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 ****************************************************** 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 AB10 7GJ E-mail: [log in to unmask] http://scholar.google.co.uk/citations?user=ty6c5IMAAAAJ ________________________________________ 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. ---------- This email has been scanned for spam and viruses by Proofpoint Essentials. Visit the following link to report this email as spam: https://eu1.proofpointessentials.com/index01.php?mod_id=11&mod_option=logitem&mail_id=1484851183-GdD71DEM0q3s&r_address=w.meier-augenstein%40rgu.ac.uk&report=1 ________________________________ Robert Gordon University is the top university for graduate jobs in the UK HESA July 2015 Robert Gordon University, a Scottish charity registered under charity number SC 013781. This e-mail and any attachment is for authorised use by the intended recipient(s) only. It may contain proprietary material, confidential information and/or be subject to legal privilege. It should not be copied, disclosed to, retained or used by, any other party. If you are not an intended recipient then please promptly delete this e-mail and any attachment and all copies and inform the sender. Please note that any views or opinions presented in this email are solely those of the author and do not necessarily represent those of Robert Gordon University. Thank you.