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5. Electron Capture Detector

For the electron capture detector in a gas chromatograph, what ADC should one choose?

Analysis of the problem: In an electron capture detector, either photoionization or β decay of 63Ni causes sustained ionization of a sensitizer, typically CH4. A current is sustained by imposing an electric field on the detector. The presence of a compound that can capture electrons (typically a halogen-containing molecule) lowers the mobility of the electrons, requiring a higher field to maintain constant current. Thus, the departure of a voltage from its quiescent value signals the presence of analyte. Instantaneous concentration is proportional to (V - V0) over a wide concentration range, with typical response ~ 1000 V s pg-1 for the highly electronegative compound CCl4. That is, a 100 fg sample in a GC peak would produce a peak with an area of 100 V s. If the peak were 1 s wide, the voltage change would be 100 V. If the peak stretched to 2 s, the voltage excursion would be 50 V. For CCl4, the dynamic range is from ~ 3 fg to ~ 200 pg for common GC parameters. See H. Cai, S. D. Stearns, and W. E. Wentworth, "Pulsed Discharge Electron Capture Detector Operating in the Constant-Current Mode by Means of Feedback dc Bias Voltage," Anal. Chem. 70, 3770-3776 (1998) for details.

Questions and choices:

  1. What is the quantity that needs to be digitized here?
  2. How does this differ from other situations in these exercises?
  3. If you had to choose one of the ADC approaches discussed in this set of modules to digitize the detector signal, what would it be and why?
  4. Can you devise an approach to digitization that is a variation on the methods we have discussed that might work more easily than the scheme you chose in question 3?

Go to next page for answers.



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