Dear All,

We struggle with a strange phenomenon with our new delta + XP & Gasbench 
III. It worked perfectly with carbonates, but about a month ago the d13C 
and d18O values started to shift during analyses. In a set of 40 samples 
(including standards) the d13C and d18O values shifted by about 1 
permill, in opposite directions (d18O positive, d13C negative). Then, 
the shift turned to a sinusoidal drift, again C and O isotopes moving in 
opposite directions. We checked the system with pure CO2, the analyses 
were perfect. We changed the acid, no improvement. After a set of 
analyses, the samples were re-analysed the next day, the shift was much 
less significant (about 0.3 permill, nice linear, correctable).

The system was tuned to linearity, and there is no correlation with 
sample size.
Originally we analysed 7 sample peaks, then changed to analysing 4 peaks 
(to reduce GC & Nafion saturation). No improvement, the analyses are 
getting worse and worse.

d18O and dD on waters (equilibration with CO2 and H2) work nicely, so 
the problem is related to the carbonates.

Can you give any hint what the cause of this strange behaviour may be?
Thanks a lot in advance.
Attila

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Dr. Attila Demeny
Geokemiai Kutatointezet - Institute for Geochemical Research
Magyar Tudomanyos Akademia - Hungarian Academy for Sciences
Budapest, Budaorsi ut 45, H-1112, Hungary
Tel: (361-)309-2600/ ext. 1223 (office), 1394 (lab)
Tel/fax: (361-)319-3137
e-mail: [log in to unmask]
Http://www.geochem.hu/
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