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Hi Jeff,


Further to John's valid comments there is also the potential for some "compound specific" sample loss and thus fractionation during injection. This will show up as delta values not being what one would expect and peak sizes that are less than what they should be given the amount of hydrogen (or carbon) present.

If one injects in split/splitless mode there is a risk (depending on material transfer rate) of loss of sample constituents depending on formula weight and volatility (bp).

As a pre-emptive approach I always inject samples in splitless/split mode.  For example, inject with the split closed and keeping it closed for 60s while opening the split thereafter for the rest of the run.

Not sure if this will cure your problem.  Just something to look out for and consider.


Best,

Wolfram


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Prof. Dr W Meier-Augenstein, CChem, FRSC
Stable Isotope Forensics & Analytical Sciences

Robert Gordon University
School of Pharmacy and Life Sciences
The Ian Wood Building
Garthdee Road
Aberdeen
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________________________________________
From: Stable Isotope Geochemistry <[log in to unmask]> on behalf of John Howa <[log in to unmask]>
Sent: 19 January 2017 18:37:37
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] dD and linearity after H3

Jeff,

I am not an expert in compound-specific isotope measurements, but I do know there are several other factors to consider besides H3+ and nonlinearity in the detectors. As mentioned before, incomplete conversion of your alkanes to H2 would cause an apparent fractionation effect correlating to peak height, if a greater fraction is lost at lower sample mass. If you can discount the reactor, the GC column may be retaining sample that does not get integrated into your peaks. This would likely cause poor chromatography, which can effectively bias your samples based on peak size. Even if your peaks are sharp, some sample may be retained on the inlet, which also would cause a similar-looking delta-to-size correlation. I'm not saying you should give up on the goal of wide size dynamic ranges, but it may be easier to just run standards that can correct your data given the potential biases, or pre-screen samples for manual dilutions or concentrations.

John Howa

IsoForensics, Inc.

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