I may recommend looking for some leaks.
N2 and O2 can react in the ion source producing some NO2, m/z is 46.

Sergey



Quoting Penny Higgins <[log in to unmask]>:

> Hi all,
>
> We've been running carbonates (CO2) on our Delta Plus XL, via the Gasbench
> and PAL, for nearly a year now.  Everything has been running smoothly.
> Alas, in the past few weeks I've noticed that the mass 46 background has
> increased dramatically from 10-20mV to nearly 50mV (sometimes more). Our
> standard deviation on d18O (based on ten peaks) has increased to 0.15 or
> much higher. Carbon is still running very well, and I am certain that water
> in the sample is not getting to the mass spectrometer source. We check
> linearity every day before running analyses, so I know that the instrument
> is performing magnificently and is linear. The mass 46 background on the
> reference peaks is generally much lower than that of the sample peaks,
> though these days still tops 20mV.
>
> Does anyone know where the excess mass 46 is coming from. There appears to
> still be a clear separation between the nitrogen peak and the carbon
> dioxide peak coming out of the GC column. However, I wonder if the GC
> column may yet be bad. We did once draw phosphoric acid into the GasBench
> (a procedural problem we have since fixed). I wonder if this may have
> damaged the GC column?
>
> Any other thoughts?
>
> Thanks,
>
> ~Penny
>
> *******************************************************************
>                        Dr. Pennilyn Higgins
>                         Research Associate
>
>                              "SIREAL"
>    Stable Isotope Ratios in the Environment Analytical Laboratory
>
>          Department of Earth and Environmental Sciences
>          University of Rochester
>          227 Hutchison Hall
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>
>                     [log in to unmask]
>
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