Archive for October, 2007

ISOTOPE DILUTION FOR THE ANALYSIS OF METALS IN PLASTICS

Monday, October 8th, 2007

ISOTOPE DILUTION FOR THE ANALYSIS OF METALS IN PLASTICS  Ralph Obenauf, Vanaja Sivakumar, Laszlo Ernyei, Bill Driscoll, SPEX CertiPrep, Inc. 203 Norcross Avenue, Metuchen, NJ 08840, robenauf@spexcsp.com 

www.spexcsp.com

     The European Directive, Restriction of Hazardous Substances (RoHS) & Waste Electrical and Electronic(WEEE) restricts the use of hazardous substances such as  lead, cadmium, chromium(VI), mercury and brominated polybihphenyl ethers  in electronic and other equipment.  The RoHS & WEEE directives set only a minimum standard.  Different countries have set their own regulations and requirements.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             Various analytical methods are employed to determine the concentration of restricted substances.  Most methods require some sample preparation such as digestion or extraction.  Many of the methods currently used are not appropriate for all products tested, and results differ from method to method.  It is vital to develop standard methods that produce reliable results that can be implemented by all laboratories.  We have developed a method for accurate analysis of the above metals in plastic.       Methods based on direct analysis of solid samples by XRF and Laser Ablation ICP-MS are limited due to the lack of calibration standards for various complex plastic matrices.  Methods based on destructive sample preparation usually employ heating the polymers with mineral acids or dissolution in organic solvents [1].  However, heating with acids may lead to the loss of analytes due to volatilization and many polymers are sparingly soluble in organic solvents.  Here we employed two techniques for sample preparation, namely, conventional ashing and microwave digestion.       Isotope dilution Mass Spectrometry (IDMS) is based on the addition of a known quantity of an enriched isotope to a sample. Equilibration of the spike isotope with the natural element in the sample alters the isotope ratio that is measured. Knowing the isotopic abundance of both spike and sample, the amount of spike added to the known amount of sample, the measured concentration of the spike added and the altered isotope ratio, then the concentration of the element in the sample can be calculated [2].  Since elements of interest such as Cd, Cr and Pb in the RoHS directive have two or more naturally occurring isotopes, the isotope dilution technique presents an interesting solution for the determination of these elements.       Ashed polyethylene polymer dissolved in nitric acid was analyzed for cadmium, chromium and lead. The polymer was spiked with a known amount of cadmium enriched with Cd-106, chromium enriched with Cr-50 and lead enriched with Pb-206; then ashed.  Both spiked and un-spiked polyvinyl chloride and polyethylene polymers were also digested in a combination of nitric, hydrochloric, hydrofluoric acid along with hydrogen peroxide.     The IDMS technique results in higher accuracies in the determination of lead, chromium and cadmium than conventional calibration methodology.   Partial loss of analyte due to sample preparation or the complexity of the matrix did not affect the accuracy of determination.   References: 1. Digestion of Plastic Materials for the Determination of Toxic Metals with a Microwave Oven for Household Use: Hiroki Sakurai et al, Analytical Sciences, February 2006, Vol.2.2. USEPA Method 6800: Elemental and Speciated Isotope Dilution Mass Spectrometry.