In time-domain detection of a double-beam spectrophotometer, how many detectors and choppers are used?

• A. 1 detector and 1 chopper
• B. 2 detectors and 1 chopper
• C. 1 detector and 2 choppers
• D. 2 detectors and 2 choppers

Answer: (A)

Time-domain detection relies on modulating the light with a chopper to convert a steady signal into a time-varying one, so the detector signal can be demodulated and drift or low-frequency noise rejected. In a double-beam setup, this modulation can be accomplished with a single detector by routing either the sample or the reference beam to the detector in alternating time slots controlled by one chopper. Because the same detector and electronics see both beams in turn, common drift and noise affect both measurements equally and can be canceled out during demodulation, giving an accurate comparison of sample versus reference. Using more detectors isn’t necessary for this time-domain approach and would add complexity; similarly, extra choppers aren’t needed when a single chopper already provides the required time multiplexing.