Which statement best describes how a photoionization detector (PID) works and a common limitation?

• A. It uses infrared spectroscopy to ionize VOCs.
• B. It uses UV light to ionize VOCs and gives real-time readings but may respond to some non-target compounds and requires calibration.
• C. It measures changes in humidity to infer VOC presence.
• D. It relies on colorimetric tubes to detect VOCs with no calibration.

Answer: (B)

Photoionization detectors rely on UV light to ionize volatile organic compounds in the air, and the resulting current from the ionized molecules is measured in real time to indicate VOC levels. This gives quick, continuous readings that reflect changes in concentration. A key limitation is that the signal can come from non-target compounds with similar ionization energies, so the reading may overestimate or misrepresent the exact VOC mix unless you calibrate the instrument with a reference gas. Different VOCs ionize with different efficiencies, so calibration is essential to translate the current into an accurate concentration. In addition, inorganic gases or compounds with ionization energies above the lamp’s energy may not be detected, and humidity can influence the response.