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Comparing the Advantages and Limitations of Different Techniques Used to Quantify Methane Emissions From Municipal Solid Waste Landfills -  2.2 Photoionization Detection

 2.2 Photoionization Detection

Photoionization detectors (PIDs) are another very common gas sensor. The sample enters the device which is equipped with an ultraviolet (UV) lamp emitting high energy short-wavelength UV photons. The sample absorbs the UV light, which causes the volatile organic compounds to ionize and eject electrons, thus becoming positively charged. These positively charged ions are attracted to a negative electrode within the device. Similar to FIDs, an electrical current is generated as the ions hit the electrode. This current is amplified and output to an ammeter. Also, similar to FIDs, PIDs are based on an empirical, directly-proportional relationship between the ion concentration and the concentration of the gas species (Hernandez Bennetts et al. 2012; RAE Systems 2013).

 PID Diagram

PID Diagram (EQUIPCO)

Although the PID technique is still a single-point measuring technique, it is often preferred over FID because it is non-destructive to the sample (RAE Systems 2013), it does not require a constant hydrogen supply, and it is 50 times more sensitive to aromatic compounds compared to FID (Li et al. 2016); however, these sensors are incapable of measuring compounds with less than 2 carbon atoms, such as methane (CH4), regardless of which type of UV lamp is used (TSI 2013).  The only way that PIDs can be used to give an indication of methane concentration is through a process called surrogate compound measurement.  The methane concentration must be assumed constant, and a surrogate concentration measurement is made based on the detectable compounds within the gas mixture (RAE Systems 2013).  

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