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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?

andy

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




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Andrew Schauer
206.543.6327
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http://isolab.ess.washington.edu/