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. 2014 Mar 28;14(4):6165–6206. doi: 10.3390/s140406165

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© 2014 by the authors; licensee MDPI, Basel, Switzerland.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/).

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Figure 19. — NNEA for categories of gas detection techniques as a function of optical path-length. Key: BB-CEAS – broadband cavity-enhanced spectroscopy, CRDS – cavity ring-down spectroscopy, OA-ICOS – off-axis integrated cavity output spectroscopy, OF-CEAS – optical feedback cavity-enhanced absorption spectroscopy, NICE-OHMS – noise-immune cavity-enhanced optical heterodyne spectroscopy, PAS photoacoustic spectroscopy, QEPAS – Quartz-enhanced photoacoustic spectroscopy, and I-QEPAS – Intracavity quartz-enhanced photoacoustic spectroscopy.