Application of isotope dilution mass spectrometry to the determination of cu, cd, pb, zn, ni, fe and cr in seawater

Abstract

Isotope dilution using a thermal source mass spectrometer (TSMS) has been applied to the investigation of natural levels of Cu, Cd, Pb, Zn, Ni, Fe and Cr in seawater. Multielement analysis can be performed in a single experiment as the release spectra of the elements tend to be staggered with filament current. A single Re filament with a silica gel/phosphoric acid substrate allowed high precision (0.1–0.5%) in the determination of a single ratio and low detection limits (from 0.02 pg for Cd or Pb to 4 pg for Fe). Two isotopic spikes per element were used where possible to allow yield and concentration to be determined in a single experiment. Blanks, assessed as the summation of individually measured components ranged from 0.02 mg kg −1 for Cd, 0.1–0.5 ng kg −1 for Pb, 1 ng kg −1 for Cu, 2 ng kg −1 for Zn to 40 ng kg −1 for Fe, a level made possible by ultra-clean room techniques. Seawater, collected from relatively low lead, coastal waters, was stored acidified in rigorously cleaned, conventional polyethylene carboys at pH 2.0, pH 1.6 and pH 1.1. Extraction of seawater stored at a pH of 1.6–2.0 by dithizone in chloroform and/or by APDC/DDC in Freon TF appeared to yield values representing the dissolved fraction. A rigorous aqua regia digestion of seawater was required for the determination of total values. The precision in the determination of Cd and Zn in seawater was ca. 1%, a level approaching the precision attainable in a single ratio determination (approximately 0.1–0.5%). Moreover, this precision includes the contamination levels incurred in storing seawater for over one year. The precision for lead results varied from 5% at concentrations of 9 ngPb kg −1 to approximately 1% at levels of 30 ng kg −1. The determination of Cu, Ni and Fe were hampered by isobaric interferences. These interferences could be minimized or eliminated by a purification of the final extract with a ‘mini’ ion exchange step and/or by the evaluation of trends in mass spectrometrically determined ratios with time and with filament current. Analyses of comparable amounts of Cd, Pb and Zn in NBS standard 1643a corroborated the accuracy of the IDMS approach/dithizone extraction procedure for these metals at least. Comparison of the seawater results with FAA and ASV methods indicated that for Cd, Pb and Zn, the IDMS approach was the most precise and the most accurate. For Cu, Ni, Fe and Cr extraction difficulties or isobaric interferences have yet to be resolved.