Continuous-wave terahertz spectroscopy by photomixing is applied to the analysis of mainstream cigarette smoke. Using the wide tunability of the source, spectral signatures of hydrogen cyanide (HCN), carbon monoxide (CO), formaldehyde (H2CO), and water (H2O) have been observed from 500 to 2400 GHz. The fine spectral purity allows direct concentration measurement from the pure rotational transitions of HCN and CO. The quantification of the measurement was validated by the means of a calibration gas containing CO. The potential of this technique for trace gas detection is demonstrated with an estimated detection limit of HCN equal to 9 parts in 10(6).

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We report on the development of a Faraday rotation spectroscopy (FRS) instrument using a DFB diode laser operating at 2.8 µm for the hydroxyl (OH) free radical detection. The highest absorption line intensity and the largest gJ value make the Q (1.5) double lines of the 2Π3/2 state (υ = 1 ← 0) at 2.8 µm clearly the best choice for sensitive detection in the infrared region by FRS. The prototype instrument shows shot-noise dominated performance and, with an active optical pathlength of only 25 cm and a lock-in time constant of 100 ms, achieves a 1σ detection limit of 8.2 × 10(8) OH radicals/cm3.

Spectroscopic detection of short-lived gaseous nitrous acid (HONO) at 1254.85 cm−1 was realized by off-beam coupled quartz-enhanced photoacoustic spectroscopy (QEPAS) in conjunction with an external cavity quantum cascade lasers (EC-QCL). High sensitivity monitoring of HONO was performed within a very small gas-sample volume (of ∼40 mm3) allowing a significant reduction (of about 4 orders of magnitude) of air sampling residence time which is highly desired for accurate quantification of chemically reactive short-lived species. Calibration of the developed QEPAS-based HONO sensor was carried out by means of lab-generated HONO samples whose concentrations were determined by direct absorption spectroscopy involving a ∼109.5 m multipass cell and a distributed feedback QCL. A minimum detection limit (MDL) of 66 ppbv (1 σ) HONO was achieved at 70 mbar using a laser output power of 50 mW and 1 s integration time, which corresponded to a normalized noise equivalent absorption coefficient of 3.6 × 10−8 cm−1 W/Hz1/2. This MDL was down to 7 ppbv at the optimal integration time of 150 s. The corresponding 1σ minimum detected absorption coefficient is ∼1.1 × 10−7 cm−1 (MDL ∼ 3 ppbv) in 1 s and ∼1.1 × 10−8 cm−1 (MDL ∼ 330 pptv) in 150 s, respectively, with 1 W laser power.

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