Penny,

I had on one occasion a high mass 46 signal, and normal 44,45. It was 
due to something coming off our Nafion water trap: it had become 
saturated with water, and we heated it a bit to drive it off. The 
side effect was some sort of thermal decomposition product with mass 
46.... Replacing the Nafion fixed the problem.

Paul Eby
University of Victoria


At 06:57 AM 06/03/2006, you wrote:
>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
>
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>                       Dr. Pennilyn Higgins
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