Hi Paul,


It would seem we have to agree to disagree on the thermal desorption point. 
Extracting adsorbed material from a Tennax tube with solvent to 100% 
recovery is possible but with some compounds this means using CS2, which is 
not nice.

I suppose it's horses for courses, there is no hard and fast rule what 
works and what not. It would seem I have been unlucky in my choice of which 
compounds I was interested in whereas you seem to have struck the mother 
lode with compounds that desorb without any problems.

A good example is SPME, it is possible to marry a particular SPME phase to 
a particular group of VOCs and get fraction free adsorption and desorption 
(fraction free = not measurable or not significant) but in my experience 
this is only true for 13C. However, try the same and analyse CSIA for 2H 
and let me know what you see.

I agree I am bit anal retentive when it comes to these things. Knowing the 
effect to be there even if it happens to disappear in the uncertainty noise 
already irritates me no end. So, for my personal feel-good factor, I'd 
rather use a method where I know nothing untowards happens.

Also, in a case when one doesn't know what to expect or doesn't want to 
discriminate towards or against a particular group of compounds, evacuated 
container (canister) sampling is still the safest bet (see EPA method 18 
for VOCs from ambient air); but it's neither the fastest nor the most 
convenient it has to be admitted.


Regards,

Wolfram



On Jul 31 2006, Paul Eby wrote:

> 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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