Questions, Myths and Misconceptions - about using photoionization detectors - Robert E. Henderson

Solvent, fuel and other VOC vapours are pervasively common in many workplace environments. Increased awareness of the toxicity of these common contaminants has led to lowered exposure limits, and increased requirements for direct measurement of these substances at their Occupational Exposure Limit concentrations. Photoionization detector (PID) equipped instruments are increasingly viewed as the best choice for measurement of VOCs at exposure limit concentrations.

Understanding the capabilities as well as the limitations of photoionization detectors is critical to interpreting test results and making decisions based on the use of this important atmospheric monitoring technology.

What are VOCs?
Volatile organic compounds (VOCs) are organic compounds characterised by their tendency to evaporate easily at room temperature. Familiar substances containing VOCs include solvents, paint thinner and nail polish remover, as well as the vapours associated with fuels such as gasoline, diesel, heating oil, kerosene and jet fuel. The category also includes many specific toxic substances such as benzene, butadiene, hexane, toluene, xylene, and many others. Most VOC vapours are flammable at surprisingly low concentrations. For most VOCs however, the toxic exposure limit is exceeded long before readings reach a concentration high enough to trigger a combustible range alarm.

How do PIDs detect VOCs?
Photoionization detectors use high-energy ultraviolet light from a lamp housed within the detector as a source of energy used to remove an electron from neutrally charged VOC molecules, producing a flow of electrical current proportional to the concentration of contaminant. The amount of energy needed to remove an electron from the target molecule is called the ionisation energy (IE) for that substance. The larger the molecule, or the more double or triple bonds the molecule contains, the lower the IE. Thus, in general, the larger the molecule, the easier it is to detect. On the other hand, small hydrocarbon molecules such as methane are not detectable by means of PID. A PID is only able to detect substances with ionisation energies lower than the energy of the ultraviolet photons produced by the PID lamp. The energy required to detect methane exceeds the energy of the ultraviolet light produced by the PID lamp.