Easy way round: Enter zeros for the gas into Isodat and the intercept of your normalisation equation is the value of the gas. That's assuming that you using a two/multi point normalisation. Otherwise with NBS 19 alone: two steps, raw delta vs gas converts to gas vs raw delta with (1/(d/1000+1)-1)x1000; gas vs raw delta converts to gas vs PDB with da-db+(daxdb)/1000 Cheers > -----Original Message----- > From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On > Behalf Of Robert van Geldern > Sent: Friday, 9 July 2010 6:38 p.m. > To: [log in to unmask] > Subject: [ISOGEOCHEM] Calculating the delta of the working gas > > Greetings, > I assume this is an easy question but it seems my last math class was a > long time ago. > Usually I don't care about the exact value of my CO2 ref gas on our > gasbench system since everything is normalizied to reference materials > with known values (and working gas cancels out). I just type in a value > for the working gas in the isodat standard editor which will give a > delta value close to the correct value. > > I need to calculate the CO2 value of the working gas now and my idea was > just to caluclate the things backwards. Assuming that my reference > material (e.g. NBS19) will produce a CO2 with a known value it should be > possible the calculate the delta value of the working gas etiher from > the raw delta values or the ratios given in Isodat by treating the ref > gas as the sample and the NBS 19 as the standard gas. However, sitting > in front of Isodat and the equations it is not so obvious anymore...can > anyone point me in the right direction? > > regards, > Robert > > -- > Dr. Robert van Geldern > GeoZentrum Nordbayern / Applied Geosciences > University of Erlangen-Nuremberg > Schlossgarten 5 > 91054 Erlangen, Germany > > mailto:[log in to unmask] > www.gzn.uni-erlangen.de > fon: +49(0)9131-85-24780 (office) > fax: +49(0)9131-85-29294 > Room: O 0.106 (office)