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]