Bill, This is interesting! When I carried out an Inter-lab comparison exercise earlier this year (subscribers may recall my asking for volunteers) the one outstanding result was the total lack of agreement on 15-N values for the KNO3 I sent out. This ties in with my experience here using a Europa roboprep/tracermass system where I get a different result for this material every time I run it. Again, other materials (Ammonium sulphate, Acetanilide etc) pose no problems. I shall try the sugar idea. Darren Sleep ITE Merlewood Grange-over-Sands Cumbria UK