Which statement about PID operation is true?

• A. It uses UV light to ionize VOCs and provides real-time readings, but requires calibration
• B. It counts bacterial colonies to infer VOC presence
• C. It uses infrared imaging to measure gas leaks
• D. It operates only at high humidity

Answer: (A)

Photoionization detectors detect VOCs by exposing them to UV photons. The UV energy ionizes many VOC molecules, producing charged particles that are collected to generate a current. That current is read in real time and relates to the concentration of VOCs in the air. Calibration is needed because different VOCs ionize with varying efficiency and the lamp’s intensity can drift as it ages, so a known calibration gas is used to translate the signal into an accurate concentration. The other ideas describe methods that aren’t how a PID works—counting bacterial colonies is a microbiology technique, infrared imaging is a different approach used for gas leaks, and a PID isn’t restricted to high humidity ( humidity can affect readings but isn’t a requirement).