What is the purpose of the chopper in an atomic absorption spectrophotometer?

• A. Correct for the amount of light emitted by the flame
• B. Correct for the fluctuating intensity of the light source
• C. Correct for the fluctuating sensitivity of the detector
• D. Correct for differences in the aspiration rate of the sample

Answer: (A)

In atomic absorption spectrophotometry, the flame can itself emit light at or near the measurement wavelength, which adds background to the signal. The chopper periodically interrupts the lamp light, creating a chopped signal that lets the detector distinguish between the light that actually passes through the flame (and may be absorbed by the analyte) and the flame’s own emission. By comparing the chopped-on and chopped-off signals, the instrument subtracts the flame background, giving a measurement that reflects only the absorption by the analyte.

The chopper is not primarily for correcting fluctuations in lamp intensity, detector sensitivity, or sample aspiration rate. Those aspects are addressed by other parts of the system or different correction methods.