precision and detection limits (0.01–0.18 mg/kg) were calculated. All the analytical parameters were better when Teflon filters and ProleneR foil were used than in the case of Whatman quartz fibre filters. D 2004 Elsevier B.V. All rights reserved. Keywords: Aerosol; Inorganic components; XRF; Method development
1. Introduction The most appropriate methods for the direct determination of inorganic species in atmospheric aerosols are proton-induced X-ray emission analysis (PIXE) [1–3], total reflection X-ray fluorescence spectrometry (TXRF) [1,4], X-ray fluorescence spectrometry (XRF) [1–6] and instrumental neutron activation analysis (INNA) [7]. The morphology of airborne particulates could be determined by Xray diffraction analysis (XRD) [8,9], analytical scanning electron microscope (ASEM) or analytical transmission electron microscope (ATEM) [10]. Generally, Nuclepore filters are used for PIXE analysis [7,11] and cellulose nitrate filters for TXRF [12]. For other analyses, Whatman [5,13] and Teflon [4] filters are used. The inductively coupled plasma emission spectroscopy (ICP) [14] and atomic
* Corresponding author. Tel./fax: +36 88423023. E-mail addresses: [email protected], [email protected] (V. Szila´gyi). 0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.09.002
absorption spectroscopy (AAS) techniques [13,15] can also be used for determining the elemental composition of airborne particulates but these methods involve a sample preparation step. The greatest advantage of the direct XRFS determination of aerosol samples is that the sample preparation procedures can be avoided in the case of trace amounts of sample. Therefore, errors arising from the sample preparation procedures are eliminated. In addition, depending on the concentration, a wide range of elements (O–U) can be determined with these methods. Nowadays, the XRFS technique, as an analytical tool, is capable of measuring samples even of small quantity and thickness. Due to the development of spectrometers, the distance between the X-ray tube window and the sample target has decreased, resulting in increased sensitivity. In addition, the available energy has also increased. It is well known that in the X-ray fluorescence technique differences in the thickness of the sample and the sampling conditions severely detract from the accuracy of the analysis. Samples can be classified as: (a) samples of infinite thickness for X-ray penetration; (b) samples of finite
38
V. Szila´gyi, Z. Hartya´ni / Microchemical Journal 79 (2005) 37–41
thickness below the critical value; (c) samples of infinite thickness placed over a sample of finite thickness [16]. A simple thin-layer sample preparation method was described for XRFS analysis [17]. A portion of vehicle exhaust particulate or atmospheric dust certificate reference material was suspended in a small volume of a fluid on a hydrophilized polytetrafluoroethylene (PTFE) membrane filter, dispersed by air blown from a pump and dried by IR irradiation. A knowledge of the X-ray penetration depth of the measured element, which depends on the density of the matrix [18], is also very important. The main objectives of the research were the comparison of different aerosol supports (Whatman filter, Teflon filter and ProleneR foil) and the development of a quantitative method for the analysis of soil-derived elements in aerosol samples.
2. Materials and methods 2.1. Instrument, measurement parameters A PHILIPS PW 2404 X-ray spectrometer equipped with a 4-kW Rh anode tube was used in the present work. Duplex, scintillation and proportional counters were used as detectors, while the analysing crystals were LiF (200), PE 002-C and PX1. The target elements were Na, Al, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr and Pb. In general, the Ka lines of the elements were measured except for Pb, when the La line was chosen for quantification. In all cases, the intensity of the background was determined. Unfortunately, the determination of Zn was not possible using a Re plate, because the Zn Ka line interfered with the Re Lh line. Similarly, the determination of Cu was not possible with Ta plates. In the case of ProleneR foil, which was developed by Philips, the sample holder set consisted of a C-shaped sample holder and a steel cup. The sample supports had a 32-mm outside diameter and a thickness of 0.4 mm.
