Hi Chancy-

  It doesn't seem that anyone has replied yet, so I'll take a shot.
  I suspect you are seeing the impact of the slow detector response for 
the m/z=3 cup resulting in spectral broadening.  Due to the low natural 
abundance concentration of D, a very high value feedback resistor is 
selected (300 G-Ohm, I think).  I don't have the 253 diagrams in front 
of me, but I believe the capacitance of the feedback circuit is 2pF.  
This yields an RC time constant, tau, of 600 ms.
 One can calculate the degree of broadening and time shift from 
Butterworth, Journal of Scientific Instruments, p1165 (1968).    
Assuming your initial peak on the m/z=2 trace is 2s (full width half 
max), then then the mass 3 cup should trail mass 2 by ~500 ms, and 
should also demonstrate tailing and be wider than the mass 2 peak.  
These effects will be slightly mitigated by the occurrence of the 
chromatographic time shift.

  To handle the problem, you could:
- make sure you have excellent separation of peaks, and set the 
integration window wider manually to ensure you are capturing the entire 
m/z=3 tail
- Dumb down your chromatography: use a wider bore column that will 
generate broader peaks.  Unfortunately, this will also lead to longer 
runs and a loss of sensitivity
- Swap out the 300 G-Ohm resistor for something lower, although this may 
limit your precision for trace components as you'll be fighting 
background noise.

Hope this helps,
Gavin

 

> we're doing GC-pyrolysis on a MAT-253 for H2 isotope analysis. However, our 
> chromatography looks a little odd in a sense that the mass 3 trace tails behind 
> the mass 2 trace of the chromatography peaks and the mass 2 ad 3 apeces 
> do not always match: an inverse time-shift so-to-speak. Any word of wisdom 
> on how to handle this problem?
>   


-- 
Gavin Sacks
Assistant Professor of Enology

Dept. of Food Science & Technology
NYS Ag. Experiment Station
Cornell University
Geneva, NY 14456

315-787-2458 (w)
607-592-1504 (c)