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4. Flame Ionization Detector

For the flame ionization detector (FID)in a GC, what ADC should one choose?

Analysis of the problem: When biased appropriately and when support gases are clean, the background current for the FID is about 1 pA. Peak widths are commonly ~ 1 s, so signals need to be digitized at at least 10 Hz. Maximum useful current before nonlinearity sets in is about 1 μA.

Questions and choices:

  1. List the possible types of ADCs that are good for signal averaging at the speeds required.
  2. If the background current is 1 pA and one integrates for 0.1 s, what is the best possible (shot noise limited) standard deviation of the background? What is the shot noise limit for current detection (3 times standard deviation of the background)? Thus, what is the dynamic range required for the ADC?
  3. Suggest an appropriate ADC for this situation.

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