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 primary method of measurement

The development and implementation of a method for the certification of cadmium 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 dilution mass spectrometry (ICP-IDMS) applied as a primary method of measurement. Two different sample digestion methods, an optimized microwave digestion procedure using HNO3 and H2O2 as oxidizing agents and a high-pressure asher digestion procedure, were developed and compared. The very high salt content of the digests and the high molybdenum content, which can cause oxide-based interferences with the Cd isotopes, were reduced by a chromatographic matrix separation step using an anion-exchange resin. All isotope ratio measurements were performed by a quadrupole ICP-MS equipped with an ultrasonic 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-processing 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 European Commission (BCR-194, BCR-195, BCR-196). Cadmium concentrations ranged between similar to0.2 ng g(-1) and similar to 12 ng g(-1). For these materials, SI-traceable certified values including total uncertainty budgets according to ISO and Eurachem guidelines were established.