Determination of hexavalent chromium in plastic certified reference materials by X-ray absorption fine structure analysis

Spectrochimica Acta Part B 93 (2014) 14–19

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Technical Note

Determination of hexavalent chromium in plastic certified reference materials by X-ray absorption fine structure analysis Masaki Ohata a,⁎, Nobuyuki Matsubayashi b a b

Inorganic Standard Section, Inorganic Analytical Chemistry Division, National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), Japan Super-Spectroscopy System Research Group, Research Institute of Instrumentation Frontier (RIIF), National Institute of Advanced Industrial Science and Technology (AIST), Japan

a r t i c l e

i n f o

Article history: Received 1 April 2013 Accepted 9 December 2013 Available online 3 January 2014 Keywords: Hexavalent Chromium (Cr(VI)) Cr-K edge XANES spectrum XAFS analysis with transmission mode RoHS directive Plastic disk CRM

a b s t r a c t X-ray absorption fine structure (XAFS) analysis with transmission mode was used to determine the percentages of hexavalent chromium {Cr(VI)} in total Cr in plastic certified reference materials (CRMs). Cr-K edge X-ray absorption near-edge structure (XANES) spectra were observed and the normalized pre-edge peaks of the spectrum where absorption data was summed was acquired for the determination of Cr(VI). Examination of different number of data point and range of photon energy for summed absorption of the pre-edge peak resulted in reproducible absorption data, though the measurements were carried out at different beam time and beam line. The concentrations of Cr(VI) in the plastic CRMs were also estimated from both the certified value of total Cr and the determined percentage of Cr(VI). The analytical procedure and the estimated concentrations can be useful for the determination of Cr(VI) in plastics with respect to RoHS (restriction of the use of hazardous substances in electrical and electronics equipment) directive. © 2014 Elsevier B.V. All rights reserved.

1. Introduction EU (European Union) legislated the RoHS (restriction of the use of hazardous substances in electrical and electronics equipment) directive since July, 2006 [1,2]. The directive restricted the concentration of such hazardous substances as Cd, Cr(VI), Hg, Pb, PBB (poly-brominated biphenyl) and PBDE (poly-brominated diphenyl ether) in electrical and electronics equipments produced in EU and transported from other areas. As well known, plastic resins are used widely in electrical and electronics equipment. It is also well recognized that hazardous substances mentioned above have been added into plastics purposely for the sake of noncombustibility and the coloring. In addition, new plastics without hazardous substances are concerned to be adulterated with used plastics with hazardous substances during recycling process. From these points of view, standard analytical procedures and certified reference materials (CRMs) of hazardous substances in plastics are still of great interest and are demanded to check the conformity to the RoHS directive [3–18]. Though several plastic CRMs with respect to the RoHS directive have been developed so far for both chemical analysis [3,4,6,11,14,18] and direct analysis [8,10,12,13,15–17], no CRMs were found to be certified the concentration of Cr(VI) in plastics even though the RoHS directive regulated. Several analytical methods were proposed to determine Cr(VI) in solid samples such as a high performance liquid chromatography (HPLC) couple with inductively coupled plasma mass

spectrometry (ICPMS) and UV–visible spectrophotometry using extraction procedures [19,20]. However, the extraction procedures of Cr(VI) from solid samples implies concern about the extraction efficiency as well as changing chemical form of Cr(VI) to e.g. Cr(III). X-ray absorption fine structure (XAFS) analysis should be the one to achieve the determination of Cr(VI) in solid samples directly without any concerns during extraction process [21–27]. In the present study, we determined the percentage of Cr(VI) in total Cr in plastic disk CRMs by XAFS analysis with transmission mode. The Cr–K edge X-ray absorption near-edge structure (XANES) spectra were observed and the normalized pre-edge peak of Cr–K edge XANES spectra where absorption data were summed were acquired to determine the percentage of Cr(VI) because the pre-edge peak was concluded to be suitable for quantitative analysis [21,23]. Different number of data point and range of photon energy for summed absorption of the pre-edge peak were examined to achieve accurate determination of the percentage of Cr(VI) in plastic disk CRMs and the variation of summed absorptions were also evaluated at different beam time and beam line. Moreover, the concentrations of Cr(VI) in the plastic disk CRMs were estimated from both the certified value of total Cr and the determined percentage of Cr(VI). 2. Experimental 2.1. Instrumentation