Sample supports are designed to hold up to 500 Al of liquid, which is subsequently dried in the central dimpled area. The patented dimpling process creates a 2-mm dimple inside a 5-mm outer dimple. These dimples allow gravitational forces to pull the liquid and residue to the lowest point in the support (the central dimple) while drying. For the Whatman and Teflon filters, a liquid sample holder with Mylar film was placed in a steel cup with an inner diameter of 27 mm. Since the filters are so thin that the X-rays can penetrate through the filter material, the radiation is able to excite the elements in the sample holder and in various parts of the instrument. The characteristic wavelengths of these elements adulterate the results, being measured together with the elements in the sample. This stray radiation can be eliminated by covering the sample with an binfinite thicknessQ material, for example a Ag plate [19], or by painting the platform and the sample holder with high purity silver paint [5]. In the present case, either a tantalum or a rhenium plate was chosen to eliminate the back-scattered radiation. Ta and Re are heavy elements, so plates of these metals are capable of absorbing the primary radiation passing through the aerosol filters. In addition, the characteristic lines of Ta and Re are very close to the absorption edges of some of the elements analysed (Fig. 1). The consequences are that these lines excite elements having K absorption edges directly on the long wavelength side of the Re and Ta lines (namely Fe, Co, Ni, Cu and Zn). This means that Ta and Re are not only absorbers for backscattered background radiation but also secondary targets. Applying Ta or Re plates results in the increased sensitivity of Ka lines for Fe, Co, Ni, Cu and Zn and a lower detection limit for these elements.
3. Results Three different filters (Whatman, Teflon and ProleneR) were tested as blank supports for the samples. Weak Ca,
Fig. 1. Absorption edges of elements and characteristic lines of Ta, Re and Rh.
V. Szila´gyi, Z. Hartya´ni / Microchemical Journal 79 (2005) 37–41
Fe and Ni signals were identified, all of which originated from the instrument. The highest signals for these elements, and also for K and Zn, were obtained using Whatman filters. These elements are contaminants of the Whatman filter. Two problems were encountered when testing these supports. The Whatman filter is made of quartz fibres and has a thickness of only about 300 Am. Since the aerosol particles become embedded in the filter material, it is necessary to calculate the X-ray penetration depth for the Whatman filter. Calculations indicate that the penetration depths of the measured elements are b100 Am (Fig. 2). Information is obtained for a depth of 100 Am or less, depending on the atomic numbers of the elements. Therefore, as the concentration of the solution on the filters increases, the signals will not increase linearly. The standard solution dripped onto the filter soaked into the filter material. The secondary X-rays from elements at a depth of more than 100 Am were not detected, because they were absorbed by the filter material. The penetration depth of the X-ray beam (x) was calculated as follows: x¼
2:69 l¯ q
ð1Þ
As shown in Eq. (1), x depends on the matrix, the density of the sample (q) (g cm3) and a constant characteristic of the instrument (2.69). The matrix is characterised by the average mass absorption coefficient (l¯ ), presented in Eq. (2). X l¯ ¼ Wj l j ð2Þ where W j and l j are the mass percentage and mass absorption coefficient of the element. l j depends on the element as well as on the wavelength (units: cm2 g1).The second problem encountered was the heat sensitivity of the Teflon filter, which became brittle and cracked after irradiation. Consequently, the X-ray beam was capable of destroying the Teflon filter depending on the intensity of
39
primary excitation and the measurement time. For this reason, the total measuring time had to be decreased. This was achieved by measuring fewer elements at a time, rather than reducing the measurement time for each element. The use of a high performance X-ray tube (4 kW) is advantageous for the excitation of the elements, but destroys the Teflon filter. 3.1. Calibration The calibration procedures can be evaluated in several manners. For example, a solution prepared from the soluble salts of the target metal can be dripped onto the filter materials [1,6,20,21]. Another possibility is to use an aerosol generator [6,21] for the preparation of standard filter materials. The preparation of glass disc bequivalent filtersQ is a further possible calibration method [19]. In the present project, the calibration procedure used for all three (Whatman, Teflon and ProleneR) filters involved a series of home-made standards based on a certified standard solution. Seven gradually diluted model solutions were prepared, with a concentration range of 100–700 Ag/l for each element. This concentration range was chosen after preliminary measurements on the intensity of real samples and standard samples. The solutions were composed of diluted ICP multi-element standard (CertiPURR 11355) dripped onto the supports and dried in air before using them as calibration standards. The measurements were performed in a vacuum. The analytical curves were estimated with the least squares method. The R values of the analytical curves for each element are presented in Table 1, while some typical analytical curves can be seen in Fig. 3. The repeatability of the measurements was evaluated by measuring each sample three times and calculating RSD% values. The RSD% values were lower than 10% in the case of ProleneR (except for Al, Fe) and Teflon (except for Al, Pb) and b5% for Whatman filters (except for Cr, Fe and Ni). For all three supports, the difference between the Ta and Re plates was not significant. The repeatability of the sample
Fig. 2. X-ray penetration depths of elements for Whatman filter.
