In a previous posting, I mentioned that we were having problems
analyzing O2 on our GasBench, which is connected to a Delta XL mass
spectrometer. It seems that the mass 33 trace is offset from the mass
32 & mass 34 traces and as such measures the mass 33 peak off to the
side of the peak. This results in relatively poor counting for O17
(33/32). We believe that the culprit is with the resistor-capacitor
pair that we manually install in our memco housing in order to
amplify the mass 33 signal.
We currently swap out the N2-CO2 resistor/capacitor combo in cup 3
(3x10e10 resistor and 5 pF capacitor) with a 1x10e11 resistor and a 2
capacitor, to amplify the O2 mass 33 signal.
We've noticed for a while the relative instability of the mass 33 peak
and the subsequent poor counting in the 33/32 O17 signal. In the
chromatograms, we see an offset in the 33 peak relative to
the 32 & 34 peaks which we attribute to time differences in the
capacitor that is used to measure O2 in cup 3. This is a 2 pF
capacitor. In the old days we ran O2 on a FMat 251 with a 1 pF capacitor
and I don't recall seeing this problem. Now maybe I'm comparing apples
and oranges here, but I'm wondering if we could replace the 2 pF
capacitor with a 1 pF capacitor and improve our mass 33 stability.
Does anyone know where we can get a 1 pF capacitor that we can use in
our Delta Plus? There is no reference to these resistor-capacitor
pairs in the Thermo manuals and I don't have enough information on
these capacitors to simply order one from an electronic supply house.
Any information would be greatly appreciated.