You can get 1 pF capacitors from a guy's garage in Canada called "Just
Make sure you buy a few because I noticed the tolerance is 1 +/- 0.5 pF on
the ones sent to me. But they are very cheap. I used them with success
in a MAT252 and MAT253, and published work using them (Tobias et.al. Anal.
Chem. 2008, 80, 8613-8621). Someone may suggest a better source, but this
is what I found.
The capacitors I bought:
500 Volt Dipped Silver Mica Capacitors
* Rated at 500 Volts (WVDC). Test voltage 1000V.
* For use where high reliability and capacitance stability are
required, such as in tuning/oscillator circuits, DC voltage blocking,
high frequency, logic and transmission circuits and filters.
* Long 35mm radial leads for easy under chassis installation.
* Capacitance tolerance of +/- 5%.
* Wide operating temperature range: -55 to +150C at full rated voltage.
* Available in large selection of pF(MMFD) sizes from 1 pF to 5000 pF,
including "hard to find pre-WWII mmfd sizes for tube electronics
* Meets specifications of MIL-C-5 and RS-153 and ISO 9002 certified.
> 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.
> Bernie Genna