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Safeguarding the environment - XRF analysis of heavy metals in polyethylene
Safeguarding the environment - XRF analysis of heavy metals in polyethylene The introduction of European Union directives on Waste Electrical and Electronic Equipment and Reduction of Hazardous Substances has highlighted the need for precise and repeatable elemental analyses of heavy metals in the plastics production process. Fortunately, XRF analysis of heavy metals in polyethylene has emerged as an effective and economical tool to do this, argues PANalytical's Joanna Wolksa. Greater awareness of the importance of recycling continues to grow among the general population. Recently, however, the focus has turned to the problems posed by waste electronic equipment, particularly computers. Improvements in technology experienced during the
past decade have had the effect of creating a growing volume of obsolete products. During 2002 for example, an estimated 55.4 million computers became redundant in the US alone. Not surprisingly, the disposal of such
Table 1: Calibration data based on 600 seconds live time measurements. Element
Calibration RMS [ppm]
Concentration range [ppm]
Cr
0.15
0-24.8
Ni
0.08
0-10.6
Cu
0.11
0-25.1
Zn
0.10
0-5.1
As
0.04
0-6.4
Br
0.84
0-163.3
Cd
0.08
0-28.4
Ba
2.7
0-600.7
Hg
0.15
0-5.3
Pb
0.17
0-22.3
Plastics Additives & Compounding January/February 2005
36
products is now a prime concern. Partly this is because of their volume, but more importantly because they contain significant amounts of heavy metals and other toxic substances. In an incinerator or landfill, these can be released into the environment to contaminate the air, ash and ground water. Issues such as these have driven the introduction of the European Union's recent broad directives on Waste Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substances (RoHS). By reducing the use of heavy metals, particularly cadmium, at source; and by recycling as much as possible, the directives aim to avoid problems associated with the disposal of hazardous materials. In the plastics industry, compliance with the directives requires manufacturers to carry out precise and repeatable elemental analyses of heavy metals down to sub-ppm levels at all stages of the production process. X-ray fluorescence (XRF) spectroscopy has emerged as the most
ISSN1464-391X/05 © 2005 Elsevier Ltd.All rights reserved.
Environment
(a)
(b)
Ba-Kα1
Ba-Lα1 Cu-Kα1
Ba-Kα2
2
2 Zn-Kα1 Ba-Lβ1
1
Ti-Kβ1
Na-Kα1 Ni-Kβ1 W-Lα1
Cr-Kα1
Cd-Kα2 Cd-Kα1
0
Cps/mAch
Cps/mAch
4
Cd-Kβ1
25
Cr-Kβ1 Zr-Kα1-EKα
0
30
5
Fe-Ka1
6
Energy (keV)
Fe-Kβ1
7
8
Energy (keV)
Figure 1: Spectrum (a) for Ba and Cd obtained using a Barkla secondary target and (b) Cr, Ni, Cu and Zn using a Ge secondary target. economical and effective analytical tool for achieving this. The routine use of XRF analysis in plastics manufacturing has been hampered by the lack of suitable measurement standards. However, a set of certified standards - known as TOXEL - is now available to facilitate XRF analyses in polyethylene. Developed jointly by DSM Resolve and PANalytical, each TOXEL set comprises five standards (one blank and four multielement standards) containing the regulated elements Cr, Cd, Hg, Pb, As, Ni, Cu, Zn, Ba and Br. Calibration with TOXEL standards is simplified by the fact that XRF is a multielement technique. Therefore a single set of the new standards can be used to calibrate several heavy elements, covering concentrations from trace level to several hundred ppm. As the following example illustrates, the availability of the new TOXEL standards means that plastics manufacturers now have the necessary resources to meet the challenges arising from the WEEE and RoHS directives. This case study is the analysis of heavy metals in polyethylene using an Epsilon 5 XRF spectrometer.
Analytical conditions A set of TOXEL standards was used to calibrate an Epsilon 5 XRF spectrometer. The measurement program was defined in the software using the Epsilon 5 Wizard. This guides the user through the steps required to calibrate the application.
Table 2: Results of the analysis of standard 3 as an unknown. Element
Certified concentration [ppm]
Measured concentration [ppm]
Cr
5.9
5.87
Ni
2.57
2.65
Cu
6.0
6.22
Zn
1.25
1.4
As
1.44
1.5
Br
38.5
40.13
Cd
6.6
6.88
Ba
145.2
150.45
Hg
1.29
1.2
Pb
5.2
5.29
Figure 2: Calibration graph for Cd. Once the elements have been selected, the software proposes the polarizing secondary targets, together with the voltage and current of the tube providing the best excitation for the
37
elements of interest. For this application the Wizard defined three measurement conditions, each involving 600 seconds of detector live time.
