Optical gas detection techniques for air-borne VOCs

Monitoring of indoor air quality by detecting individual airborne pollutant is essential for maintaining a healthy indoor environment. Indoor air usually contains pollutants such as volatile organic compounds (VOCs), and a long term exposure can pose a threat to human health even at a low concentration of ppm-level. Detection and identification of air-borne VOCs require a highly sensitive detection method. Optical sensors offer a sensitive and non-destructive approach with a quick time response for identification and detection of gas molecules.

The objective of the research work in this thesis is the development of an optical gas detector for BTEX molecules. The desired features include high sensitivity with a limit of detection in a ppb-range, good selectivity, portability and ease-of-integration with other analytical devices, for instance, micro gas chromatography (µGC). Optical sensing techniques, i.e. interferometry and UV absorption spectrophotometry, were investigated for VOCs detection. A comprehensive literature review of interferometry and absorption spectrometry was conducted, and it was found that UV absorption spectrophotometry provides a highly sensitive and reliable approach with good selectivity for the identification and detection of air-borne BTEX.

Initially, a simple deep-UV absorption spectrophotometer was developed using LED, spectrometer and aluminium-based Hollow Core Waveguides (HCW) as a gas cell. 3D printed optofluidics connectors were designed to integrate the gas flow with UV light. Two types of HCWs were tested as a gas cell: glass capillary tube with aluminium-coated inner walls and an aluminium capillary tube. The setup was tested for different toluene concentrations (10 - 100 ppm), and a linear relationship was observed with sensitivities of 0.20 mAU/ppm and 0.32 mAU/ppm for the glass and aluminium HCW, respectively. The corresponding limit of detectin were found to be 8.1 ppm and 12.4 ppm, respectively.

The study was extended to the development of low-cost absorption spectrophotometer with a low-volume gas cell for application of µGC. The spectrophotometer was developed using a low-volume gas cell made of PolyEther Ether Ketone polymer (PEEK) tube, connected with a portable deep-UV LED and photomultiplier tube. The performance of the detection unit was evaluated with different concentrations of toluene - N2 (5 - 100 ppm) and sensitivity of 0.11 mAU/ppm with a limit of detection of 1.41 ppm was obtained. The detector was incorporated into a µGC setup which was already developed by InAir Solution France. Thus the µGC was not developed as part of this thesis. However, the integration of the µGC and all of the associated testing was performed as part of this thesis and solely by the author. High-quality chromatograms having all the peaks separated with good repeatability were obtained for BTEX molecules. The deep-UV absorption spectrophotometer developed demonstrates low-volume (50 - 135 µL), low-cost, and ease of development and integration.

Finally, a novel, self-referenced deep-UV spectrophotometer with good sensitivity and stability was developed by directly measuring their absorbance values in volts. The design was based on a log-ratio amplifier which uses the photocurrents from a reference and test photodiodes to compute the absorbance according to Beer-Lambert law. The design features the reduction of noise and instability from the UV emission source by the use of the reference photodiode. The spectrophotometer was evaluated with different concentration of toluene, and good linearity was obtained (1.5 ppm to 50 ppm). A sensitivity and limit of detection of 1.76×10−4 AU/ppm and 196 ppb respectively were obtained for toluene concentration. The spectrometer was tested with µGC setup to identify and detect BTEX molecules. All the BTEX molecules were separated and detected with an individual peak for each BTEX molecule for a concentration down to 2.5 ppm with good linearity ( R2 ≃ 0.99). Whilst demonstrated for BTEX in a nitrogen carrier gas, the spectrometer has the potential to be applied to the chromatographic analysis of different analytes in gas or liquid media. The spectrometers developed in this study can be readily extended for detection of gases like ozone S02 and NOx as these molecules absorb strongly in the deep UV range (240 - 300 nm). Owing to its modular structure, the spectrophotometer can be applied for detection of gas molecules having absorption spectrum other than the UV-region by replacing the LED and photodiodes according to the desired spectral range.