Dear Willi Brand

Let me intervene the discussion about the SMOW-SLAP differences at this 
point and clarify Jim O'Neil's statement that one cannot "get anywhere near 
the correct dD value when it is large" with a modern machine (Delta-S).
        Below are the values for our last calibration at Michigan (or at 
least the last calibration I performed when I was still at Michigan = Sept. 
1994). Calibrating our reference gas (-125.8 permil) against a value for 
SMOW of 0 permil, our SLAP (extracted on the same day, in the same manner - 
using Zn in silica tubes) had an average value of -421.8 permil. GISP (also 
extracted the same day, etc.) had an average value of -187.7 permil. The H3+ 
factor was measured before each sample and was found to vary very little 
between samples (range of values was between 22.930 and 23.017 (mA^-1) for 
that day). To bring values of SLAP to -428 permil we, therefore, used a 
multiplicative correction factor of 1.014 which then gives a value for GISP 
of -190.4 permil.
The previous calibration (Feb. 1993) was very similar, even though the value 
for the reference gas was distinctly different (-51 permil). Values for SLAP 
averaged -421.6 permil, while GISP averaged -186.1 permil, again requiring a 
correction factor bejond the H3+ correction. 
We have heard that the values we get for SLAP using our Delta-S (purchased 
in 1989) are very similar to those obtained in other laboratories which use 
the 251. Indeed, gases (large batches of which have been prepared from 
mixtures of commercially available H2 and HD) that we have sent to four 
other laboratories for D/H measurement gave values that were generally 
within 2-3 permil of our values IF THOSE GASES WERE ANALYZED ON FINNIGAN 
MAT'S, but could differ by 10 to 70 permil from our values when analyzed on 
VG machines.The VG-machines always had lower dD values for the gases and the 
difference was a function of the dD value of the gas - the more depleted the 
gas was in D, the bigger the difference. The large differences of 70 permil 
were thus observed for gases of dD = -860 permil while a 10 permil 
difference was observed for a gas of about -336 permil. All labs used SMOW 
and SLAP for their calibration and while some used Zn as reductant for H2O, 
others used uranium. No differences were observed between labs using either 
zinc or uranium, AS LONG AS these labs analyzed the H2 with the same mass 
spectrometer.

Does that clarify matters somewhat? 
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Dr. Torsten W. Vennemann
Institut fuer Geochemie
Wilhelmstr. 56
D-72074 Tuebingen
Germany
phone ++49-(0)7071-294992
Fax ++49-(0)7071-296870
e-mail: [log in to unmask]
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