The accuracy and precision of micro-analysis of sulfur (S), chlorine (Cl) and bromine (Br) in quartz-hosted fluid inclusions by laser ablation (LA) inductively coupled plasma mass spectrometry (ICP-MS) was tested. A scapolite mineral sample (Sca17) can be used as standard reference material (SRM) for the determination of Cl and Br, and NIST610 glass for S determination in fluid inclusions. We found that laser ablation of quartz and UV irradiation in the ablation cell produced elevated background signals of S and to a lesser amount of Cl and Br due to remobilization of these elements from the inside surfaces of the ablation Page 2 chamber. Careful cleaning of the ablation chamber with nitric acid and by UV irradiation results in a 10 times lower S contamination signal and improves fluid inclusion analysis by reducing the detection limit for S by 50%. Micro-analysis of liquid and vapor inclusions synthesized in two different laboratories produce good accuracies of S, Cl, and Br. Analytical uncertainties based on numerous analyses of individual synthetic fluid inclusions in one assemblage are 17-44 % (RSD) for the sulfur concentration, and 6-26 % for Br/Cl ratios. Limits of detection (LOD) in 30 μm diameter liquid inclusions with densities of 0.99-1.02 g/cm 3 are in the range of 60 μg/g for S, 250 μg/g for Cl, and 15 μg/g for Br. LOD's in similar sized vapor inclusions with a density of 0.33 g/cm 3 are at least an order of magnitude poorer. Based on the investigated natural brine inclusion assemblages, the precisions of Br/Cl ratios (4-9 % RSD) is adequate to determine the source of salinity in different ore-forming fluids.

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