Plastics Additives & Compounding January/February 2005
Environment Table 3: Repeatability and reproducibility. Element
Cr
Ni
Cu
Zn
As
Br
Cd
Ba
Hg
Pb
Repeatability (20 consecutive measurements) Mean [ppm]
6.08
2.63
6.27
1.39
1.47
40.01
6.76
149.63
1.4
5.39
St.Dev [ppm]
0.16
0.04
0.05
0.04
0.06
0.11
0.3
0.66
0.21
0.12
Rel. St.Dev [ppm] 2.68
1.33
0.85
3
3.75
0.27
4.44
0.44
14.88
2.26
Reproducibility (measurements carried over 3 weeks) Mean [ppm]
5.98
2.63
6.21
1.31
1.43
39.84
6.88
149.33
1.59
5.43
St.Dev [ppm]
0.23
0.03
0.09
0.07
0.07
0.19
0.24
0.89
0.33
0.1
Rel. St. Dev [ppm] 3.82
1.16
1.51
5.33
4.63
0.48
3.53
0.6
20.69
1.89
Counting Statistical Error (CSE) CSE
0.12
0.18
0.36
0.13
0.16
0.74
0.17
0.67
0.17
0.17
CSE rel %
1.37
0.91
0.46
1.21
1.02
0.22
0.99
0.24
0.98
0.98
Figure 3: Short and long term stability measurements for Cd in polyethylene.
Performance
Accuracy
The Epsilon 5 software features a powerful deconvolution algorithm, which automatically analyzes the sample spectrum and determines the net intensities of element peaks as soon as the measurement is complete. The accuracy and precision with which this is carried out is crucial when analyzing heavy elements in polymers at trace concentration levels, particularly when peaks overlap. An example of the spectra used for the analysis of (a) Ba, Cd, and (b) Cr, Ni, Cu, Zn in polyethylene is illustrated in Figure 1.
The accuracy of the results is shown in Table 1. The calibration RMS value is a statistical comparison (1 sigma) of the certified chemical concentrations of the standards with the concentrations calculated by regression in the calibration procedure. A single calibration standard was analyzed as an unknown to test the accuracy of the analytical program. The data in Table 2 show that the agreement between the measured and certified concentrations is excellent. The calibration plot for Cd shown in Figure 2 provides a graphical representation of the method.
Plastics Additives & Compounding January/February 2005
38
Precision and instrument stability The precision of the Epsilon 5 is demonstrated in both short and long term reproducibility tests (Table 3). A single standard analyzed 20 consecutive times shows very good short-term repeatability. Figure 3 gives a graphical illustration of the analytical precision of Cd. Twenty consecutive measurements of a polyethylene sample demonstrate standard deviations better than 4.5% relative at the 7 ppm level, for example. 6.76 ± 0.3 ppm Cd. The data do not show any trend and plot
Environment Table 4. Detection limits. Element
Cr
Ni
Cu
Zn
As
Br
Cd
Ba
Hg
Pb
LLD (100 s)
0.51
0.17
0.12
0.51
0.15
0.19
0.11
0.30
0.93
2.22
Application LLD (600 s)
0.21
0.07
0.05
0.21
0.06
0.08
0.05
0.12
0.38
0.91
around the statistical mean, further demonstrating the stability of the instrument. This level of precision was maintained for measurements carried out over a period of three weeks without any drift correction.
