HI Dana--

Refocus?  So it's not just a peak-centering issue..hmmm...  And what are 
the background gases like?

It may be time to clean the source.  Alternatively, if there are large 
changes in the water background, things can get screwy.  In that case, 
you may need to clean the nafion tubing. 

good luck...

gerry

Dana Biasatti wrote:

>Hello,
>
>I am having a problem with our Delta plus XP, and I am hoping that someone
>may be able to offer some helpful insight.
>
>Last year, we were able to run 70+ carbonate samples at a time on the gas
>bench, with no problems, meaning that we were getting good results for
>almost all samples (SD <0.1 per mil).
>
>Over last summer, we had some problems with leaky SGE valves and that
>problem was resolved.
>
>Now, we can no longer run 70+ samples without encountering problems. During
>a run, the first 10 to 30 samples will run fine (SD <0.1 per mil), but then
>the noise suddenly increases (samples have SD's >0.1 per mil and sometimes
>  
>
>>1.0 per mil). So we have to stop the run, re-focus the mass spec, and
>>    
>>
>restart the run. After the mass spec is refocused, it will run fine again
>for the first 10 to 30 samples (the number is highly variable), and then we
>have to stop again and repeat the above process.
>
>Has anyone else had this same experience? Any advice would be greatly
>appreciated.
>
>Dana Biasatti
>
>  
>