⁎ Corresponding author at: Tsukuba Central 3-9, 1-1-1, Umezono, Tsukuba, Ibaraki 3058563, Japan. E-mail address: [email protected] (M. Ohata). 0584-8547/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sab.2013.12.005

The Cr–K edge XANES spectra were observed by transmission mode with ionization chambers using a Si(111) double crystal

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Table 1 Energy range, step, measurement time, and number of data for measured XAFS spectra. Block

Initial energy (eV)

Final energy (eV)

Step (eV/step)

Measurement time (s)

Number of data

1 2 3

5659.30 5959.30 6059.30

5959.30 6059.30 6200.30

6.00 0.40 3.00

2.0 2.0 2.0

50 250 47

Total measurement time was 694 s for each spectrum.

Table 2 Certified values of total Cr and their expanded uncertainties in plastic CRMs.

NMIJ CRM 8106-a NMIJ CRM 8116-a(02) NMIJ CRM 8136-a

Expanded uncertaintya (mg kg−1)

Relative expanded uncertainty (%)

268.1 912 890.6

2.1 46 10.7

0.8 5.0 1.2

Each expanded uncertainty was determined with the coverage factor k = 2; it defines an interval estimated to have a level of confidence of approximately 95%.

2.2. Samples and reagents The plastic disk CRMs for heavy metal analysis with respect to RoHS directive such as NMIJ CRM 8106-a, 8116-a(02) and 8136-a as listed in Table 2 which were developed by National Metrology Institute of Japan (NMIJ) were used in the present study [14,16]. The plastic resins for NMIJ CRM 8106-a as well as 8116-a(02) and 8136-a are acrylonitrilebutadiene-styrene (ABS) and polypropylene (PP), respectively. The diameter and the thickness of these disks are 30 mm and 2 mm, respectively. The concentrations of total Cr in these plastic disk CRMs are certified as listed in Table 2 [14,16,29]. The percentages of Cr(VI) in total Cr was expected to be 100% in NMIJ CRM 8106-a because PbCrO4 was only added. On the other hand, expected percentages of Cr(VI) were about 25% for NMIJ CRM 8116-a(02) and 8136-a because both

PbCrO4 and Cr-acetylacetonate as Cr(VI) and Cr(III), respectively, were added with the Cr mass ratio of 1: 4. The blank plastic disks which did not contain any heavy metals were also used to correct XANES spectra for each disk CRM. In order to determine the percentage of Cr(VI) in the plastic disk CRMs, pellet form of calibration standards which consisted of both PbCrO4 and Cr-acetylacetonate (Wako pure chemical industries Inc., Osaka, Japan) as Cr(VI) and Cr(III), respectively, were prepared by mixing with methyl cellulose (Wako pure chemical industries Inc., Osaka, Japan) as a binder, because PbCrO4 and Cracetylacetonate were contained in the plastic disk CRMs as mentioned above. The percentages of Cr(VI) in plastic disk CRMs were determined by a calibration curve obtained from the calibration standards, which have different percentage of Cr(VI) in total Cr such as 0, 5, 10, 15, 20, 25, 30, 50, 75, 90, 95 and 100% prepared by pressing after mixing with three reagents by mass ratio. The addition amount of total Cr in each

1.5

Normalized absorption

monochromator of the XAFS beam lines of BL-7C, 9A, 9C, and 12C at KEK-PF (Tsukuba, Japan) [28]. The electron storage ring was operated with the energy of 2.5 GeV and a current of 300–450 mA. The photon energy of the Cr–K edge XANES spectrum between 5959.30 eV and 6200.30 eV was measured. A copper foil (6 μm thickness) was used as a standard to calibrate photon energy for the Si(111) double crystal monochromator. The hump at the absorption edge of copper (8980.30 eV) was regarded as 12.7185°. The non-focused monochromated beam was collimated to 1 mm × 6 mm with respect to the sample surface by four slits. The measurement time for each energy range was listed in Table 1 and total measurement time was 694 s for one Cr–K edge XANES spectrum.

a

1.0 100% 90% 70% 50% 25% 0%

0.5

0.0 5980

5990

6000

6010

6020

6030

6040

6050

Photon energy (eV) Background subtracted absorption obtained at the photon energy from 6098.3 to 6200.3 eV was averaged and normalized to be 1.

