Background

The monitoring of the heavy metals restricted within the plastics industry, such as cadmium, lead, mercury, chromium, and arsenic is an important aspect of environmental protection. The body responsible for most of the stringent new regulations is the European Union. For instance, an EU Packaging Directive in the mid-nineties did much to ensure the responsible handling of plastics from production, through to disposal and recycling.

Another EU Directive, 91/338/EC, sets the maximum allowable concentration of cadmium in plastics used for consumer goods at 100 mg/kg. As early as 1993, Denmark banned all products in which cadmium is used either as a surface treatment agent (cadmium plating), color pigment or plastic stabilizer with more than 75 ppm in the homogeneous components of the product.
Directive Governing Toxic Heavy Metals

The EU’s broad Restriction of Hazardous Substances (RoHS) directive will eliminate the use of cadmium in electronic products by July 1, 2006. The EU’s recent End-of-Life Vehicles (ELV) Directive bans lead, cadmium, mercury, and hexavalent chromium from products without specific exemptions. Even in the US, California’s Proposition 65 bans the use of cadmium.

With its WEEE (Waste Electronic and Electrical Equipment) directive, the EU requires manufacturers to organize collection and proper recycling of their old equipment e.g. computers. It also indicates that all batteries need to be removed from that waste.

The aim of all this regulation is to reduce the use of heavy metals at source, then recycle as much as possible to avoid the problems associated with disposal. Manufacturers exporting to countries where these regulations apply have no alternative but to comply with them, and such countries already check the heavy metals content of goods in customs.
X-Ray Fluorescence for Elements Analysis of Heavy Metals in Plastics

In order to comply with these new regulations, plastics manufacturers require precise and repeatable measurements of additives – at all stages of the production process. Given the rigorous demands of new regulation, X-ray fluorescence spectroscopy (XRF) has emerged as the optimal solution for elemental analysis of heavy metals in plastics.
Standards and Their Preparation

A set of TOXEL (Toxic heavy elements in polyethylene) standards was used to set up the calibrations. These standards were produced and jointly developed by DSM Resolve and PANalytical. A set contains five different standards made of polyethylene that was blended, extruded and granulated to achieve a high degree of homogeneity. Subsequently discs were pressed. The trace element composition and homogeneity of these standards was assessed using one, two or more independent and reliable analytical methods for example: ICP-MS, NAA, ICP-AES and CV-AAS.
Analytical Conditions

The measuring program was set up in the software using the Epsilon 5 Wizard, which guides the user through the steps required to calibrate the application. Once the elements are selected, the software proposes the polarizing secondary targets, together with the voltage and current of the tube, which provides the best excitation for the elements of interest. For this application the Wizard defined 3 measurement conditions, each of which was measured for 600 seconds of detector live time.
Performance

The Epsilon 5 software features a very 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 with which this is carried out is essential to the analysis of heavy elements in polymers at the trace concentration levels, and particularly when elements overlap one another. 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, which shows the successful deconvolution of these elements.

AZoM - Metals, Ceramics, Polymer and Composites - Spectrum (a) for Ba and Cd obtained using a Barkla secondary target and (b) Cr, Ni, Cu and Zn using a Ge secondary target.

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.
Accuracy

The accuracy of the calibrations is presented in Table 1 and illustrated graphically for Cd, Pb, Hg and Cr in Figures 2-5. 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.

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


0.03


0-6.4

Br


0.50


0-163.3

Cd


0.04


0-28.4

Ba


1.40


0-600.7

Hg


0.09


0-5.3

Pb


0.15


0-22.3

AZoM - Metals, Ceramics, Polymer and Composites - Calibration graph for lead

Figure 2. Calibration graph for Cd

AZoM - Metals, Ceramics, Polymer and Composites - Calibration graph for lead

Figure 3. Calibration graph for Pb

AZoM - Metals, Ceramics, Polymer and Composites - Calibration graph for mercury

Figure 4. Calibration graph for Hg

AZoM - Metals, Ceramics, Polymer and Composites - Calibration graph for Chromium

Figure 5. Calibration graph for Cr.

A single calibration standard was analyzed as an unknown sample 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. Calibration plots for Cd, Pb, Hg and Cr shown in Figures 2, 3, 4 and 5 provide a graphical representation of the method.

Table 2. Results of the analysis of standard 3 as an unknown

Element


Certified
concentration (ppm)


Measured
concentration (ppm)

Cr


5.9


5.9

Ni


2.57


2.65

Cu


6.0


6.2

Zn


1.25


1.4

As


1.44


1.5

Br


38.5


40.1

Cd


6.6


6.9

Ba


145.2


150.4

Hg


1.29


1.2

Pb


5.2


5.3
Precision and Instrument Stability

The precision of the Epsilon 5 is unrivalled and 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 6 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 e.g. 6.76 ± 0.3 ppm Cd. The data do not show any trend and plot around the statistical mean, further demonstrating the stability of the instrument. This level of precision is maintained for measurements carried over a period of 3 weeks. This excellent level of reproducibility was achieved without any drift correction, again demonstrating the stability of Epsilon 5.

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

RMS


0.16


0.04


0.05


0.04


0.06


0.11


0.3


0.66


0.21


0.12

RMS rel%


2.68


1.33


0.85


3


3.75


0.27


4.44


0.44


14.88


2.26

Reproducibility (Measurements carried out 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

RMS


0.23


0.03


0.09


0.07


0.07


0.19


0.24


0.89


0.33


0.1

RMS rel%


3.82


1.16


1.51


5.33


4.63


0.48


3.53


0.6


20.69


1.89

Comparison (with 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

AZoM - Metals, Ceramics, Polymer and Composites - Short and long term stability measurements for Cd in PE

Figure 6. Short and long term stability measurements for Cd in PE.
Detection Limits

Detection limits for the elements examined in this study are given in Table 4.

Table 4. Detection limits.

Element


Cr


Ni


Cu


Zn


As


Br


Cd


Ba


Hg


Pb

LLD (100s)


0.78


0.17


0.17


0.42


0.29


0.27


1.10


2.69


0.51


0.64

Application LLD (600s)


0.32


0.07


0.07


0.17


0.12


0.11


0.45


1.10


0.21


0.26

The lower limits of detection (LLD) is calculated from:

Where:

s = sensitivity (cps/ppm)
rb = background count rate (cps)
tb = live time (s)
Conclusions

The Epsilon 5 is fully capable of analyzing heavy metals in polyolefins such as Cd, Pb, Hg, Cr, As, Ni, Cu, Zn and Br at the low levels required by RoHS and WEEE directives governing the plastic industry. Measurements are accurate and precise and the method benefits from simple, essentially hazard-free, sample preparation. Establishing a calibration with TOXEL standards and Epsilon 5 is simplified by the fact that XRF is a multi-element technique. This means that a single set of TOXEL calibration standards can be used to calibrate several heavy elements, covering concentrations from trace level to a few hundreds of ppm. The stability of the Epsilon 5 is such that individual calibrations can be used for months. Time consuming re-standardizations are unnecessary and the resulting data are highly consistent over time. By adopting the XRF analysis, plastics manufacturers will be ready to meet the challenges – and opportunities – involved with the manufacturing of plastics in the 21st century.