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hI Jeff--

I haven't played with this in a while but some problems that can happen....(yes, an excuse to give general advice)

1.  Loss of reactor conditioning.  The carbon reacts with water/air that migrates thru reactor and/or the reactor itself.  The conditioning runs coat the reactor and the sample runs also help recondition it.  Eventually the reactor becomes so permeable that the carbon coating burns off quickly--and it's no longer a carbon reducing tube.  When a reactor no longer holds the conditioning--time for a new one.  That is, runs are fine just after conditioning--but keep getting worse during the day.  Note--high temps are more problematic.

2. Baseline shifts as temp ramps--and it can be difficult to get the "dynamic background" to do what you want.   Dirty samples can be difficult--as can failing GC columns.

3.  Fractionation in the injector port.  I'm guessing that is not an issue here, but can be a problem.  Cored septa, dirty liners, leaky valves. can all do weird stuff.

4. "tee" in oven and back flush valve--and the "tee" might be a four way union.  Depending on set up, your valve could be inside or outside the GC oven (pnuematic controls outside oven in all cases).  The fun thing is, sometimes leaks open up at temp.  You can see bubbling by sticking end of vent line into vial of water.  The occasional bubble when GC ramping up will probably just be thermal expansion of the air in the line.

5.  Leaks down stream--though some intentional, e.g. there should be a tiny leak out the backflush line at the other end, where the He comes in.  The tiny capillary at that valve is a bleed out line--and keeps air from getting into main flow.  You might also have a leak at the open split--the feed capillary might be sliding out of the epoxy (not likely, but it does happen).

Tests:

Standards available from Arndt Schimmelmann: http://pages.iu.edu/~aschimme/hc.html

for alkanes, Mix A is a series of compounds at similar conc.  Mix B has a saw tooth pattern in conc.

Similar conc. mix means latter peaks can run wider and at small amplitude (depends on temp gradient)--so same area, different amplitude.  In that case, a linearity problem will show up as a shift in D/H over time.  Saw tooth pattern mixture can help tease out difference of issue with linearity and issue with drift over time.  If expect areas drop over time, then you are losing sample as system heats up--which might point to an issue with the injector amongst other issues. And of course Ar leak checking.

good luck,

gerry



On 1/19/2017 1:41 PM, Jeff Salacup wrote:
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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,

Jeff


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.



--
Jeff




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
UMass-Amherst