SI-traceable certification of Cu, Cr, Cd and Pb in sediment and fly ash candidate reference materials

Many fields in environmental analytical Chemistry deal with very low limits and thresholds as set by governmental legislations or transnational regula tions. The need for the accuracy, comparability and traceability of analyti cal measurements in environmental analytical chemistry has significantly in creased and total uncertainties are even asked for by accreditation bodies of environmental laboratories. This paper addresses achieving these goals t o guarantee accuracy, quality control, quality assurance or validation of a method by means of certified reference materials. The assessment of analyt ical results in certified reference materials must be as accurate as possib le and every single step has to be fully evaluated. This paper presents the SI-traceable certification of Cu, Cr, Cd and Pb contents in geological and environmentally relevant matrices (three sediments and one fly ash sample) . Certification was achieved using isotope dilution (ID) ICPMS as a primary method of measurement. In order to reduce significantly the number of anal ytical step's and intermediate samples a multiple spiking approach was deve loped. The full methodology is documented and total uncertainty budgets are calculated for all certified values. A non-element specific sample digesti on process was optimised. All wet chemical digestion methods examined resul ted in a more or less pronounced amount of precipitate. It is demonstrated that these precipitates originate mainly from secondary formation of fluori des (essentially CaF2) and that their formation takes place after isotopic equilibration. The contribution to the total uncertainty of the final value s resulting from the formation of such precipitates was in general <0.1% fo r all investigated elements. Other sources of uncertainty scrutinised inclu ded the moisture content determination, procedural blank determination, cro ss-contamination from the different spike materials, correction for spectra l interferences, instrumental background and deadtime effects, as well as t he use of either certified values or IUPAC data in the IDMS equation. The a verage elemental content in the sediment samples was 30-130 <mu>g g(-1) for Pb, 0.5-3 mug g(-1) for Cd and 50-70 mug g(-1) for Cu. Cr was measured in one sample and was about 60 mug g(-1). The concentrations in the fly ash sa mple were up to 2 orders of magnitude higher. Expanded uncertainty for the investigated elements was about 3% (coverage factor k = 2) except for Cr, ( measured by high resolution ICPMS), for which the expanded uncertainty was about 7% (k = 2).