High resolution quantitative multi-species hydrocarbon gas sensing with a cw external cavity quantum cascade laser based spectrometer in the 6-11 μm range

We report multi-species spectroscopy of hydrocarbons with a continuous wave external-cavity quantum cascade laser based spectrometer providing tunability from 6 to 11μm to measure direct absorption spectra of the first 7 alkanes and their mixtures. The gas spectra were acquired in the range from 1440 to 1480cm⁻¹ at a reduced pressure of 50 mbar and at a temperature of 323 K. By linearization of the measured wavelengths with a custom-made highly temperature stable air spaced etalon, a high spectral accuracy of ±0.001cm⁻¹ is achieved for the whole spectral range. The simultaneous high resolution of 0.001cm⁻¹ yields spectra of unprecedented richness of detail for the heavier alkanes (C₃–C₅) and allows the discrimination of narrow spectral features for the lighter ones (C₁–C₂). Thereby, the measured spectra reveal the influences of collisional broadening effects among the measured species. Quantitative spectroscopic multi-species gas sensing relies on the comprehension of the extent of spectral broadening. Studying the spectral broadening in combination with highly accurate reference spectra is mandatory for highly sensitive and specific multi-species gas analyzers. The quantitative results that can be obtained with our approach are presented for an exemplary calibrated mixture of all 7 components and reveal an absolute accuracy below 0.5 vol. % for the determination of the mole fraction of each gas.