Sometimes the issue is mechanical; i.e. the pnuematic piston to actuate the reference. The first time it is turned on after a bit of idle time, it can be slow to engage. You can easily check with the cover off your interface.
If this is the case, you may not see a problem after several on-offs in a row. But in the amount of time spent idle for peak center, sample drop, N2 peak, and CO2 peak, a crusty actuator could be more noticeable. We always prefer to use more than one reference pulse and evaluate with the latest pulse to account for allow for conditioning of the actuator. You could look at your std on offs early in the day or week and see if the first peak looks more disparaging.
It seems I replace one SMC piston in our Thermo interfaces every few years. They are about $30.
Hi Rich et al.,
Have you checked how your peak backgrounds are set? I'm only familiar with Isodat, but depending on your software and its peak background detection settings you might find the cause of your disparity. How much time do you have between your monitoring gas peak injections? The first injection might still be coming down (zoom in a lot on the spectrogram) when the second injection goes in....even a mV or two difference can influence the values.
Hope this helps!
From: Stable Isotope Geochemistry [[log in to unmask]]
on behalf of Dabundo, Richard C [[log in to unmask]]
Sent: Thursday, January 26, 2017 5:40 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] Elemental Analyzer CO2 reference peaks
I'm having an issue with CO2 reference peak deviation during an Elemental Analyzer N and C run. Two CO2 reference peaks are produced following the C sample peak. Originally, the first peak was consistently ~1.1‰ lighter than the second peak. After creating a new N2 to CO2 jump calibration, the first reference peak became consistently heavier than the second by about 0.6‰. We have sufficient distance between the sample and reference peaks, and the same issue results even if no samples are loaded. CO2 on-offs are fine, with a standard deviation of about 0.04‰. Does anyone know what may be causing our disparity?
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