The detection limits of the wet chemical techniques are in the ppbv range; however they suffer from interference from the environmental conditions (temperature, humidity), are expensive and have to be performed by highly specialized personnel. These methods also exhibit slow response times, typically on the order of minutes, related with the chromatographic separation and this prevents application requiring real�Ctime and continuous gas monitoring.To overcome these limitations several laser-based spectroscopic systems have been developed. Among them, direct absorption and cavity enhancement spectroscopies (i.e. cavity ring down spectroscopy) take advantage of long optical path in multi-pass cell and high finesse optical cavities, respectively.
These techniques allow high sensitivities (up to sub-ppbv), however they need sophisticated and/or cumbersome equipments, not suitable in applications which require compact and transportable sensors [11,12]. For example, multi-pass absorption spectroscopy requires high volume multi-pass cell and sensitive IR detectors like thermoelectrically cooled or room-temperature photoconductive detectors or even liquid nitrogen cooled mercury cadmium telluride detectors. Instead, the major drawbacks of the cavity ring down spectroscopy are the requirement for high-reflectivity mirrors and high-quality laser beam.On the other hand, photoacoustic spectroscopy (PAS) has the potentiality to result in simple, robust, cheaper and easy to maintain designs, less sensitive to the problems of interference fringes and optical misalignments, giving PAS a competitive advantage over other sensitive techniques and the possibility to obtain a man-portable sensor.
Moreover, while the sensitivity of direct absorption technique is Drug_discovery independent from laser optical power, PA spectroscopy benefits from the use of high intensity sources to reach lower detection limits, since its sensitivity scales linearly with the laser power.In the last few years, efficient quantum cascade laser (QCL) sources, emitting in the mid-IR molecular fingerprint region, have become available. These lasers work at room temperature with emitted power up to several Watts  and thus represent ideal sources for PA gas sensing; detection limits of few ppbv [14�C16] have been already demonstrated.In this work we report the development and calibration of a PA trace gas sensor for the monitoring of formaldehyde with a detection limit of 150 ppbv, based on a resonant cell and a commercial QC laser source emitting at 1,778.9 cm?1. The sensor easily meets the international environmental regulations in terms of minimum detectable CH2O concentration.2.?ExperimentalThe experimental set-up is depicted in Figure 1.