Hi Jennifer,

Is the flow rate changing over the course of the run? We observe drift over
the course of a run as the magnesium perchlorate trap accumulates more and
more of a densely packed plug and the flow rate decreases. Presumably you
have baked the GC over a weekend? Why do you suspect the GC column? Do the
peak shapes change or the time between N and C?


On Thu, Oct 20, 2016 at 3:04 PM, Jen McKay <[log in to unmask]>

> Hi Everyone,
> I’ve notice something a bit strange and was hoping to get some input.
> Recently, I’ve noticed a large drift in the raw d13C values from the
> standards at the start to those at the end. Both standards show the same
> amount of drift from the start to the end and so do the check standards
> that are at the start and end. However, the check standard in the middle is
> always a bit (~0.2 permil) lighter.  Now if I correct the run just using
> the standards at the end the middle and end check standards are perfect but
> the standards at the start of way off.  This suggests to me that the drift,
> if you can call it that, is only happening at the beginning of the run and
> by the time half the samples have run it has disappeared.  Now, I should
> mention that this happens if I’m running C and N together or just C, the
> d15N do not exhibit a drift, and the stabilities look OK.
> Any ideas about what may be going on would be greatly appreciated.
> Thanks,
> Jennifer
> P.S. I am about to purchase a new GC column for the EA because that is the
> only thing I can think to blame.
> Jennifer McKay
> Assistant Professor (Senior Research)
> Stable Isotope Lab Manager
> CEOAS, Oregon State University
> Corvallis, Oregon 97331
> 541-737-4054

Andrew Schauer
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