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
- 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
Hope this helps,
> 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?
Assistant Professor of Enology
Dept. of Food Science & Technology
NYS Ag. Experiment Station
Geneva, NY 14456