Detection limits Detection limits for the elements examined in this study are given in Table 4. The lower limit of detection (LLD) is calculated from: LLD = 3/s sqrRb/tb where: s = sensitivity (cps/ppm)
Rb = background count rate (cps) tb = live time (s)
Conclusions Clearly, the avoidance of toxic waste is an important issue. As the data presented here show, XRF in combination with the use of the new TOXEL standards is fully capable of analyzing heavy metals such as Cd, Pb, Hg, Cr, As, Ni, Cu, Zn and Br at the low levels required by the RoHS and WEEE directives. Measurements are accurate and precise and the method benefits from simple,
39
essentially hazard-free, sample preparation. By adopting XRF analysis, plastics manufacturers will be able to play their part in reducing toxic waste and safeguarding the environment. Contact: PANalytical B.V. P.O. Box 13 7600 AA Almelo The Netherlands Tel: +31 546 534 444 Fax: +31 546 534 598 E-mail: [email protected] Website: www.panalytical.com
Plastics Additives & Compounding January/February 2005
past decade have had the effect of creating a growing volume of obsolete products. During 2002 for example, an estimated 55.4 million computers became redundant in the US alone. Not surprisingly, the disposal of such
Table 1: Calibration data based on 600 seconds live time measurements. Element
Calibration RMS [ppm]
Concentration range [ppm]
Cr
0.15
0-24.8
Ni
0.08
0-10.6
Cu
0.11
0-25.1
Zn
0.10
0-5.1
As
0.04
0-6.4
Br
0.84
0-163.3
Cd
0.08
0-28.4
Ba
2.7
0-600.7
Hg
0.15
0-5.3
Pb
0.17
0-22.3
Plastics Additives & Compounding January/February 2005
36
products is now a prime concern. Partly this is because of their volume, but more importantly because they contain significant amounts of heavy metals and other toxic substances. In an incinerator or landfill, these can be released into the environment to contaminate the air, ash and ground water. Issues such as these have driven the introduction of the European Union's recent broad directives on Waste Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substances (RoHS). By reducing the use of heavy metals, particularly cadmium, at source; and by recycling as much as possible, the directives aim to avoid problems associated with the disposal of hazardous materials. In the plastics industry, compliance with the directives requires manufacturers to carry out precise and repeatable elemental analyses of heavy metals down to sub-ppm levels at all stages of the production process. X-ray fluorescence (XRF) spectroscopy has emerged as the most
ISSN1464-391X/05 © 2005 Elsevier Ltd.All rights reserved.
Environment
(a)
(b)
Ba-Kα1
Ba-Lα1 Cu-Kα1
Ba-Kα2
2
2 Zn-Kα1 Ba-Lβ1
1
Ti-Kβ1
Na-Kα1 Ni-Kβ1 W-Lα1
Cr-Kα1
Cd-Kα2 Cd-Kα1
0
Cps/mAch
Cps/mAch
4
Cd-Kβ1
25
Cr-Kβ1 Zr-Kα1-EKα
0
30
5
Fe-Ka1
6
Energy (keV)
Fe-Kβ1
7
8
Energy (keV)
Figure 1: Spectrum (a) for Ba and Cd obtained using a Barkla secondary target and (b) Cr, Ni, Cu and Zn using a Ge secondary target. economical and effective analytical tool for achieving this. The routine use of XRF analysis in plastics manufacturing has been hampered by the lack of suitable measurement standards. However, a set of certified standards - known as TOXEL - is now available to facilitate XRF analyses in polyethylene. Developed jointly by DSM Resolve and PANalytical, each TOXEL set comprises five standards (one blank and four multielement standards) containing the regulated elements Cr, Cd, Hg, Pb, As, Ni, Cu, Zn, Ba and Br. Calibration with TOXEL standards is simplified by the fact that XRF is a multielement technique. Therefore a single set of the new standards can be used to calibrate several heavy elements, covering concentrations from trace level to several hundred ppm. As the following example illustrates, the availability of the new TOXEL standards means that plastics manufacturers now have the necessary resources to meet the challenges arising from the WEEE and RoHS directives. This case study is the analysis of heavy metals in polyethylene using an Epsilon 5 XRF spectrometer.
Analytical conditions A set of TOXEL standards was used to calibrate an Epsilon 5 XRF spectrometer. The measurement program was defined in the software using the Epsilon 5 Wizard. This guides the user through the steps required to calibrate the application.
Table 2: Results of the analysis of standard 3 as an unknown. Element
Certified concentration [ppm]
Measured concentration [ppm]
Cr
5.9
5.87
Ni
2.57
2.65
Cu
6.0
6.22
Zn
1.25
1.4
As
1.44
1.5
Br
38.5
40.13
Cd
6.6
6.88
Ba
145.2
150.45
Hg
1.29
1.2
Pb
5.2
5.29
Figure 2: Calibration graph for Cd. Once the elements have been selected, the software proposes the polarizing secondary targets, together with the voltage and current of the tube providing the best excitation for the
37
elements of interest. For this application the Wizard defined three measurement conditions, each involving 600 seconds of detector live time.
