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:

> 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.


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

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