40
V. Szila´gyi, Z. Hartya´ni / Microchemical Journal 79 (2005) 37–41
Table 1 R values of analytical curves Elements Na Al K Ca Cr Mn Fe Co Ni Cu Zn Sr Pb
ProleneR
filters and lower than 10% except for Cr, Fe and Ni on Whatman filters. Teflon
Whatman
Ta plate
Re plate
Ta plate
Re plate
Ta plate
Re plate
0.401 0.930 0.943 0.917 0.978 0.987 0.958 0.951 0.950 – 0.690 0.686 0.877
0.775 0.928 0.943 0.877 0.990 0.963 0.950 0.935 0.971 0.993 – 0.967 0.983
0.172 0.270 0.902 0.823 0.986 0.858 0.954 0.987 0.888 – 0.978 0.757 0.917
0.489 0.837 0.771 0.055 0.982 0.979 0.941 0.974 0.977 0.967 – 0.992 0.870
0.173 0.237 0.848 0.711 0.987 0.177 0.937 0.521 0.804 – 0.874 0.080 0.054
0.149 0.736 0.963 0.939 0.167 0.572 0.346 0.589 0.666 – 0.735 0.807 –
preparation was also investigated using three simultaneously prepared samples for each support and calculating RSD% values after the measurements. The RSD% values were lower than about 15% except for Fe and Pb on ProleneR foil, lower than 15% except for Na, Al, Cr and Pb on Teflon
3.2. Detection limit The lower limit of detection (LLD) was calculated using the SuperQ software, which takes into account the measuring time for each element (t), the background intensity (Rb) and the slope of the analytical curve (m). The following equation is applied: rffiffiffiffiffiffi 3 Rb LLD ¼ ð3Þ m t The LLD values obtained for the elements on the three different sample supports were compared (Fig. 4). The LLD values ranged from 0.01 to 0.13 mg/kg on ProleneR foil and from 0.03 to 0.18 mg/kg on Teflon filters except for Al. For both supports, the difference between the LLD values obtained using Ta and Re plates was not significant. The LLD values calculated for Whatman filters depended on the elements to be analysed and varied from 0.03 to 0.20 mg/kg.
Fig. 3. Analytical curves of Al, Cr and Ni.
V. Szila´gyi, Z. Hartya´ni / Microchemical Journal 79 (2005) 37–41
41
Acknowledgements The authors are indebted to Judit Szauer and La´szlo´ Mere´ nyi for their professional help in analysing the ´ gnes Molna´r atmospheric aerosol and soil samples, and to A and Andra´s Gelencse´r for their useful advice. This work was supported by the grants from the Higher Educational Scientific Research and Development (FKFP 0606/1997) and the Hungarian Scientific Research Fund (OTKA T026423 and T043574). Fig. 4. Lower limit of detection of measured elements using ProleneR foil.
For a few elements (Na, Al, Mn, Zn and Sr), however, the LLD values were not calculated, as the analytical curves were not acceptable due to line interferences.
