Precise sulfur isotope ratio measurements in trace concentration of sulfur by inductively coupled plasma double focusing sector field mass spectrometry

Sulfur isotope ratios (34S/32S) were determined by means of inductively coupled plasma double focusing sector ®eld mass spectrometry (ICP-SMS) operated in the medium resolution mode (m/Dm=4000) using a torch with a platinum guard electrode and a microconcentric nebulizer combined with a membrane desolvation unit. The guard electrode together with the nebulizing unit increased the signal intensity of the measured isotopes by two orders of magnitude. The use of the membrane desolvation unit decreased the signal intensity of the corresponding interference (mainly oxygen containing species) signi®cantly. Detection limits in solution of 0.01 ng g−1, limited only by blank levels, could be achieved. Moreover, sulfur isotope ratios could be determined at concentration levels down to 1 ng g−1 with a precision of better than 0.1% relative standard deviation (RSD) (n=10). A precision of 0.04% RSDcould be achieved at higher concentration levels. ICP-SMS has been shown to be an excellent tool for fast and precise isotope ratio measurements in combination with a high sample throughput and minimum sample preparation prior to analysis.