Hello gracious purveyors of isotope wisdom,

Following up:

- Starting with a dry and tight instrument (mass 18 = 352; mass 40 = 56 BF off, split in)

- I took the reactor temp from 1450 to 1420 in an attempt to better hold the deactivation.

- I deactivated by injecting 1 uL of hexane twice, separated by 20 mins. Splitless, BF off, split out

- About and hour later I did my H3 (3.183) and started a run of a dilution series (an actual dilution series, not just changing the split flow. All runs are splitless.)

- The 'area effect' is still there. From 1.7 V to 7.5 V there was a ~ 20 permil depletion (bigger peak = more depleted). This works out to ~ 3.4 permil/V.

- This feature is present across reactors... I've changed several.

Aaaand go.

Thanks for any thoughts.

Jeff

On Tue, Jan 17, 2017 at 1:21 PM, Jeff Salacup <[log in to unmask]> wrote:

Thanks, Paul.

We're currently running at 1450, so I'm hesitant to get hotter. Deactivation seems like a possibility though, especially given the high temp we're running. Based on some of the reading I've done in the archives (too much vs. too little graphite in the reactor), it seems like an easy way to mess up the results. I'm going to recondition (deactivate) the reactor and see if that changes anything.

Best,

On Tue, Jan 17, 2017 at 12:09 PM, Paul Eby <[log in to unmask]> wrote:
Jeff,

If your sample peaks show a size effect and your ref peaks don’t, then the answer could be in the reactor. If the conversion process is incomplete, you could easily get this result. You could try increasing the reactor temperature or looking at the whether the tube is sufficiently deactivated.

I’m also curious about the dilution series that you run: are you changing the concentration of the standard, or perhaps are you changing the split flow to change the peak height?

Paul Eby

Stable Isotope Lab Manager
InnoTech Alberta

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From: Stable Isotope Geochemistry [mailto:[log in to unmask]U] On Behalf Of Jeff Salacup
Sent: January-17-17 8:07 AM
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] dD and linearity after H3

Thanks, Matheus.

I've got an older GCC III that I have to manually adjust.

So, if I do a series of on/offs, 8 refgas peaks, of increasing size from 1 V to 10 V I get a much smaller effect of ~ 0.3 permil/V with an H3 of ~ 3.2 applied. This same H3 is applied to my alkane standards, but I still have the effect I described above.

Grumble...

--
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

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