Hi Penny,


There is another, perfectly 'normal' reason for high m/z 46, namely N2 and
O2 entering the source at the same time leading to the in-situ formation of
NO2; in other words a tiny leak somewhere.

Of course, in such a case N2O will also be formed (m/z 44) but the different
resistor settings cause a 'disproportionate' amplification of the m/z 46
signal (and m/z 45, and, hence a nice transient on the 45/44 ratio trace).

For those interested replicating this effect under controlled conditions;
inject a bit of air onto a PLOT column that separates CO2 from N2 and O2,
but not N2 from O2.


Cheers,

Wolfram




> -----Original Message-----
> From: Stable Isotope Geochemistry 
> [mailto:[log in to unmask]] On Behalf Of Penny Higgins
> Sent: 06 March 2006 14:57
> To: [log in to unmask]
> Subject: High Mass 46 background
> 
> 
> 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
>          Rochester, NY 14627
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