Cantilever-enhanced photoacoustic spectroscopy in the analysis of volatile organic compounds: Dissertation

Accurate and reliable measurement of volatile organic compounds (VOCs) is an important need in many application areas in industry, air pollution and atmosphere, health and well-being, defense and security as well as in many other fields. In this thesis, cantilever-enhanced photoacoustic spectroscopy (CEPAS) has been applied for the measurement of VOCs. A key feature in CEPAS is the non-resonant operational mode of the detector, which enables the broadly tunable wavelength ranges needed to resolve the spectral interferences that are typical in VOC measurement applications. Due to the large variation in VOC applications, the objective of this work was to build several, differently optimized CEPAS measurement systems and characterize their performance in certain applications. The Fourier transform infrared (FT-IR) technique was applied for multicompound VOC mixtures because of its capability to resolve spectral interference between the compounds. A compact, industry-ready FT-IR-CEPAS system was tested and reached multivariate detection limits (3??, 25 s) at the single ppm level with the average sum of the cross-selectivity numbers in a four compound mixture being <0.01 ppm ppm-1. To achieve better analytical sensitivity, the CEPAS detector was set up with a quantum cascade laser (QCL). The QCL-CEPAS system provides a univariate detection limit (3??, 0.951 s) of 1.3 ppb for formaldehyde, which is ~1000 times better than the FT-IR-CEPAS system. However, in case of several compounds, spectral interferences are usually difficult to resolve because the mode hop-free tuning range of QCLs is limited to a few wavenumbers. For sensitive and selective trace gas detection, a compact optical parametric oscillator (OPO) was combined with CEPAS and applied to the multi-compound measurement of benzene, toluene, p-, m- and o-xylene (BTX). The achieved multivariate detection limits (3??, 3237-3296 nm, 591 spectral points each 0.951 s) were around 10 ppb and the average sum of the cross-selectivity numbers <0.04 ppb ppb-1. Another achievement was the construction of a CEPAS measurement system capable of measuring at gas temperatures up to 180 °C. This enables applications where gases can only be measured in the hot state, e.g. the monitoring of many industrial emissions. Since the cantilever pressure transducer can withstand 180 °C, it was in direct contact with the hot sample gas and the need for cooling the gas or for using a signal tube was eliminated. In summary, this thesis shows that modern CEPAS is a suitable technique for measuring VOCs. CEPAS is now robust and reliable enough for industrial and other applications outside the laboratory. Several measurement systems based on CEPAS and relevant for VOC applications have been demonstrated in this thesis.