I think Sergey has described the main point why we measure the stable 
isotope ratios relative to standards (mass spec long-term instability).

Regarding the calculated absolute ratio: if the reported relative value 
is accurate, then the calculated ratio must be accurate (correct) as 
well. This case we have to calculate the uncertainty as well.

Regards
Istvan Forizs

Sergey Assonov wrote:

> Tim,
>
> I was and may will be still in this kitchen.
> The difference between mas-specs for absolute measurements is that 
> these should have very stable ion source and one to be sure in absence 
> of any discrimination effects in the ion source.
> Namely, that all produced ions are gone through the slits with the 
> same efficiency. For normal machine that is just opposite - you can 
> re-tune your gas machine a bit and you  will see that e.g. ratio of 
> voltages for m/z 45 to that on m/z /44 will be changed, may be for few 
> 0.1% or more. You can  trace this ratio for your ref. gas over say a 
> year, and you will see fluctuations.
> Thus relative measurements are always more stable than absolute ones. 
> Absolute calibration of ratios is 1000 or 1000 000 times more difficult.
>
> Regards,
> Sergey
>
> PS: some refs:
>
> Assonov, S. S. and Brenninkmeijer, C. A. M., A re-determination of 
> absolute values for 17RVPDB-CO2 and 17RVSMOW. Rapid Commun. Mass 
> Spectrom., 17, 1017-1029, 2003.
>
> Norgaard, J. V. et al., The International Measurement Evaluation 
> Programme, IMEP-8: carbon and oxygen isotope ratios in CO2. Anal. 
> Bioanal. Chem., 374, 1147-1154, 2002.
>
> Ding, T. et al., Preparation of two synthetic isotope mixtures for the 
> calibration of isotope amount ratio measurements of sulfur. 
> International Journal of Mass Spectrometry, 197, 131-137, 2000.
>
> Valkiers, S., Kipphardt, H., Ding, T., Damen, R., De Bievre, P. and 
> Taylor, P. D. P., A primary isotopic gas standard for sulfur in the 
> form of SF6 with Systeme International d'Unites traceable values for 
> isotopic composition and molar mass. International Journal of Mass 
> Spectrometry, 193, 1-8, 1999.
>
> The accuracy - Pier de Groot will certainly reply, he likes that.
>
>
>
> tcp121 wrote:
>
>> Greetings...
>>
>> I have a question of interest...maybe someone can point me to a 
>> reference, or will be able to answer this question directly.  My 
>> understanding is that typical gas mass spectrometers are designed to 
>> measure isotope ratios relative to some reference gas, hence the data 
>> reported are relative data.  We may calibrate the reference gas 
>> directly, or run standards with our samples and calibrate offline, 
>> but the end result are delta values that are reported relative to 
>> some international standard.  I've been told that we can more 
>> accurately make such a relative measurement than we can an absolute 
>> measurement.
>>
>> My questions are: if we know the absolute ratio of the international 
>> standard, can we not back calculate (through the definition of the 
>> delta value) the absolute ratio for the sample in question?  How 
>> accurate is this process?  How are gas mass specs designed 
>> differently to make absolute measurements directly, and how much more 
>> accurate are such measurements over doing the above mentioned 
>> calculation?
>>
>> Thanks...
>>
>> Tim
>>
>> -----------------------------------------------
>> Tim Prokopiuk
>> B. Sc. Geology/Technician
>> Saskatchewan Isotope Laboratory
>> Room 241
>> Department of Geological Sciences
>> University of Saskatchewan
>> 114 Science Place
>> Saskatoon, Saskatchewan, Canada
>> S7N 5E2
>> Phone:    (306) 966-5712
>> Fax:        (306) 966-8593
>> Email:    [log in to unmask]
>>
>>
>

-- 
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Dr. FÓRIZS István ......... Istvan FORIZS Ph.D.
Magyar Tudományos Akadémia, Geokémiai Kutatóintézet
Institute for Geochemical Research, Hungarian Academy of Sciences
Cím/Address: H-1112 Budapest, Budaörsi út 45.   Hungary
Tel./Phone: (36-1)-309-2600/ mellék/extension 1151
Fax: (36-1)-319-3137
Http://www.geokemia.hu/people/forizs_hu.html
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