Laser absorption spectroscopy, air monitoring by tunable mid-infrared diode

Infrared (IR) spectroscopy is a proven tool for gas analysis, and tunable mid-IR diode laser absorption spectroscopy is a special technique giving optimum sensitivity, selectivity and measurement speed for most molecules of interest. Molecules such as the NOx family (N2O, NO, NO2, HONO, and HNO3) can be detected selectively down to well below part per billion levels. The technique works well with all molecules having line spectra and thus is quite universal. It is conceptually simple and detects gases by repetitively scanning absorption lines, which is possible due to the narrow laser line width and easy laser tuning. The scanning procedures of diode laser spectroscopy optimally reduce background fluctuations. Basic operation principles are explained in the article. Alternative techniques are well established and often allow detection of single components at lower system cost, but in general need specific sample pretreatment that makes the results often difficult to compare. Mass spectroscopy is an alternative multicomponent technique that reaches comparable selectivity, but only with preselection processes which induce calibration problems of their own, such as matrix-dependent sensitivity. In order to give an insight into the capabilities of tunable mid-IR diode laser absorption spectroscopy, three exemplary systems are treated in detail for air monitoring from airplanes, from rockets, and gas analysis of combustion exhaust. System parameters are explained, and system performance is compared with alternative techniques.