Greetings... I have a question of interest...maybe someone can point me to a reference, or will be able to answer this question directly. My understanding is that typical gas mass spectrometers are designed to measure isotope ratios relative to some reference gas, hence the data reported are relative data. We may calibrate the reference gas directly, or run standards with our samples and calibrate offline, but the end result are delta values that are reported relative to some international standard. I've been told that we can more accurately make such a relative measurement than we can an absolute measurement. My questions are: if we know the absolute ratio of the international standard, can we not back calculate (through the definition of the delta value) the absolute ratio for the sample in question? How accurate is this process? How are gas mass specs designed differently to make absolute measurements directly, and how much more accurate are such measurements over doing the above mentioned calculation? Thanks... Tim ----------------------------------------------- Tim Prokopiuk B. Sc. Geology/Technician Saskatchewan Isotope Laboratory Room 241 Department of Geological Sciences University of Saskatchewan 114 Science Place Saskatoon, Saskatchewan, Canada S7N 5E2 Phone: (306) 966-5712 Fax: (306) 966-8593 Email: [log in to unmask]