SI-traceable certification of the amount content of cadmium below the ng g(-1) level in blood samples by isotope dilution ICP-MS applied as a primarymethod of measurement

The development and implementation of a method for the certification of cad mium in blood samples at low ng g(-1) and sub ng g(-1) levels is described. The analytical procedure is based on inductively coupled plasma isotope di lution mass spectrometry (ICP-IDMS) applied as a primary method of measurem ent. Two different sample digestion methods, an optimized microwave digesti on procedure using HNO3 and H2O2 as oxidizing agents and a high-pressure as her digestion procedure, were developed and compared. The very high salt co ntent of the digests and the high molybdenum content, which can cause oxide -based interferences with the Cd isotopes, were reduced by a chromatographi c matrix separation step using an anion-exchange resin. All isotope ratio m easurements were performed by a quadrupole ICP-MS equipped with an ultrason ic nebulizer with membrane desolvator. This sample introduction set-up was used to increase sensitivity and minimize the formation of oxides (less MoO + interference with the Cd isotopes). Because of the very low Cd concentrations in the samples and the resulting need to minimize the procedural blank as much as possible, all sample-proce ssing steps were performed in a clean room environment. Detection limits of 0.005 ng g(-1) Cd were achieved using sample weights of 2.7 g. The method described was used to recently the cadmium content of three different blood reference materials from the Community Bureau of Reference (BCR) of the Eu ropean Commission (BCR-194, BCR-195, BCR-196). Cadmium concentrations range d between similar to0.2 ng g(-1) and similar to 12 ng g(-1). For these mate rials, SI-traceable certified values including total uncertainty budgets ac cording to ISO and Eurachem guidelines were established.