The CAPS unit of SERCON GEO 20-20 IRMS developed some problems. Now we 
are doing the carbon and oxygen isotope analyses using the WES for the last 
40 days or so. The instrument as such was not used for carbonate analysis for 
quite sometime and due to recurring power problems the instrument was not 
used continuously for over six months. Now the power problem has been 
sorted out and we have started analysing carbonate samples using WES.

Consistently we have been getting inaccurate values for 13C while 18O seems 
alright for NBS 18. Another internal standard also gave erroneous values. The 
GC was baked, the mass spec was also baked externally. Supposing the 
orthophosphoric acid used is not of the right concentration will it have any 
preferential effect on 13C but not on 18O? (sounds illogical though). The drift 
in 13C for subsequent analyses in a batch increases consistently while 18O is 
alright. The drift correction does not do anything to the data. Is it possible 
that somewhere there is contamination and if so where could that be from? 
Will the water scrubber/Nafion contribute to carbon by any chance? Can 
somebody help me on this?

NB. My experience is with radiogenic isotopes and am pretty new to stable 
isotopes.


Dr. R. Anand
Mineralogist
PPOD Division
Geological Survey of India
40th Cross, Eshwar Nagar
Bengaluru 560 070
Phone: +91 80 26715686
Mobile: +91 96207 28953