4. Discussion Three different supports were measured as blank samples. Weak Ca, Fe, and Ni signals originating from the instrument were obtained for all of them. In the case of Whatman filters, higher blank signals were obtained for these elements and also for K and Zn. These elements are contaminants of the Whatman filter and must be taken into account in the calibration procedure. Comparative tests were performed on three different filters as possible substrates for atmospheric aerosol sampling. It could be seen during sample preparation that the standard solutions dried on the surface of ProleneR foil and Teflon filters, but in the case of Whatman filters the solution soaked into the filter material. Because the X-ray penetration depth of the measured elements is smaller than the thickness of the Whatman filter, this filter cannot be used for this sample preparation procedure. ProleneR foil and Teflon filters are equally applicable for XRF analysis, having appropriate precision and accuracy, though the Teflon filters may be damaged during the measurement, so they cannot be used for repeated measurements. It was thus concluded that the best results could be achieved using ProleneR foil or Teflon filter as sample supports. The technique developed for aerosol analysis using the XRFS method is novel because, to the best of our knowledge, Ta and Re plates have not been used before to eliminate the back-scattered radiation originating from various parts of the spectrometer. The emitted radiation of Ta or Re also causes the excitation of elements as a secondary target. The energies of the Ta and Re lines are slightly higher than the K absorption edges of some of the measured elements (Fe, Co, Ni, Cu, Zn).
References [1] U. W7tjen, H. Cave´, Nuclear Instruments & Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 109/110 (1996) 395. [2] J. Miranda, E. Andrade, A. Lo´pez-Sua´rez, R. Ledesma, T.A. Cahill, P.H. Wakabayashi, Atmospheric Environment 30 (20) (1996) 3471. [3] F. Aldape, J. Flores, M.R.V. Dı´az, B. Herna´ndez-Me´ndez, J.M.Z. Montoya, E.E. Blanco, A.F. Fuentes, L.M. Torres-Martı´nez, Nuclear Instruments & Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 150 (1999) 439. [4] K. Moloi, A. Viksna, E. Selin Lindgren, P. Standzenieks, X-ray Spectrometry 31 (2002) 27. [5] L. Samek, J. Injuk, P. Van Espen, R. Van Grieken, X-ray Spectrometry 21 (2002) 84. [6] O. Haupt, K. Linnow, R. Harmel, C. Schaefer, W. Dannecker, X-ray Spectrometry 26 (1997) 79. [7] I. Salma, W. Maenhaut, E´. Zemple´n-Papp, G. Za´ray, Atmospheric Environment 35 (2001) 4367. [8] B.L. Davis, Guo Jixiang, Atmospheric Environment 34 (2000) 2703. [9] V. Esteve, J. Rius, L.E. Ochando, J.M. Amigo´, Atmospheric Environment 31 (23) (1997) 3963. [10] A. Chabas, R.A. Lefe`vre, Atmospheric Environment 34 (1999) 225. [11] W. Maenhaut, F. Francois, J. Cafmeyer, O. Okunade, Nuclear Instruments & Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 109/110 (1996) 476. [12] M. Schmeling, R. Klockenk7mper, D. Klockow, Spectrochimica Acta, Part B: Atomic Spectroscopy 52 (1997) 985. [13] N. Kubilay, S. Yemenicioglu, A.C. Saydam, Marine Pollution Bulletin 30 (7) (1995) 475. [14] N.A.H. Janssen, D.F. Van Manson, K. Van Der Jagt, H. Harssema, G. Hoek, Atmospheric Environment 31 (8) (1997) 1185. [15] C. Bilos, J.C. Colombo, C.N. Skorupka, M.J. Rodriguez Presa, Environmental Pollution 111 (2001) 149. [16] P. Frigieri, R. Trucco, X-ray Spectrometry 3 (1) (1974) 40. [17] T. Kyotani, M. Iwatsuki, Analyst 123 (1998) 1813. [18] P. Frigieri, R. Trucco, E. Caretta, X-ray Spectrometry 4 (1) (1975) 28. [19] P. Quisefit, P. De Chateaubourg, S. Garivait, E. Steiner, X-ray Spectrometry 21 (2) (1994) 59. [20] O.V. Shuvayeva, P.K. Koutzenogii, V.B. Baryshev, V.I. Rezchihov, A.I. Smirnova, L.D. Ivanova, F.V. Sukchorukov, Nuclear Instruments & Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 405 (1998) 553. [21] O. Haupt, B. Klaue, C. Schaefer, W. Dannecker, X-ray Spectrometry 24 (5) (1995) 267.