Dear Paul,
Did you set this -350 permil in Isodat? If I am correct, Isodat is using 
a simple normalization equation versus tank value: dtrue = deltameas + 
deltalabgas + (deltameas x deltalabgas)/1000. If you are doing this way 
and then you are using regression line basis on 2 or 3 points, you are 
doing somehow double normalization. I prefer to have in software 0 for 
tank reference gas and use a multipoints regression line for normalization.

There is a big difference between normalization methods, just due to 
math, see e.g. Paul D., Skrzypek G., Forizs I, 2007, Normalization of 
measured stable isotope composition to isotope reference scale -- a 
review, Rapid Commun. Mass Spectrom 21: 3006-3014.


best regards


Greg



-------------------------------------------------------------
Dr Grzegorz Skrzypek, Research Assistant Professor

West Australian Biogeochemistry Centre
John de Laeter Centre of Mass Spectrometry
School of Plant Biology, The University of Western Australia
35 Stirling Highway (MO90), Crawley WA 6009, Australia

Phone +61 8 6488 4584, Fax +61 8 64887925
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Office - room G16; Lab - room G15; Botany Building



Paul Eby wrote:
> Listmembers,
>
> I've had a few off-list responses to this issue that suggested that 
> the problem was the dD value of the reference gas: and they were right.
>
> If I set the value of the reference gas to be 0.0, SMOW measures at 
> +539 and SLAP at -118 (difference of 657, slope of 0.6). If I 
> recalculate those same runs with a ref gas value of -350 permil, SMOW 
> measures at 0 permil and SLAP at -427 (difference of 427). That's a 
> slope of 1.0 .....
>
> I'm still trying to wrap my head around this mathematical vulgarity.
>
> Paul Eby
> Alberta Research Council
>
>
>
>
>
>> Paul Eby wrote:
>>> Listmembers,
>>>
>>> Has anyone ever seen a calibration slope (known and measured values 
>>> of two standards) deviate significantly from the expected 0.95 to 
>>> 1.05? We are consistently seeing 0.67.
>>>
>>> This is a new Delta Advantage. d13C and d18O on a pair of OzTech CO2 
>>> standards by dual inlet both have a slope of approximately 1, so the 
>>> instrument is otherwise working fine (similar results by GasBench).
>>>
>>> Hydrogen is a different story: a pair of OzTech standards by dual 
>>> inlet has a consistent slope of 0.67, and SMOW/GISP/SLAP by H/Device 
>>> shows similar results. H3 is consistently around 0.6, and the 
>>> calibration performed on the H/Device seems stable enough that I 
>>> could probably ignore the issue and run.
>>>
>>> Any suggestions out there on how this could be?
>>>
>>> Paul Eby
>>>
>>>
>>> [log in to unmask]
>>> Alberta Research Council
>>>
>>> Vancouver Island Technology Park
>>> 3 - 4476 Markham St.
>>> Victoria, BC, Canada V8Z 7X8
>>>
>>> Phone: 250-483-3290
>>> Fax: 250-483-1989
>