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Dear Bethany,


What you describe suggests the tune / peak centre settings are fine for stability but do not yield good isotopic linearity once an EA-IRMS methods loads the gas configuration settings. For a system meeting isotopic linearity specs (SD for multiple cylinder gas peaks of different size being 0.06 or better).

Given m/z28 signal is comparable between stability test and EA-IRMS runs but m/z29 signals are not sounds like the EA-IRMS method loads peak centre settings  different  to those used for stability testing.  While the ratio between 29/28 signal ratio based on the mV value for stability test sounds perfectly normal, the 29/28 signal ratio for cylinder gas peaks in EA-IRMS runs is not.  The most likely explanation would be for the peak centre settings applied during EA-IRMS runs resulting for the peak window to be centred on either the left or the right flank of the peak where the plateaus of 28 and 29 no longer run parallel but 29 is either in the ascend or the descend.

The screen shots you attached would seem to support the above hypothesis.  The stability test shows an HV setting of 2892 V while the EA-IRMS analysis is run at an HV setting of 2994 V yet for the same magnet setting; a difference of 102 V.

Conversely, the EA-IRMS sequence is being run without the Peak Centre cell being ticked (while the stability test is) and HV has drifted by 102 V and is no longer centred on the plateau (something that "should" not happen, esp. in a temperature controlled lab).

Like new tune settings, new peak centre settings need to be saved / passed on to the corresponding Gas Configuration as this is the location where run methods look for instrument information.


Hope this helps.


Best,

Wolfram

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Prof. Dr W Meier-Augenstein, CChem, FRSC
Stable Isotope Forensics & Analytical Sciences

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Stable Isotope Forensics book, 2nd edition:
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From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Theiling, Bethany
Sent: 04 December 2018 19:30
To: [log in to unmask]
Subject: [ISOGEOCHEM] ref gas ratios vary between on-off and analysis


Hi everyone,

I recently cleaned our Delta V source, replaced the filament, and re-tuned. Everything appeared to be OK. Using my new tune file, the reference gas d13C and d15N are spot-on, and stdev for multiple ref gas peaks = 0.01 or better.

I tried to run some standards on the EA today, and the ref gas isotope ratios are several per mil off, resulting in completely nonsense isotope ratios for my standards. I've tried using our old tune file, a new tune file (from last week), and an even newer tune file from today, to no improvement.

I finally realized that the mass intensities were varying for my reference gases between the on-off tests and my analysis. For example, mass 28 is ~6570mV for my on-off test and during a typical analysis, but mass 29 is ~4798 during the on-offs and ~2170 during an analysis. Both of these tests were performed with the source open to the conflo and Flash HT EA--the exact same conditions. The focus delta shows the same parameters are consistent for on-offs and the analysis. I can run several on-offs and several standards with the same results. Further, the same problem occurs if I use a different tune file. The same problem occurs for CO2 isotopologues.

Any ideas? I haven't this this one before...

Thanks,

Bethany








Bethany Theiling, Ph.D.
Assistant Professor, Geochemistry
Department of Geosciences
University of Tulsa
800 Tucker Drive
Tulsa, OK 74104







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