In order to accomplish it, detection limit measurements were carried out. A value of 20 ppbv was achieved for the CO2 laser system and a value promotion information of 50 ppbv was achieved for the QC laser system. The motivation of our research comes from the necessity for simple, sensitive, and spectrally selective devices capable of measuring this important greenhouse gas at trace levels.2.?Methodology2.1. CO2 Laser Photoacoustic SpectroscopyThe gas samples were analyzed Inhibitors,Modulators,Libraries with a technique based on the photoacoustic method (Figure 1). In conventional absorption spectroscopy, one measures the absorption of the radiation power transmitted through the sample. On the contrary, in photoacoustic spectroscopy, the absorbed power is determined directly via its heat and hence the sound produced in the sample.

Actually, several laser-based methods have been reported because they are very sensitive, as for instance photoacoustic and cavity-ring-down spectroscopy Inhibitors,Modulators,Libraries [30,31]. Photoacoustic spectroscopic methods offer important advantages with respect to contaminant gas monitoring. This technique is based on pressure changes in the gas sample, which is induced by ro-vibrational excitation of molecules and, subsequent; relaxation by collisions (heat). The pressure change is detected by one or more microphones placed inside a resonator pipe of a resonant photoacoustic cell (Figure 2), through which, the air sample; containing the molecules under consideration was flown. An acoustic signal is produced at the resonance frequency of about 2,400 Hz of our resonant cell, by a chopper modulation of the excitation laser beam.

This resonance frequency value corresponds to the first longitudinal vibration mode. Our photoacoustic resonator is 67 mm long and has 18 mm in diameter.Figure 1.Scheme Inhibitors,Modulators,Libraries of the photoacoustic Inhibitors,Modulators,Libraries experimental setup.Figure 2.Diagram showing the design of the resonant photoacoustic cell used.The measurement was performed initially using a 1.1 ppmv certified gas mixture of ethylene in N2 flowing into the cell at a rate of 5 L/h. The ethylene gas is used as a calibrator Cilengitide in CO2 laser photoacoustic spectroscopy. The acoustic signal is detected by a microphone that generates an electric signal. This electric signal, in turn, is pre-amplified and then detected by a Lock-In amplifier (Stanford SR850). The Lock-In response is registered in a microcomputer.

A continuous wave CO2 infrared laser (Lasertech Group Inc.,��LTG, model LTG150 626G), tunable over about 80 different lines between 9.2 and 10.6 ��m and delivering a power of 1.9 W at the emission line of 10P(14), was employed as the excitation source. At this power level, no saturation effect of the photoacoustic signal was observed. The CO2 laser lines can be swept by read me a step motor controlled by the microcomputer. Within this spectral region many small molecules show a unique fingerprint.