Simultaneous monitoring of 32S, 33S and 34S isotopes of H2S using cavity ring-down spectroscopy with a mid-infrared external-cavity quantum cascade laser

Abstract

The simultaneous monitoring of three stable isotopes of hydrogen sulphide (H₂S), i.e. H₂³²S, H₂³³S, and H₂³⁴S, within a single laser scan of 0.4 cm⁻¹ has been achieved by means of a continuous-wave (cw) external-cavity quantum cascade laser (EC-QCL) coupled to high-resolution cavity ring-down spectroscopy (CRDS) near 7.5 μm. The measurement precisions of S³², S³³, and S³⁴ isotopes of H₂S were found to be 1190 ppb, 13 ppb, and 124 ppb for the integration times of 264 s, 261 s, and 247 s, respectively. The mode-hop-free (MHF) tunability (1257 cm⁻¹ to 1340 cm⁻¹) of the EC-QCL was subsequently exploited to identify the interference-free 9 transition lines of H₂³²S and H₂³³S arising from the fundamental bending vibration (ν₂) for the trace detection of H₂S isotopes. A limiting sensitivity (1σ) of 20 ppb of total H₂S concentration was achieved for an integration time of 250 s by targeting the rotational transition [i.e.] of (4, 4, 1) → (5, 5, 0), centred at ∼1292.857 cm⁻¹. Finally, simultaneous monitoring of multiple trace species such as CH₄, N₂O, and H₂S in a single laser scan of 0.05 cm⁻¹ was demonstrated with high sensitivity and molecular specificity. The present study may lead to several potential future applications ranging from understanding of sulphur isotope geochemistry and fractionations to environmental sensing.