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]
>
>

-- 

----------------------------------------
Dr. S. S. Assonov,
Max-Planck -Institute for Chemistry,
J.J.Becher Weg 27,
55128 Mainz, Germany

Snail mail: POBox 3060       55020 Mainz  


Phone:   +49 (0)6131-305-495
Fax:      +49 (0)6131-305-436