Ammonia detection by use of quartz-enhanced photoacoustic spectroscopy with a near-IR telecommunication diode laser

A gas sensor based on quartz-enhanced photoacoustic detection and a fiber-coupled telecommunication distributed-feedback diode laser was designed and characterized for trace NH3 monitoring at a 1.53-microm wavelength (overtone absorption region). Signal and noise dependence on gas pressure were studied to optimize sensor performance. The ammonia concentration resulting in a noise-equivalent signal was found to be 0.65 parts per million by volume with 38-mW optical excitation power and a lock-in amplifier time constant of 1 s. This corresponds to a normalized absorption sensitivity of 7.2 x 10(-9) cm(-1) W/Hz1/2, comparable with detection sensitivity achieved in conventional photoacoustic spectroscopy. The sensor architecture can be the basis for a portable gas analyzer.     

Optically multiplexed multi-gas detection using quantum cascade laser photoacoustic spectroscopy

We report high-throughput, nondispersive optical multiplexing of laser beams using a scanning galvanometer. We have utilized this technique for multispecies trace-gas detection using multiple quantum cascade laser photoacoustic spectroscopy. We demonstrate switching from one laser to another in less than 1 s, a performance level needed for a comprehensive multispecies sensor, and a high signal-to-noise ratio detection of five gaseous components, NH(3), NO(2), dimethyl methyl phosphonate (DMMP, a simulant for nerve agents), acetone, and ethylene glycol, in a room air gas mixture containing approximately 3 ppb of NH(3), approximately 8 ppb of NO(2), approximately 20 ppb of DMMP, approximately 30 ppb of acetone, and approximately 40 ppb of ethylene glycol.     

Sub-parts-per-billion level detection of dimethyl methyl phosphonate (DMMP) by quantum cascade laser photoacoustic spectroscopy

The need for the detection of chemical warfare agents (CWAs) is no longer confined to battlefield environments because of at least one confirmed terrorist attack, the Tokyo Subway [Emerg. Infect. Dis. 5, 513 (1999)] in 1995, and a suspected, i.e., a false-alarm of a CWA in the Russell Senate Office Building [Washington Post, 9 February 2006, p. B01]. Therefore, detection of CWAs with high sensitivity and low false-alarm rates is considered an important priority for ensuring public safety. We report a minimum detection level for a CWA simulant, dimethyl methyl phosphonate (DMMP), of <0.5 ppb (parts in 10(9)) by use of a widely tunable external grating cavity quantum cascade laser and photoacoustic spectroscopy. With interferents present in Santa Monica, California street air, we demonstrate a false-alarm rate of 1:10(6) at a detection threshold of 1.6 ppb.     

Analysis of isotopic CO(2) mixtures by laser photoacoustic spectroscopy

Photoacoustic spectroscopic studies on a mixture of six CO(2) isotopes ((12) C(16) O(2), (12) C(18) O(2), (13) C (16) O(2), (13) C(18) O(2), (16) O(12) C(18) O, and (16) O(13) C(18) O) in the wavelength range of 9-11 mum by use of a home-built high-pressure continuously tunable CO(2) laser with a bandwidth of 0.017 cm(-1) are discussed. The concentrations of all CO(2) isotopes present in the mixture could be determined with good accuracy. Furthermore, the previously unknown absorption cross sections of some important lines of the(12) C(18) O(2), (13) C(18) O(2), and (16)O (13) C(18) O isotopes in the 9-11-mum range are reported.     

Characterization of soils using photoacoustic mid-infrared spectroscopy

This study investigates the use of photoacoustic spectroscopy (PAS) for rapid soil analysis. Photoacoustic spectroscopy requires very minimal sample preparation (air-drying), which is a major advantage compared to the more traditional transmittance technique, which requires time-consuming preparation of pellets. The amount of information contained in the PAS spectra appears to be similar to that contained in transmittance spectra, and the PAS spectra exhibit a large number of bands that can be associated with various soil constituents such as quartz, calcium carbonate, and various types of clay. Comparison with attenuated total reflection (ATR) spectra of saturated soil pastes shows that the PAS spectra provide much more information than the ATR spectra due to the strong water bands present in the latter. PAS quantitative analysis of clay, calcium carbonate, and organic matter is presented, with respective determination errors of approximately 12% clay, approximately 5% CaCO(3), and approximately 0.2% organic matter.     

Cavity ringdown spectroscopic detection of nitric oxide with a continuous-wave quantum-cascade laser

A spectroscopic gas sensor for nitric oxide (NO) detection based on a cavity ringdown technique was designed and evaluated. A cw quantum-cascade distributed-feedback laser operating at 5.2 mum was used as a tunable single-frequency light source. Both laser-frequency tuning and abrupt interruptions of the laser radiation were performed through manipulation of the laser current. A single ringdown event sensitivity to absorption of 2.2 x 10(-8) cm(-1) was achieved. Measurements of parts per billion (ppb) NO concentrations in N(2) with a 0.7-ppb standard error for a data collection time of 8 s have been performed. Future improvements are discussed that would allow quantification of NO in human breath.     

