Dear Devon, Jakov, Hagit and other interested in 2H or 13C analysis of halogenated organics,


Not seeing an immediate negative effect is not the same as it's not happening. In the words of Scotty, "you cannie change the laws of physics, capt'n".

In an ox/red system as used for 13C isotope analysis, chlorine will not (or should not) enter the source because primarily the ox reactor will mop up chlorine (and fluorine) as copper (and nickel) halide. Under conditions typically employed for running ox reactors, re-oxidising does zilch for reversing this effect because copper chloride (and even more so copper fluoride) is extremely stable and once formed will not revert to copper oxide and chlorine gas (the normal potential for the half reaction of 2Cl- to Cl2 and 2 e- is +1.36 V, which tells you how [un]-likely this reaction is). So, as time goes one the ox capacity of the ox reactor is reduced leading to a non-quantitative conversion of analyte into CO2; non-quantitative = mass discrimination = isotopic fractionation.

To put it another way, run a few injection of, say benzene followed by a few injections of dichloro-benzene (say 1 nmol on column) and then compare the peak areas. Since both compounds contain the same amount of carbon the peak areas should match. See for how long that's the case and then ask yourself if the evolved CO2 is still a true isotopic representation of the parent material if the CO2 peak area is less than 90% of what it should be.

For 2H the problem is caused by a different reaction but the outcome is the same, namely non-quantitative conversion of target compound into target gas. Here the competing side reaction/s reduce the yield of H2 and hence lead to isotopic fractionation, which impacts first on precision and over time leads to an increased off-set from the true value. A colleague and I reviewed a paper earlier this year or at the end of last year giving 2H data of DCE showing exactly that. By now the paper should have appeared in print (Rapid Commun Mass Spectrom).

To maker matters worse, here halogens will end up in the source thus potentially resulting in further unwanted effects impacting on H2 ionization (and H3 formation) plus reducing the life span of the filament.



Best,

Wolfram



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"Facts do not cease to exist because they are ignored."

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Dr W Meier-Augenstein
Senior Lecturer in Stable Isotope Forensics

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________________________________________
From: Stable Isotope Geochemistry [[log in to unmask]] On Behalf Of Hagit Affek [[log in to unmask]]
Sent: 02 November 2007 12:09
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] residual chlorine - a problem?

Following up on Devon's question: do people have problems running
similar samples, containing halogenated organics, for compound specific
Hydrogen isotopes?

thanks,
hagit.



Bolotin, Jakov wrote:
> Hi Devon,
>
> Not really. We have to reoxidise the reactor from time to time.
>
> Regards
>
> Jakov
>
> ------------------------------------------------------------------------
> *From:* Stable Isotope Geochemistry [mailto:[log in to unmask]]
> *On Behalf Of *Devon Rowe
> *Sent:* Mittwoch, 31. Oktober 2007 06:16
> *To:* [log in to unmask]
> *Subject:* [ISOGEOCHEM] residual chlorine - a problem?
>
> To any of you who run samples (e.g., industrial ground waters) with
> appreciable concentrations of chlorinated ethenes for carbon (via
> GC-C-IRMS), have you seen any negative effects (short term/long term)
> due to chlorines entering into your mass spec?
>
>
>
> Devon
>
> ------------------------------------------------------------------------
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