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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:
> 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] 
> <mailto:[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*
>