Dear Kim,
     
        Addressing the Hydrogen sulfide gas contamination to your carbonate 
     samples.... try Silver Phosphate trap:
     
     Depending on the amount of H2S gas to be removed and the number of 
     samples, as well as your sample prep system ( in your case I assume 
     you have a basic vacuum line), try to use about 10g of Silver 
     Phosphate  (Aldrich Chem. Co., part no. 33,738-2, 25g bottles, light 
     sensitive) in a glass vacuum tube with a suitable tap.
     Simply freeze the contaminant CO2 gas into this tube, allow it to come 
     to the room temp for few minutes and than transfer the now cleaned CO2 
     into your sample tube.
     
     Other suitable methods might be, as you have mentioned, lead acetate, 
     but also lead oxide, n-pentane or a GC.
     
     Hope this helped!
     
     Zarko Roksandic
     AGSO
     Isotope & Organic Geochem Lab.
     Canberra ACT 
     Australia
     
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