Accurate and Precise Analysis of Trace Levels of Cu, Cd, Pb, Zn, Fe and Ni in Sea Water by Isotope Dilution Mass Spectrometry

Abstract

Accurate and precise determinations of natural levels of trace metals in sea water are highly reliant upon the size and variability of the analytical blank, the method for determining the yield, and, to a lesser extent, the inherent precision of the instrument used. Thermal source, isotope dilution mass spectrometry together with ultra-clean room techniques were successfully used in the determinations of Cu, Cd, Pb, Zn, Ni and Fe in sea water. Multi-element analyses were performed in a single experiment owing to the differing release with filament current for each element. A single Re filament loaded with a substrate of silica gel and phosphoric acid gave high precision (0.1% to 0.5%) for the determination of a single ratio, and allowed low detection limits (from 0.02 fmol Cd to 0.07 pmol Fe). Yields were accurately and uniquely determined in the same sample by the addition of two isotopically enriched spikes, one before, and one after an extraction. Blanks were assessed as the summation of individual contributions determined in separate experiments prior to sample analysis. Simply obtaining one value for a total blank contribution can be misleading and can generate larger errors. Rigorous clean room procedures allowed very low blanks; 0.0002 nmol Cd kg−1, 0.0005-0.002 nmol Pb kg−1, 0.02 nmol Cu kg−1, 0.03 nmol Zn kg−1 and 0.7 nmol Fe kg−1.