We are currently working on some carbonate samples containing a relatively high S content. Thus the CO2 we extracted through phosporic acid digestion ends up with a very annoying S contamination, which was confirmed by the mass scan of the sample. At this moment, the exact nature of the contaminant hasn't been identified, presumably H2S or SO2. We have already tried several purification methods, including cryogenic separation, reacting with silver wire, GC separation etc. So far, the results are not satisfactory. I wonder whether anyone can give me some more hints on this problem. (The sample size we are dealing with is usually around 60 micromoles.)