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The ability to obtain accurate vapour parameter information from a compound’s absorption spectrum is an essential data
processing application in order to quantify the presence of an absorber. Concentration measurements can be required for
a variety of applications including environmental monitoring, pipeline leak detection, surface contamination and breath
analysis. This work demonstrates sensitive concentration measurements of complex mixtures of volatile organic
compounds (VOCs) using broadly tunable mid wave infrared (MWIR) laser spectroscopy. Due to the high absorption
cross-sections, the MWIR spectral region is ideal to carry out sensitive concentration measurements of VOCs by tunable
laser absorption spectroscopy (TLAS) methods. Absorption spectra of mixtures of VOCs were recorded using a MWIR
optical parametric oscillator (OPO), with a tuning range covering 2.5 μm to 3.7 μm. The output of the MWIR OPO was
coupled to a multi-pass astigmatic Herriott gas cell, maintained at atmospheric pressure that can provide up to 210 m of
absorption path length, with the transmission output from the cell being monitored by a detector. The resulting spectra
were processed by a concentration retrieval algorithm derived from the optimum estimation method, taking into account
both multiple broadband absorbers and interfering molecules that exhibit narrow multi-line absorption features. In order
to demonstrate the feasibility of the concentration measurements and assess the capability of the spectral processor,
experiments were conducted on calibrated VOCs vapour mixtures flowing through the spectroscopic cell with
concentrations ranging from parts per billion (ppb) to parts per million (ppm). This work represents as a first step in an
effort to develop and apply a similar concentration fitting algorithm to hyperspectral images in order to provide
concentration maps of the spatial distribution of multi-species vapours. The reported functionality of the novel fitting
algorithm makes it a valuable addition to the existing data processing tools for parameter information recovery from
recorded absorption data.
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