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
> -----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
> 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?
> Dr. Robert van Geldern
> GeoZentrum Nordbayern / Applied Geosciences
> University of Erlangen-Nuremberg
> Schlossgarten 5
> 91054 Erlangen, Germany
> mailto:[log in to unmask]
> fon: +49(0)9131-85-24780 (office)
> fax: +49(0)9131-85-29294
> Room: O 0.106 (office)