Determination of cadmium in environmental and biological reference materials using isotope dilution analysis with a double focusing ICP-MS: a comparison with quadrupole ICP-MS

A detailed examination of the performance of a double focusing (DF) sector field ICP-MS (Element, Finnigan MAT) for the ID-ICP-MS determination of cadmium in biological and environmental samples is described and compared with a quadrupole-based instrument (HP4500, Hewlett Packard). Experimental parameters governing the instrumental precision and accuracy for isotope ratio measurements of cadmium in the DF-ICP-MS are first characterized. Systematic errors, including dead time, mass bias effects and spectroscopic interferences, can be easily corrected in the two instruments studied. Detector dead time has only to be taken into account using the DF-ICP-MS. Typical isotope ratio precisions of 0.2-0.3% can be obtained routinely with both instruments. However, the precision of the measurements in the DF-ICP-MS can be improved (< 0.1%) by increasing the number of scans. Moreover, for very low cadmium concentrations the DF-ICP-MS at low resolution setting (R=300) can provide better isotope ratio precision than the quadrupole ICP-MS because of its higher sensitivity. Both ICP-MS instruments were successfully applied to the determination of very low levels of cadmium by ID-ICP-MS (by both conventional and on-line ID modes) in biological and environmental certified reference materials, including Freeze-Dried Urine NIST SRM-2670, Horse Kidney IAEA H-8, Lichens BCR TP-25, Marine Sediment PACS-1 and Riverine Water SLRS-3. Combined uncertainty associated with the determinations is addressed.