I would have to disagree with Wolfram.

We have used both thermal desorption and solvent 
extraction with sorbent tubes for isotopic 
analyses quite successfully. Fractionation is 
always a concern, as Wolfram says. However, each 
such project for us begins with extensive tests 
establishing that we can sample a standard mix of 
known isotopic composition and get reliable 
measurements after sorption/desorption without an appreciable shift.

We have had similar experience with SPME.

Paul Eby
University of Victoria


At 02:07 AM 29/07/2006, you wrote:
>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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