Plastics Additives & Compounding January/February 2005
Environment Table 3: Repeatability and reproducibility. Element
Cr
Ni
Cu
Zn
As
Br
Cd
Ba
Hg
Pb
Repeatability (20 consecutive measurements) Mean [ppm]
6.08
2.63
6.27
1.39
1.47
40.01
6.76
149.63
1.4
5.39
St.Dev [ppm]
0.16
0.04
0.05
0.04
0.06
0.11
0.3
0.66
0.21
0.12
Rel. St.Dev [ppm] 2.68
1.33
0.85
3
3.75
0.27
4.44
0.44
14.88
2.26
Reproducibility (measurements carried over 3 weeks) Mean [ppm]
5.98
2.63
6.21
1.31
1.43
39.84
6.88
149.33
1.59
5.43
St.Dev [ppm]
0.23
0.03
0.09
0.07
0.07
0.19
0.24
0.89
0.33
0.1
Rel. St. Dev [ppm] 3.82
1.16
1.51
5.33
4.63
0.48
3.53
0.6
20.69
1.89
Counting Statistical Error (CSE) CSE
0.12
0.18
0.36
0.13
0.16
0.74
0.17
0.67
0.17
0.17
CSE rel %
1.37
0.91
0.46
1.21
1.02
0.22
0.99
0.24
0.98
0.98
Figure 3: Short and long term stability measurements for Cd in polyethylene.
Performance
Accuracy
The Epsilon 5 software features a powerful deconvolution algorithm, which automatically analyzes the sample spectrum and determines the net intensities of element peaks as soon as the measurement is complete. The accuracy and precision with which this is carried out is crucial when analyzing heavy elements in polymers at trace concentration levels, particularly when peaks overlap. An example of the spectra used for the analysis of (a) Ba, Cd, and (b) Cr, Ni, Cu, Zn in polyethylene is illustrated in Figure 1.
The accuracy of the results is shown in Table 1. The calibration RMS value is a statistical comparison (1 sigma) of the certified chemical concentrations of the standards with the concentrations calculated by regression in the calibration procedure. A single calibration standard was analyzed as an unknown to test the accuracy of the analytical program. The data in Table 2 show that the agreement between the measured and certified concentrations is excellent. The calibration plot for Cd shown in Figure 2 provides a graphical representation of the method.
Plastics Additives & Compounding January/February 2005
38
Precision and instrument stability The precision of the Epsilon 5 is demonstrated in both short and long term reproducibility tests (Table 3). A single standard analyzed 20 consecutive times shows very good short-term repeatability. Figure 3 gives a graphical illustration of the analytical precision of Cd. Twenty consecutive measurements of a polyethylene sample demonstrate standard deviations better than 4.5% relative at the 7 ppm level, for example. 6.76 ± 0.3 ppm Cd. The data do not show any trend and plot
Environment Table 4. Detection limits. Element
Cr
Ni
Cu
Zn
As
Br
Cd
Ba
Hg
Pb
LLD (100 s)
0.51
0.17
0.12
0.51
0.15
0.19
0.11
0.30
0.93
2.22
Application LLD (600 s)
0.21
0.07
0.05
0.21
0.06
0.08
0.05
0.12
0.38
0.91
around the statistical mean, further demonstrating the stability of the instrument. This level of precision was maintained for measurements carried out over a period of three weeks without any drift correction.
Detection limits Detection limits for the elements examined in this study are given in Table 4. The lower limit of detection (LLD) is calculated from: LLD = 3/s sqrRb/tb where: s = sensitivity (cps/ppm)
Rb = background count rate (cps) tb = live time (s)
Conclusions Clearly, the avoidance of toxic waste is an important issue. As the data presented here show, XRF in combination with the use of the new TOXEL standards is fully capable of analyzing heavy metals such as Cd, Pb, Hg, Cr, As, Ni, Cu, Zn and Br at the low levels required by the RoHS and WEEE directives. Measurements are accurate and precise and the method benefits from simple,
39
essentially hazard-free, sample preparation. By adopting XRF analysis, plastics manufacturers will be able to play their part in reducing toxic waste and safeguarding the environment. Contact: PANalytical B.V. P.O. Box 13 7600 AA Almelo The Netherlands Tel: +31 546 534 444 Fax: +31 546 534 598 E-mail: [email protected] Website: www.panalytical.com
Plastics Additives & Compounding January/February 2005