Microfabricated modular photoionization detector for volatile organic compounds

Air quality has become a major concern worldwide. Volatile organic compounds (VOCs) are hazardous chemicals that can cause cancer and are commonly found in indoor air. In Europe, increasing pressure from regulations on the VOCs exposure drive the need for portable, sensitive and fast gas analyzers. The gas chromatograph (GC) is a common gas analyzer and can be equipped with a photoionization detector (PID) for VOC analysis. Miniaturization of the PID can enhance the performance and portability of a GC gas analyzer. This work describes the design, fabrication and characterization of a micro PID (μPID) that can be integrated into a portable GC. The μPID uses a commercial UV lamp (with energy of 10.6 eV) emitting light into a microchannel, which is where the signal is measured. The device was microfabricated using micromilling and electrical discharge machining, with a final design that is modular. This enables the easy mount-dismount operation and replacement of components, while the miniaturized PIDs state of the art uses mainly fixed components. Four microchannel geometries are fabricated with varying electrode and illumination area and ionization chamber volume (1.1 μL to 6.7 μL). Low ionization chamber volume enables a reduction in sample volume, speeding up the analysis process and improving portability of gas analyzer. The prototype was tested for the four microchannel geometries with a direct injection of toluene 100 ppm and produced a signal from 0.5 to 2.5 nA. The results showed that the main geometrical parameters influencing the signal level were illumination area and electrode area. A detection limit down to 37 ppb for toluene can be reached if a digital moving average filter is applied to the signal for noise reduction. The μPID was also integrated with the portable GC from a French start-up company (In’Air Solutions), resulting in a GC-μPID gas analyzer. The GC-μPID results were obtained for an optimized cut-off frequency from a current preamplifier (𝑓𝑐=0.3 Hz), which provided the best signal-to-noise ratio. The detection limit of the GC-μPID was 73, 138, 308, 262 and 254 ppb, for benzene, toluene, ethylbenzene, m,p-xylene and o-xylene, respectively. An analysis of the ionization chamber volume influence on the GC signal was evaluated and showed that, for the chamber volume and sample volumes used in this work, the variation in chamber size is not significant for a relevant GC signal change (measured by peak height and peak width change). Finally, a response of the GC-μPID when integrated with a preconcentrator (PC) in the sampling process is provided and showed that the whole system can have a detection limit lower than 5 ppb of toluene.