Normalized absorption

1.5

1.0

0.5

0.0 5950

6000

6050

6100

6150

Photon energy (eV) Background subtracted absorption obtained at the photon energy of 5959.3 eV was set to be 0.

Fig. 1. Cr–K edge XANES spectra obtained for NMIJ CRM 8106-a.

6200

1.5

Normalized absorption

a

Certified value (mg kg−1)

100% 90% 70% 50% 25% 0%

b

1.0

0.5

0.0 5985

5990

5995

6000

Photon energy (eV) Fig. 2. (a) and (b) Cr–K edge XANES spectra obtained for calibration standards of 0, 25, 50, 70, 90 and 100% Cr(VI) in total Cr prepared by mixing with PbCrO4, Cr-acetylacetonate and methyl cellulose observed for photon energy ranged (a) from 5980 eV to 6050 eV and (b) from 5985 eV to 6000 eV.

16

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1.000

a Correlation coefficient of calibration curve (r2)

Normalized absorption

1.5

1.0 NMIJ CRM 8106-a NMIJ CRM 8136-a

0.5

100 % 25 %

0%

0.0 5980

5990

6000

6010

6020

6030

6040

Percentage of Cr(VI) in total Cr in NMIJ CRM 8106-a

100 % 25 %

0%

0.5

5995

6000

6005

Photon energy (eV) Fig. 3. (a) and (b) Cr–K edge XANES spectra obtained for NMIJ CRM 8106-a and 8136-a as well as the calibration standards of 0, 25 and 100% Cr(VI) observed for the photon energy ranged (a) from 5980 to 6050 eV as well as (b) from 5985 to 6005 eV.

pellet was determined by estimation of the edge jump of Δμat which could be calculated from Victoreen's Eq. (1). 3

μ a ðλÞ ¼ Cλ −Dλ

4

ð1Þ

The factors of μa, λ and C as well as D are absorption coefficient (cm2 g−1), X-ray wavelength (Ǻ) and Victoreen coefficients, respectively. The factor of t (cm) is also defined as sample thickness. The Δμat which was calculated from the differences between the pre Cr–K absorption edge (μ1t) and the post one (μ2t) was also estimated. The values of 1–1.5 and less than 4 for Δμat (cm3 g−1) and μ2t (cm3 g−1), respectively, were adopted in this study.

Percentage of Cr(VI) in total Cr in NMIJ CRM 8136-a

Normalized absorption

b

NMIJ CRM 8136-a

5990

0.998

0.997

0.996

NMIJ CRM 8106-a

0.0 5985

0.999

6050

Photon energy (eV) 1.0

a

102

b

101 100 (0.6 %)

99

(0.5 %) (0.5 %) (0.6 %) (0.7 %)

98

(1.0 %) (1.6 %)

(0.7 %)

97 28

c

27 26 25 (2.6 %) (3.1 %)

24

(2.0 %) (2.1 %) (2.4 %) (4.0 %) (6.4 %)

(3.0 %)

23 0

5

10

15

20

25

30

Integrated data point Fig. 5. (a)–(c) Obtained correlation coefficients (r2) of the calibration curves (a), percentages of Cr(VI) in total Cr for NMIJ CRM 8106-a (b) and for NMIJ CRM 8136-a (c) as a function of number of data point summed listed in Table 3. The half of the bar and the value in parenthesis indicate combined standard uncertainty and relative combined standard uncertainty, respectively.

