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Sensors 2011, 11(5), 5270-5289; doi:10.3390/s110505270

FT-IR-cPAS—New Photoacoustic Measurement Technique for Analysis of Hot Gases: A Case Study on VOCs

Photonic Devices and Measurement Solutions, VTT Technical Research Centre of Finland, Kaitoväylä 1, FI-90570 Oulu, Finland
Mass and Heat Transfer Process Laboratory, Department of Process and Environmental Engineering, University of Oulu, FI-90014 Oulu, Finland
Gasera Ltd., Tykistökatu 4, FI-20520 Turku, Finland
Author to whom correspondence should be addressed.
Received: 7 April 2011 / Revised: 4 May 2011 / Accepted: 12 May 2011 / Published: 16 May 2011
(This article belongs to the Special Issue Direct and Indirect Sensing of Odor and VOCs and Their Control)
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This article describes a new photoacoustic FT-IR system capable of operating at elevated temperatures. The key hardware component is an optical-readout cantilever microphone that can work up to 200 °C. All parts in contact with the sample gas were put into a heated oven, incl. the photoacoustic cell. The sensitivity of the built photoacoustic system was tested by measuring 18 different VOCs. At 100 ppm gas concentration, the univariate signal to noise ratios (1σ, measurement time 25.5 min, at highest peak, optical resolution 8 cm−1) of the spectra varied from minimally 19 for o-xylene up to 329 for butyl acetate. The sensitivity can be improved by multivariate analyses over broad wavelength ranges, which effectively co-adds the univariate sensitivities achievable at individual wavelengths. The multivariate limit of detection (3σ, 8.5 min, full useful wavelength range), i.e., the best possible inverse analytical sensitivity achievable at optimum calibration, was calculated using the SBC method and varied from 2.60 ppm for dichloromethane to 0.33 ppm for butyl acetate. Depending on the shape of the spectra, which often only contain a few sharp peaks, the multivariate analysis improved the analytical sensitivity by 2.2 to 9.2 times compared to the univariate case. Selectivity and multi component ability were tested by a SBC calibration including 5 VOCs and water. The average cross selectivities turned out to be less than 2% and the resulting inverse analytical sensitivities of the 5 interfering VOCs was increased by maximum factor of 2.2 compared to the single component sensitivities. Water subtraction using SBC gave the true analyte concentration with a variation coefficient of 3%, although the sample spectra (methyl ethyl ketone, 200 ppm) contained water from 1,400 to 100k ppm and for subtraction only one water spectra (10k ppm) was used. The developed device shows significant improvement to the current state-of-the-art measurement methods used in industrial VOC measurements.
Keywords: volatile organic compound (VOC); photoacoustic spectroscopy (PAS); science based calibration (SBC); elevated temperature measurement volatile organic compound (VOC); photoacoustic spectroscopy (PAS); science based calibration (SBC); elevated temperature measurement

This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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MDPI and ACS Style

Hirschmann, C.B.; Koivikko, N.S.; Raittila, J.; Tenhunen, J.; Ojala, S.; Rahkamaa-Tolonen, K.; Marbach, R.; Hirschmann, S.; Keiski, R.L. FT-IR-cPAS—New Photoacoustic Measurement Technique for Analysis of Hot Gases: A Case Study on VOCs. Sensors 2011, 11, 5270-5289.

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