Dear David,


I'm afraid to say either method you think of using will carry the risk of 
non-quantitative recovery and, hence isotopic fractionation.

Direct injection or cryo-focusing are the only methods that avoid this 
problem. That said, if you are only interested in a particular group of 
VOCs, say BTEX, you can try SPME. I am not saying this process is free of 
mass discrimination because it is not but you may be able to determine the 
extent of the isotope shift (for 13C in the region of 2-5 o/oo if memory 
serves) and how to control and, hence correct for it.

Incidentally, as far as I am aware the thorny issue of recovery of air 
sampling methods has prompted the EPA to revisit all air sampling methods 
(for VOCs) and instead of adsorbtive sampling followed by thermal 
desorption now recommends either bag or evacuated container (canister) 
sampling.

I have to admit I was rather pleased when I learned about this. Whenever I 
critised adsorptive VOC sampling methods (esp. Tennax) at conferences, 
workshops, meetings etc, I was made to feel as welcome as a garlic and holy 
water merchant at a vampires' convention. You can imagine how popular I was 
with the company reps.

As long as there is some recovery, one can correct for this effect using a 
standard calibration curve as long as one is only interested in presence 
and concentration thought even this can be dicey at the lower concentration 
end of things. For isotope analysis however, desorption tubes filled with 
Tennax are completely and utterly useless.


Regards,

Wolfram



On Jul 28 2006, Widory David wrote:

> Dear all, we are currently trying to set our equipment up for analysis 
> of atmospheric VOC (on a Delta plus XP). For sampling, we are tending 
> towards the use of small glass sorbent tubes. My main question would be 
> to identify what is the most appropriate device for introduction into the 
> GC-C/TC ? According to me we have to choose between a thermal desorption 
> unit and solvent extraction followed by liquid injection (with a GC-PAL 
> or similar). Can anyone share its experience on the pros and cons of each 
> technique, as well as related papers ? Thanking you in advance. 
> Cordially, David
>  
> 
> 
> Dr D. Widory
> 
> Stable Isotope Laboratory Manager
> 
> BRGM MMA/ISO
> 
> 3 avenue Claude Guillemin
> 
> BP 36009
> 
> 45060 Orléans Cedex 2
> 
> France
> 
> e-mail : [log in to unmask]
> 
> Tel : +33-(0)2 38 64 47 72
> 
> Fax : +33-(0)2 38 64 37 11
> 
>  
>  
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