2.3. Normalization procedure for XANES spectrum In order to subtract the background absorption spectrum from each XANES one, spectrum fitting was carried out by the Victoreen's equation

with constant (A) indicated in Eq. (2) using absorption spectrum in the pre-edge region of 200–300 eV in lower energy of Cr–K absorption edge. 3

4

μ b ðλÞ ¼ Cλ –Dλ þ A

The example of normalized XANES spectrum after the background subtraction is shown in Fig. 1. The background subtracted spectrum was normalized by the averaged absorption value obtained at the

Summed absorption of pre-edge peak

6

y = 0.0496x + 0.5169 R² = 0.9994

5

ð2Þ

4 Table 3 Number of data point and range of photon energy for summed absorption of pre-edge peak.

3 2

Number of data point

Range of photon energy (eV)

1 3 5 8 11 15 19 25

5991.72 5991.32–5992.11 5990.91–5992.51 5990.51–5993.31 5990.12–5994.11 5989.31–5994.91 5988.51–5995.72 5986.91–5996.52

1 0 0

20

40

60

80

100

Percentage of Cr(VI) in total Cr (%) Fig. 4. Calibration curve of the percentage of Cr(VI) in total Cr obtained by the calibration standards. The absorption of pre-edge peak obtained at photon energies between 5990.12 eV and 5994.11 eV were summed.

Summed absorption of pre-edge peak

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5.7

5.6

5.5

5.4 100 % PbCrO4 4 (pellet 1) 100 % PbCrO4 4 (pellet 2) 100 % PbCrO4 4 (pellet 3) NMIJ CRM 8106-a (disk 1) NMIJ CRM 8106-a (disk2) NMIJ CRM 8106-a (8 different disks)

5.3

5.2 2008/9/9

2009/1/7

2009/5/7

2009/9/4

2010/1/2

2010/5/2

Measurement date Fig. 6. Plots of summed absorption of pre-edge peak obtained for NMIJ CRM 8106-a and calibration standard of 100% Cr(VI).

photon energy from 6098.3 eV to 6200.3 eV. The normalized XANES spectrum at 5959.3 eV was also set to be 0 as shown in Fig. 1. In case of plastic disk CRMs, the XANES spectrum of blank plastic disk was subtracted from the XANES spectra of plastic disk CRMs before the normalization was carried out. 3. Results and discussion Fig. 2(a) and (b) show Cr–K edge XANES spectra normalized obtained for the calibration standards which have 0, 25, 50, 70, 90 and 100% of Cr(VI). The normalized absorption of the pre-edge peaks observed around 5992 eV of photon energy showed the proportional relation between percentages of Cr(VI) and corresponding normalized absorptions, which were similar to the results indicated in Ref. [21]. Fig. 3(a) and (b) show normalized Cr–K edge XANES spectra obtained for 0, 25 and 100% Cr(VI) of the calibration standards as well as the plastic CRMs. It could be seen from Fig. 3, the pre-edge peak region of the Cr– K edge XANES spectra between NMIJ CRM 8106-a and 100% Cr(VI) calibration standard as well as between NMIJ CRM 8136-a and 25% Cr (VI) one showed close to each other, respectively; therefore, the percentages of Cr(VI) in plastic CRMs could be expected to be ca. 100% or ca. 25% which were close to the preparation values. However, the XANES spectra between the plastic disk CRMs and the calibration standards differed slightly at the photon energy more than 6000 eV even though the same chemical form of Cr was contained. Based on the results, summed absorption of normalized pre-edge peak was acquired

Summed absorption of pre-edge peak

1.85

1.80

for the determination of the percentage of Cr(VI) in plastic disk CRMs, in order to avoid any uncertainties from the peak fitting calculation. Fig. 4 shows a calibration curve of the percentage of Cr(VI). The normalized absorption of pre-edge peak at photon energies between 5990.12 eV and 5994.11 eV were summed in the present study. Although these data were obtained at different beam time and beam line, linear calibration curve was obtained. Fig. 5(a)–(c) show the obtained correlation coefficients of calibration curves (r2) and determined percentages of Cr(VI) in NMIJ CRM 8106-a and 8136-a. A number of data point summed as well as a range of photon energy is listed in Table 3. As shown in Fig. 5, the linearity of the calibration curve and the results of the percentages of Cr(VI) as well as their standard uncertainties, which were estimated in accordance with GUM (Guide to the expression of Uncertainty in Measurement) [30], were dependent on the number of data point summed. The standard uncertainties could be mainly calculated by linearity of the calibration curve as well as signal to background noise ratio (S/N) which was affected by summed absorption. When the number of data point summed increased, the estimated background noise became larger due to the increase of the background. As results, the standard uncertainties estimated with respect to the results obtained by the number of data point summed more than 15 was larger even though the correlation coefficients of the calibration curves was still higher than those of data point summed less than 5. According to the results obtained in Fig. 5, number of data point summed was adopted to be 11; then, the calibration curve shown in Fig. 4 was obtained. Fig. 6 shows plots of summed pre-edge peak absorption for NMIJ

