Dear Jan
No excel file has arrived to me, but anyway, the first I would do is  
just run standard on/off in backflush on mode and see if the problem  
is related with the ion source tunning or even with your CO2 tank. Of  
course there will be influence on the delta value but this will depend  
on how good your linearity is and if you are inside the range where  
the linearity is good enough for you.
Good luck
Rafael


Quoting Jan Vermeulen <[log in to unmask]>:

>
> I have a delta XP connected to a GC/C III.
>
> I'm measuring FAME's on the GC.
> Now I've seen that the reference gas peaks are decreasing in fuction  
>  of time. Inside one run, I have ref peaks in the beginning and at   
> the end. Now the peaks at the end are lower than in the beginning.   
> (in peak area it is a difference of +- 3) I've attached an exell   
> file with the results of a sequence of 7 samples out of the same   
> vial and the same conditions. Only 3 samples are put inside the   
> exell file. nr. 3,5and 7. The delta value is not put in but seems to  
>  be oké.
>
> Now, is there an influence on the delta value if the reference peaks  
>  are decreasing??
> Is there an influence on the peak area of the samples ??
>
> If so, what can I do ?
>
> Kind regards,
> Jan
> --
> Jan Vermeulen
> ISOFYS-Laboratory Of Applied Physical Chemistry
> AMBERLab Advanced Mass spectrometry for Bioscience Engineering   
> Research Laboratory
> Ghent University
> Coupure Links, 653
> 9000 Gent
> Belgium
> http://www.isofys.ugent.be
> http://www.amberlab.ugent.be
> tel.: +32 9 264 6095
>       +32 9 264 5864
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>