Determination of total lead and lead isotope ratios in natural waters by inductively coupled plasma time-of-flight mass spectrometry after flow injection on-line preconcentration

An on-line flow injection (FI) preconcentration method is developed for the determination of total lead and lead isotope ratios in natural waters by inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS). This method yields a 20-fold enrichment compared to the direct FI determination and provides improved isotope ratio precision. Besides, the high selectivity achieved, particularly towards Hg, allows the accurate determination of 204Pb-based isotope ratios at ng l^-1 Pb levels with a good precision. Precise transient Pb isotope ratios in ion counting are affected by detector dead time; a dead time value of 20 ns was used for correction for Pb concentrations in excess of 0.4 (mu)g l^-1, and a 100 (mu)l eluent loop. Mass discrimination (MD) was evaluated for pairs of isotopes using an isotopic standard reference material (NIST 981). The precision obtained for 204Pb/206Pb, 207Pb/206Pb, and 208Pb/206Pb ratios at a Pb concentration of 1 (mu)g l^-1 was 1.87, 0.7, and 0.28% relative standard deviation (R.S.D.) respectively, using ion counting mode, 1.0 s integration time and calculations performed using full peak integration. Lead concentrations in the water samples were between 210 and 462 ng l^-1. Precisions (for n=5) were found in the ranges 1.5–4.25% and 0.1–0.8% for 204Pb/206Pb and for 207Pb/206Pb, 208Pb/206Pb ratios.