Fast, accurate, and in-situ detection of volatile organic compounds (VOCs) is beneficial for quality control in many industrial fields, environmental monitoring, and medical diagnostics. Laser-absorption spectroscopy (LAS) has the potential to fulfil these requirements, even though VOCs exhibit broad and spectrally overlapped absorption spectra, however it requires light sources with broad spectral coverage. Extended-tuning quantum-cascade (QC-XT) lasers [1] are a promising candidate that can combine rapid, high-resolution spectral scanning with broad coverage.

In this work, we present the development and characterisation of a compact mid-IR spectrometer for the high-precision and simultaneous measurement of small VOCs. The analyser uses a QC-XT laser (Alpes Lasers) coupled to a multi-pass cell with 76 m of optical path. By means of the Vernier effect, the laser can be operated in six different spectral windows (each ~1.5 cm⁻¹ wide) distributed between ~1163 cm⁻¹ and ~1102 cm⁻¹. A complete spectrum is measured every 360 ms by rapid switching between and tuning within the six windows.

VOC measurements were performed in a flow-through configuration at low pressure (~50 mbar) to minimise the broadening of spectral lines. The spectra were recorded at high resolution (< 10⁻⁴ cm⁻¹), which is not available in the literature for most VOCs. Thus, we generated our reference database and developed a concentration-retrieval algorithm.

Our instrument is well-suited for the detection of small oxygen-containing VOCs at amount fractions down to tens of ppb. We have achieved a typical precision of ~1 ppb for 25 s averaging time, demonstrated for methanol at an amount fraction of 10 ppm, and a large linearity range over 3 orders of magnitude. Excellent selectivity of the method that enables multi-compound measurements with a relative expanded uncertainty of <2% (k=2) was achieved thanks to the broad measuring range, high spectral resolution, and the unique spectral fingerprints of the investigated VOCs.

The system is currently being further developed for breath analysis in the framework of Zurich Exhalomics [2].