In trace-gas analysis of chemical species in the atmosphere, besides sensitivity and selectivity the response time is of increasing interest for the real-time detection of temporal concentration changes. In order to achieve the necessary sensitivity and selectivity, the use of high-resolution laser techniques in IR and near-IR (NIR) fingerprint region is of special interest. The methodologies based on the photothermal techniques, mainly photoacoustic spectroscopy, have suitable characteristics for trace gas detection. Actually several laser-based methods have been reported because they are very sensitive [12�C14]. For instance, photoacoustic spectroscopy is widely used in the detection of several gases in the ppbv and sub-ppbv concentration range [15�C19].
A homemade CO2 laser photoacoustic spectrometer has been developed to monitor gas emissions of several sources [20]. A continuous wave CO2 infrared laser tunable over 80 different lines, between 9.2 and 10.6 ��m, has been employed at the emission line of 10P(16) as excitation source for sulfur hexafluoride gas detection [21�C23].With the recent development of quantum-cascade lasers (QCLs), compact solid-state radiation sources are available, covering the important infrared (IR) region with specific molecular absorption lines. In addition, spectral regions known as atmospheric windows can be selected in which water vapor has a very low absorption coefficient.
Another important advantage of QCLs in practical applications is that they work at near room temperature, Entinostat whereas diode lasers such as lead salt lasers, which emit in the fundamental IR region, have to be cryogenically cooled.
Recent Anacetrapib applications of QCLs clearly indicate their potential as tunable light sources in the mid-infrared, especially between 3 and 13 ��m, with strong fundamental absorption bands. Current interest is based on the lack of other convenient coherent laser sources. In fact, it can be expected that QCLs will open new possibilities for real-time diagnostics of various molecular species in the 3�C5 ��m and 8�C13 ��m atmospheric windows [24].Pulsed quantum-cascade distributed-feedback (QC-DFB) lasers provide quasi room temperature operation, combined with high spectral selectivity and sensitivity, real-time measurement capabilities, robustness, and compactness. For this reason, QCLs are ideal for the development of compact trace gas analyzers that are also suitable for field measurements. In recent years the detection of a series of important trace gases has been demonstrated with these devices [24�C29].