25 % PbCrO44 NMIJ CRM 8116-a(02) (disk 1) NMIJ CRM 8116-a(02) (disk 2) NMIJ CRM 8116-a(02) (8 different disks) NMIJ CRM 8136-a (disk 1) NMIJ CRM 8136-a (8 different disks)

1.75

1.70

1.65 2008/9/9

2008/12/18

2009/3/28

2009/7/6

2009/10/14

2010/1/22

2010/5/2

Measurement date Fig. 7. Plots of summed absorption of pre-edge peak obtained for NMIJ CRM 8116-a(02), 8136-a and calibration standard of 25% Cr(VI).

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Table 4 Analytical results of the percentage of Cr(VI) in total Cr in plastic CRMs.

NMIJ CRM 8106-a NMIJ CRM 8116-a(02) NMIJ CRM 8136-a

Percentage of Cr(VI) in total Cr (%)

Expanded uncertaintya (%)

Relative expanded uncertainty (%)

100.5 24.3 25.4

3.9 1.8 1.8

3.9 7.5 7.0

a Standard uncertainty (k = 1) was estimated from standard uncertainty of calibration curve as well as standard deviation from replicated measurements of samples which included sample homogeneity. Each expanded uncertainty was determined with the coverage factor k = 2; it defines an interval estimated to have a level of confidence of approximately 95%.

Table 5 Estimated concentration of Cr(VI) and Cr(III) in plastic CRMs.

NMIJ CRM 8106-a NMIJ CRM 8116-a(02) NMIJ CRM 8136-a

Cr(VI) Cr(VI) Cr(III) Cr(VI) Cr(III)

Estimated concentration (mg kg−1)

Expanded uncertaintya (mg kg−1)

Relative expanded uncertainty (%)

268 222 690 226 664

11 20 71 16 48

4.0 9.0 10.3 7.2 7.3

a Standard uncertainty (k = 1) was estimated from standard uncertainty of calibration curve as well as standard deviation from replicated measurements of samples which included sample homogeneity. Each expanded uncertainty was determined with the coverage factor k = 2; it defines an interval estimated to have a level of confidence of approximately 95%.

CRM 8106-a and the calibration standard of 100% Cr(VI) as a function of different measurement date. It could be seen from Fig. 6 that difference between the lowest summed absorption and the highest one was about 6%, even though the measurements were carried out at different beam time and beam line, for different specimens of plastic disk CRM as well as different pellets for same percentage of Cr(VI); if the normalized absorption showed about 1 as shown in Fig. 3. Fig. 7 shows plots of summed pre-edge peak absorption for NMIJ CRM 8116-a(02), 8136-a and the calibration standard of 25% Cr(VI). It could be seen from Fig. 7 that discrepancy between the lowest and the highest summed absorption was about 9%, if the normalized absorption showed about 0.3 as shown in Fig. 3. These variations include the uncertainties estimated from the repeatability of measurements and sample homogeneity. Table 4 shows the analytical results of the percentages of Cr(VI) in NMIJ CRM 8106-a, 8116-a(02) and 8136-a with their expanded uncertainties. The main sources of the uncertainties estimated were the uncertainties from calibration curve and homogeneity of samples which included repeatability of measurements. In case of the percentage of Cr(VI) in NMIJ CRM 8106-a, the contributions on the estimated uncertainties from the calibration curve and the homogeneity were ca. 30% and ca. 70%, respectively. On the other hand, ca. 50% each contribution was estimated for both NMIJ CRM 8116-a(02) and 8136-a. This is, because the X-ray radiation from KEK-PF (2.5 GeV, 300–450 mA) was strong enough and total Cr concentrations of these plastic disk CRMs were more than 250 mg kg−1 as listed in Table 3, stable and reproducible XANES spectra could be obtained by transmission mode which could achieve less than 8% of expanded uncertainties. Table 5 shows the estimated concentrations of Cr(VI) and Cr(III) in the plastic disk CRMs. The expanded uncertainties were estimated by the combined uncertainties for both analytical results (Table 4) and certified values (Table 2). Similar analysis of Cr(VI) in plastics was only reported by Miyauchi et.al. by a laboratory XAFS spectrometer [26]. They acquired the pre-edge peaks of Cr–K edge XANES spectra for plastics and calibration standards by fluorescence mode and transmission one, respectively, because the sensitivity of their laboratory XAFS spectrometer was not high enough for the determination of the percentage of Cr(VI) in plastics. It is considered that the same measurement mode (transmission mode or fluorescence one) for both calibration standards and samples is ideal for accurate quantitative analysis. From this point of view, the analytical results extended in the present study is considered to be accurate, and it can be used for validation of Cr(VI) analysis in plastics with respect to RoHS directive as well as calibration of analytical instruments such as laboratory XAFS spectrometer, because the concentration of Cr (VI) is accurate and high enough for the spectrometer.

