Dear Dr Sarkar and others,

I cannot resist to answer on your informative question on the list.
Changing from dual inlet into CF mode should not be the problem. Changing from one set of masses into another is just the routine operation with normally also not too many problems – specially with modern machines. What really should worry you more is the swapping from analyzing one type of material to others – here commonly are more problems. It is not ideal to have a number of different ‘periferals’ devoted to the same machine. It is possible, but may give nasty problems which are not always easy to understand why they occur. But mostly they are not MS problems, but from the ‘preparation line’ producing the gases  for analysis (It is informative to look into the archives of this list for questions on problems in such systems...). If a machine is devoted to just one technique things are easier but often far from ideal or simple as some want us to believe – depends a lot on experience and skills of the operator(s). With limited money you might choose for the ‘difficult solution’, or you want too much for just a little bit of money...

About the ‘precision’ of measuring isotopes in carbonate as you mention, this is certainly NOT an external precision. It just presents the precision of the CO2 measured on the MS and does not include the uncertainty (rather than precision metrologists use uncertainty; see for definition the GUM: Guide on Uncertainty Measurement) of the preparation, which basically is any pretreatment of the sample, including eventual removal of organic compounds in the carbonate and the acid digestion as most important factors. Moreover, the (corrected) delta values are not disputed, but the (combined)uncertainty of the measurement is. Unfortunately this has become common practice in stable isotope (and not only there) analysis. A small uncertainty (precision) only is good if it is realistic...
The precision (uncertainty) as reported for measuring CO2 gas on a MS is very realistic and no problem with that, as long as it is given as ‘machine precision/uncertainty’.
I think it is sort of similar tendency as happened with radiometric age dating in earth sciences: the analyst with the oldest date was the best...!?! With the stable isotopes it seems to be the tendency who has the smallest precision is the best – who is fooling whom?!?
For hydrogen it is even more true – ever seen anybody include the relatively large uncertainty related to the H3+ correction? (Even more a question mark if considering this correction for CF-IRMS systems, where it is an ‘intelligent approximation’).
Finally I like to state not to criticize anyone specifically or any company. It is not a question who started this wrong concept, but rather how to get it right...
I expect quite some comment on this, and will be happy to see a discussion.

Best wishes,
Pier.

 



Dear Isogeochemists

We are planning to purchase an IRMS system with peripherals where we would like to analyse carbonates (including ocean cores), water (both O & H) and sedimentary organic matter (C& N). Money is limited and hence we have to make a choice. GVI claims that their multiprep dual inlet system can be switched over through a change over to continuous flow (CF) configuration attached to an elemental analyser to perform all the above analyses. Question is does it not affect the stability of the machine or beam? Even if one plans a switch over which is not so frequent how good is the reproducibility? in other words does anyone has experience in using same dual inlet machine in CF mode?

          The other thing is the difference in precision between a normal multiprep type dual inlet carbonate system (not Kiel type) and a gas bench. Finnigan gas bench claims to get ~0.06 per mil external precision in oxygen in carbonate analyses. Is it universally true? Inherently, I thought, CF precision are much lower than DI. Any suggestion in reagrd of above points will be highly appreciated.

Regards.

---------------------------------------------------------
Dr. Anindya Sarkar
Associate Professor
Department of Geology & Geophysics
Indian Institute of Technology
Kharagpur 721 302
Tel: 0091-3222-283392 (O) 283393 / 220184 (R)
Fax: 0091-3222-282268
email: [log in to unmask]
Home page: http://anindya-sarkar.tripod.com
---------------------------------------------------------
 

*****************************************
This Mail is Certified to be Virus Free.
CIC Network Security Group, IIT Kharagpur
*****************************************