Which single device is commonly used for direct-reading measurements of gases or vapors?

• A. Photoionization detector (PID)
• B. Gas chromatography
• C. Mass spectrometer
• D. Diffusion badge

Answer: (A)

Direct-reading measurements in the field rely on instruments that give an immediate readout as air is sampled. A photoionization detector is well suited for this because it can detect a wide range of volatile organic compounds in real time and display a concentration right away on a handheld unit. It works by using ultraviolet light to ionize many VOC molecules; the resulting ions create an electrical current that is proportional to the concentration, allowing the instrument to read out in ppm or mg/m3. This fast response makes it ideal for screening, task-based monitoring, or spill response. In contrast, gas chromatography and mass spectrometry are lab-based techniques that require sample collection and analysis, and a diffusion badge is a passive sampler that accumulates contaminants over time and does not provide immediate readings.