Trace gas detection by laser intracavity photothermal spectroscopy

A novel laser intracavity photothermal detector is described. In this scheme, sample absorption of the pump laser power takes place within the cavity of a probe He-Ne laser causing modulation in the gain and in turn the output power. Comparison of this intracavity detector with two other photothermal techniques, namely, phase fluctuation optical heterodyne spectroscopy and thermal beam deflection, is made in terms of practicality and sensitivity. For in situ measurements, sensitivity of 0.5 x 10(-7) cm(-1) for a probe length of 3 cm has been achieved.     

Diode laser based photoacoustic water vapor detection system for atmospheric research

A wavelength modulated, distributed feedback diode laser based photoacoustic water vapor mixing ratio measuring system for atmospheric research applications is presented. Laser modulation parameters were optimized either at 180 or 500 mbar total pressure to enhance the system's sensitivity for low or high pressures (upper troposphere/lower stratosphere or biosphere exchange layer), respectively. A wavelength locking method was developed that ensured sub-picometer absolute (5 x 10(-7) relative) wavelength stability of the laser while consuming minimum additional measurement time. At the calibration of the system, correction factors for the pressure- and temperature-dependence of the photoacoustic signal were determined, which were in turn applied to the calculation of the water vapor mixing ratio from the measured signal during the test operation of the system. The introduced features resulted in reliable, sub-ppm-level water vapor detection even under abrupt gas pressure or temperature variations typical in open atmospheric applications.     

Determination of acidities of zeolites by photoacoustic spectroscopy

Photoacoustic spectroscopy is found to be a useful technique for determining the acidity of zeolites. The acidity so determined correlates well with temperature programmed desorption studies of ammonia and product distribution.     

Pulsed indirect photoacoustic spectroscopy: application to remote detection of condensed phases

The technique of pulsed indirect photoacoustic spectroscopy is applied to the examination of free liquid surfaces, and the prospects are assessed for remote detection and identification of chemical species in a field environment. A CO(2) laser (tunable within the 9-11-mum region) provides pulsed excitation for a variety of sample types; the resulting photoacoustic pulses are detected at ranges of the order of a few centimeters. The phenomenon is investigated as a function of parameters such as temperature, sample depth, laser-pulse energy, pulse length, and beam diameter. The results are in good agreement with a theoretical model that assumes the mechanism to be expansion of air resulting from heat conduction from the laser-heated surface of the sample under investigation. Signal and noise processing issues are discussed briefly, and the possible extension of the technique to ranges of the order of 10 m is assessed.     

Gas detection by correlation spectroscopy employing a multimode diode laser

A gas sensor based on the gas-correlation technique has been developed using a multimode diode laser (MDL) in a dual-beam detection scheme. Measurement of CO(2) mixed with CO as an interfering gas is successfully demonstrated using a 1570 nm tunable MDL. Despite overlapping absorption spectra and occasional mode hops, the interfering signals can be effectively excluded by a statistical procedure including correlation analysis and outlier identification. The gas concentration is retrieved from several pair-correlated signals by a linear-regression scheme, yielding a reliable and accurate measurement. This demonstrates the utility of the unsophisticated MDLs as novel light sources for gas detection applications.     

Trace-gas detection in ambient air with a thermoelectrically cooled, pulsed quantum-cascade distributed feedback laser

A pulsed quantum-cascade distributed feedback laser operating at near room temperature was used for sensitive high-resolution IR absorption spectroscopy of ambient air at a wavelength of ~8 mum. Near-transform-limited laser pulses were obtained owing to short (~5-ns) current pulse excitation and optimized electrical coupling. Fast and slow computer-controlled frequency scanning techniques were implemented and characterized. Fast computer-controlled laser wavelength switching was used to acquire second-derivative absorption spectra. The minimum detectable absorption was found to be 3 x 10(-4) with 10(5) laser pulses (20-kHz repetition rate), and 1.7 x 10(-4) for 5 x 10(5) pulses, based on the standard deviation of the linear regression analysis.     

Detection of ammonia by photoacoustic spectroscopy with semiconductor lasers

Sensitive photoacoustic detection of ammonia with near-infrared diode lasers (1.53 microns) and a novel differential acoustic resonator is described; a sensitivity of 0.2 parts per million volume (signal-to-noise ratio = 1) is attained. To eliminate adsorption-desorption processes of the polar NH3 molecules, a relatively high gas flow of 300 SCCM was used for the ammonia-nitrogen mixture. The results are compared with recent ammonia measurements with a NIR diode and absorption spectroscopy used for detection and photoacoustic experiments performed with an infrared quantum-cascade laser. The performance of the much simpler and more compact setup introduced here was comparable with these previous state-of-the-art measurements.     

Imaging at 3.4 Thz with a quantum-cascade laser

We have assembled a single-frequency imaging system at 3.4 THz with a quantum-cascade laser. Images of electronic and biological applications are demonstrated. We operate the laser with a peak output power of 2.5 mW at a 7% duty cycle and a 22 K operating temperature. The minimum spot size is 340 microm. The signal is detected with a single-element deuterated triglycine sulfate detector, and images are captured by scanning of the sample.     

External photoacoustic detection of a trace vapor inside a multimode laser.

We combine highly sensitive intracavity absorption spectroscopy in a multimode laser with external photoacoustic detection, providing high detectivity. Photoacoustic measurements of the intracavity iodine concentration in a Rhodamine 6G laser are compared with simultaneous spectral recordings. They demonstrate slightly improved overall sensitivity and a greatly enhanced dynamic range of 5 orders of magnitude that may be shifted along the absolute scale of absorber density.