4. Conclusions The determinations of the percentage of Cr(VI) in the plastic disk CRMs were carried out by XAFS analysis with transmission mode at KEK-PF whose X-ray irradiation was strong enough (2.5 GeV, 300–450 mA). The summed absorption of the pre-edge peak of Cr–K edge XANES spectra normalized showed good reproducibility and the linear calibration curve was obtained from summed absorptions of the pre-edge peaks even though measurements were conducted at different beam time and beam line. The concentrations of Cr(VI) and Cr(III) were also estimated from both the analytical results and the certified values of total Cr. The analytical procedures using XAFS with transmission mode as well as the analytical results extended in the present study can be useful for the determination of Cr(VI) in solid samples such as materials regulated by RoHs directive. References [1] Directive 2002/95/EC of the European parliament and of the council of 27 January 2003, on the restriction of the use of certain hazardous substances in electrical and electronic equipment, Off. J. Eur. Union 13 (2003) 2. [2] Directive 2011/65/EC of the European parliament and of the council of 8 June 2011, on the restriction of the use of certain hazardous substances in electrical and electronic equipment (recast), Off. J. Eur. Union 1 (2011) 7. [3] W.V. Borm, A. Lambery, P. Quevauviller, Collaborative study to improve the quality control of trace element determinations in polymers. Part 1. Interlaboratory study, Fresenius J. Anal. Chem. 365 (1999) 361–363. [4] J. Vogl, D. Liesegang, M. Ostermann, J. Diemer, M. Berglund, C.R. Quetel, P.D.P. Taylor, K.G. Heumann, Producing SI-tracable reference values for Cd, Cr and Pb amount contents in polyethylene samples from the polymer elemental reference material (PERM) project using isotope dilution mass spectrometry, Accred. Qual. Assur. 5 (2000) 314–324. [5] P. Quevauviller, Certified reference materials for the quality control of inorganic analyses of manufactured products (glass, polymers, paint coatings), Trends Anal. Chem. 20 (2001) 446–456. [6] A. Lambery, W.V. Borm, P. Quevauviller, Collaborative study to improve the quality control of trace element determinations in polymers. Part 2. Certification of polyethylene reference materials (CRMs 680 and 681) for As, Br, Cd, Cl, Cr, Hg, Pb and S content, Fresenius J. Anal. Chem. 370 (2001) 811–818. [7] K. Eliola, P. Peramaki, Development of a modified medium pressure microwave vapor-phase digestion method for difficult to digest organic samples, Analyst 128 (2003) 194–197. [8] C. Mans, S. Hanning, C. Simons, A. Wegner, A. Janβen, M. Kreyenschmidt, Development of suitable plastic standards for X-ray fluorescence analysis, Spectrochim. Acta B 62 (2007) 116–122. [9] IEC 62321, International Standard: electrotechnical products\determination of levels of six regulated substances (lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls, polybrominated diphenyl ethers), Edition 1.0, 2008. [10] K. Nakano, T. Nakamura, I. Nakai, A. Kawase, M. Imai, M. Hasegawa, Y. Ishibashi, I. Inamoto, K. Sudou, M. Kozaki, A. Turuta, H. Honma, A. Ono, K. Kakita, M. Sakata, Plastic certified reference materials JSAC 0611 0615 for determination of hazardous constituents using X-ray fluorescent analysis, Bunseki Kagaku 55 (2006) 501–507.

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