hi Susan, I saw your query about S linearity. We have both a Micromass Optima and a Finnigan Delta Plus XL on which we run S. The Optima is linear in a rather small range, about 1-3.5 nA, the trap current is set at 400uA. I think this is different than the "recommended" trap current. I've seen this setting make a difference for C and N linearity, so maybe that is where your S linearity problem lies as well... For the Delta, we haven't run as much S on this machine but I have had occasional linearity problems, but this seems to happen when the source gets dirty. I have compensated by turning the extraction pot back and retuning everything else. By trial and error I've been able to find an extraction setting where the linearity is decent without compromising the sensitivity. If the source is dirty this quick fix doesn't last very long. So if you have a Finnigan, maybe try turning the extraction down and retuning. The linear range for the Finnigan is better, about 1-4.5V. We run BaSO4 standards and ZnS standards. hope that might help! Cyndi __________________________ Cyndi Kester Ecologist U.S. Geological Survey DFC, Building 21, MS 963 Denver, CO 80225 phone: 303-236-0278 fax: 303-236-4930 email: [log in to unmask]