Thanks for the thoughts, John.

My backup plan is to control for this effect - using pre-screening, manual dilutions, and standard corrections. In fact it's what we've been doing. I was hoping I could get rid of the effect, but it may not be worth it or even possible.

Thanks again,


On Thu, Jan 19, 2017 at 1:37 PM, John Howa <[log in to unmask]> wrote:

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.


Two things are infinite: the universe and human stupidity; and I'm not sure about the universe. - Einstein

Jeff Salacup, Ph.D.
Stable Isotope & Biogeochemistry Lab Manager