Feasibility of isotope dilution calibration for the determination of thallium in sediment using slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry

Abstract

Isotope dilution (ID) calibration was investigated as a means to establish accurate values for thallium in a variety of sediment reference materials using slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS) as the analytical tool. The pyrolysis curve, as well as the analysis of SRM 2704 Buffalo River Sediment, suggested that a pyrolysis temperature of 700°C could be used for this determination. The analytical results for a number of river sediment reference materials confirmed this finding; however, the results for marine sediments were far too high, indicating a serious problem with these samples. Careful investigation revealed the following interference mechanism as the most likely one. The chloride content of marine sediments is some two orders of magnitude higher than that of river sediments, and the chloride leaches out very easily into the aqueous phase of the slurry. The thallium that is in solution forms TlCl upon drying, which is easily volatilized and lost at pyrolysis temperatures >400°C. As the enriched isotope spike was added in solution, and the ‘original’ thallium is retained, at least in part, in the sediment particles, the spiked thallium is preferentially lost, leading to the excessively high analytical results. Accurate results for Tl in marine and river sediments could be obtained using pyrolysis temperatures of 400 and ≤700°C, respectively. The detection limit calculated from 3×S.D. ( n=5) of the sediment with the lowest thallium content (HISS-1) was found to be 0.003 μg g −1. The precision at the 0.5–1.0-μg g −1 Tl level based on five consecutive readings was 1.3–4.0% R.S.D.