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:
- What is the quantity that needs to be digitized here?
- How does this differ from other situations in these exercises?
- 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?
- 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.