Greetings... I have been following the recent discussions on the list concerning correcting inorganic O and C isotope data for carbonate samples. We have a MAT 253 and a Kiel III device in our lab. We recently had the same discussion...why is the correction we are using additive rather than multiplicative (is that a word?). When we run NBS-19, values come out as follows: d13C = 1.82 ‰ and d18O = -2.37 ‰. I've always corrected my data by adding the necessary correction factor that brings NBS-19 in line, to all of my samples as well. Ideally, the reference gas would be calibrated so that this step is unnecessary, so I performed an experiment: Our reference gas values: d13C = -13. 36 ‰ and d18O -10.95 ‰. I pulled up an old run of NBS-19 and changed the reference gas values by the additive correction factor, so d13C = -13.36 + 0.13 = -13.23 ‰ and d18O = -10.95 + 0.17 = -10.78 ‰. Reevaluating this NBS-19 run with these values gave d13C = 1.95 ‰ and d18O = -2.20 ‰. I then took a few sample runs to see what would happen. One sample: d13C = 0.00 ‰ and d18O = 0.41‰. Changing the reference gas values for this sample and reevaluating gave d13C = 0.13 ‰ and d18O = 0.58 ‰, which is additive (this was a great test sample...0.00‰ should not have changed at all if the correction is multiplicative). I also tested a more depleted sample: original d13C = -5.60 ‰ and d18O = -8.01 ‰. With the new ref gas values this changed to d13C = -5.47 ‰ and d18O = -7.84 ‰, again which is additive. Unless there is some flaw in the way the Isodat 2.0 software is calculating delta values, this seems to me as proof that addition to correct works...at least in this range of sample data. I know of one motherland engineer who has stated that delta values are not additive in this way. Though the correction seems to hold within the data range above, as we move further away from NBS-19, such as around NBS-18 (d18O = -23 ‰), there will be an error that appears (maybe around 0.1‰ at this point). Would he care to comment with some formula examples to show the exact way of correcting data when your reference gas is not perfectly calibrated? Any other thoughts? 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]