Sulfur isotope ratio measurements of individual sulfate particles by NanoSIMS

The sulfur isotopic compositions of barite (BaSO4), anhydrite (CaSO4), gypsum (CaSO4 center dot 2H(2)O), mascagnite ((NH4)(2)SO4), thenardite (Na2SO4), boetite (K2SO4), epsomite (MgSO4 center dot 7H(2)O), magnesium sulfate (MgSO4 center dot XH2O) and cysteine (an amino acid) were determined with a Cameca NanoSIMS 50 ion microprobe employing a Cs+ primary ion beam and measuring negative secondary ions. This ion microprobe permits the analysis of sulfur isotope ratios in sulfates on 0.001-0.5 ng of sample material, enabling the analysis of individual S-bearing particles with diameters as small as 500nm. The grain-to-grain reproducibility of measurements is typically 5 parts per thousand (1 sigma) for micron-sized grains, < 5 parts per thousand for submicron-sized grains down to roughly 500 nm, and < 2 parts per thousand for polished thin sections and ultra microtome sections which were studied for comparison. The role of chemical composition (matrix effect) and sample preparation technique on the instrumental mass fractionation (IMF) of the S-34/S-32 ratio in the NanoSIMS has been investigated for different sulfates and one amino acid. The IMF varies by similar to 15 parts per thousand between the standards studied here, underlining the importance of a good understanding of the matrix-specific IMF correction in order to get precise S isotope data for very small samples such as aerosol particles. A good correlation between IMF and ionic radius of the cations in sulfates was found, permitting inference of IMF corrections for sulfates for which no isotope standards are available. (c) 2008 Elsevier B.V. All rights reserved.