Determination of total sulfur at microgram per gram levels in geological materials by oxidation of sulfur into sulfate with in situ generation of bromine using isotope dilution high-resolution ICPMS

We have developed a new, simple, and accurate method for the determination of total sulfur at microgram per gram levels in milligram-sized silicate materials with isotope dilution high-resolution inductively coupled plasma mass spectrometry equipped with a flow injection system. In this method, sulfur can be quantitatively oxidized by bromine into sulfate with achievement of isotope equilibrium between the sample and spike. Detection limits for ³²S⁺ and ³⁴S⁺ in the ideal solution and silicate samples were 1 and 6 ng mL^-1 and 0.07 and 0.3 μg g^-1, respectively. The total blank was 46 ng, so that a 40-mg silicate sample containing 10 μg g^-1 sulfur can be measured with a blank correction of < 10%. This total blank can be lowered to 8 ng if a low-blank air system is used for evaporations. To evaluate the applicability of this method, we analyzed not only silicate reference materials with sulfur content of 5.25-489 μg g^-1 and sample sizes of 13-40 mg but also the Allende meteorite with a sulfur content of 2%. The reproducibility for various rock types was <9%, even though blank corrections in some samples of low sulfur content were up to 24%. This method is suitable for analyzing geological samples as well environmental samples such as soils, sediments, and water samples.