Showing papers on "Photoacoustic spectroscopy published in 2002"

Detection of ammonia by photoacoustic spectroscopy with semiconductor lasers

Abstract: 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.

Laser-based photoacoustic ammonia sensors for industrial applications

Abstract: An industrial trace-ammonia sensor based on photoacoustic spectroscopy and CO2 lasers has been developed for measuring ammonia with a 1σ detection limit of 220 parts-per-trillion (ppt) in an integration time of 30 s. The instrument response time for measuring ammonia was 200 s, limited by adsorption effects due to the polar nature of ammonia. The minimum detectable fractional absorbance was 2.0×10-7, and the minimum normalized detectable absorption coefficient for this system was 2.4×10-7 W cm-1/\(\surd\Box H\Box \)z. The 9R(30) transition of the CO2 laser at 9.22 μm with 2 W of output power was used to probe the strong sR(5,K) multiplet of ammonia at the same wavelength. This sensor was demonstrated with an optically multiplexed configuration for simultaneous measurement in four cells.

Miniaturization and integration of photoacoustic detection

Abstract: Photoacoustic spectroscopy is an absorption spectroscopy technique that is currently used for low-level gas detection and catalyst characterization. It is a promising technique for chemical analysis in mesoscale analysis systems because the detection limit scales favorably with miniaturization. This work focuses on the scaling properties of photoacoustic spectroscopy, and on the miniaturization of gas-phase photoacoustic detection of propane in a nitrogen ambient. The detection system is modeled with a transmission line analogy, which is verified experimentally. The model includes the effects of acoustic leaks and absorption saturation. These two phenomena degrade the performance of the photoacoustic detector and must be controlled to realize the scaling advantages of photoacoustic systems. The miniature brass cells used to verify the model employ hearing aid microphones and optical excitation from a mechanically chopped, 3.39 μm He–Ne laser, transmitted into the cells with an optical fiber. These cells a...

Near-infrared diode laser based spectroscopic detection of ammonia: a comparative study of photoacoustic and direct optical absorption methods.

Abstract: A photoacoustic spectroscopic (PAS) and a direct optical absorption spectroscopic (OAS) gas sensor, both using continuous-wave room-temperature diode lasers operating at 1531.8 nm, were compared on the basis of ammonia detection. Excellent linear correlation between the detector signals of the two systems was found. Although the physical properties and the mode of operation of both sensors were significantly different, their performances were found to be remarkably similar, with a sub-ppm level minimum detectable concentration of ammonia and a fast response time in the range of a few minutes.

Resonant photoacoustic simultaneous detection of methane and ethylene by means of a 1.63-μm diode laser

Abstract: A compact multi-component trace-gas detector based on the resonant photoacoustic technique and a NIR external cavity diode laser has been developed It has been characterized using a mixture of ethylene and methane diluted in ambient air A spectroscopic investigation of combination bands and overtones between 5900 and 6250 cm-1, obtained with an IR pulsed laser photoacoustic spectrometer, allowed us to find a wavelength region where the 2ν3 overtone of CH4 and the ν5+ν9 combination band of C2H4 show uncongested rotational lines Using a single-mode scan of the diode laser in this region, around 6150 cm-1, the sensitivity for the simultaneous detection of ethylene and methane is 8 ppm/mW and 40 ppm/mW respectively Factors affecting the sensitivity and selectivity of the detection system and possible improvements suitable to reach the sub-ppm detection limit are discussed

Photoacoustic measurement of methane concentrations with a compact pulsed optical parametric oscillator.

Abstract: A pulsed periodically poled lithium niobate optical parametric oscillator operating in a cavity with a grazing-incidence grating configuration was used for sensitive and precise measurement of trace quantities of methane in nitrogen by photoacoustic spectroscopy with a novel differential photoacoustic detector. A sensitivity of 1.2 parts in 10(9) by volume of methane was obtained in direct calibration measurements (not extrapolated). With this apparatus, in situ measurement of the methane concentration in ambient air under atmospheric conditions was demonstrated.

Cylindrical mirror multipass Lissajous system for laser photoacoustic spectroscopy

Abstract: A simple optical multiple reflection system is developed with two cylindrical concave mirrors at an appropriate spacing. The two cylindrical mirrors have different focal lengths and their principal sections are orthogonal. The alternate focusing of the two cylindrical mirrors at different direction keep the reflecting spots small. The reflecting spots fall on Lissajous patterns on the cylindrical mirrors. The mathematics for this optical system is described and the calculated coordinates of beam spots are very close matches of the experimental observations. The cylindrical mirror optical system is easy to construct and align, with a suitable method for obtaining long optical paths and a large number of passes in small volumes. In a photoacoustic spectrometer the beam family enhance the effective power in the photoacoustic cell and thus the signal-to-noise ratio of photoacoustic signal. An experimental result for photoacoustic spectrum of HDSe gas is given.

Photoacoustic probes for nondestructive testing and biomedical applications

Abstract: Fiber-optic photoacoustic sources for nondestructive testing and biomedical applications are described. The photoacoustic sources consist of a pulsed laser, a fiber-optic cable, and a generation head. The generation head is a miniature hermetically sealed chamber, which can be embedded into solid structures or immersed in liquid media. The face of the chamber acts as a target for laser irradiation. Bulk ultrasonic waves generated inside of the target are transmitted into the medium. The proposed systems offer wide ultrasonic range (0.5-15 MHz), easy control over directivity of the ultrasonic beam, high efficiency of generation, and the ability to operate in a harsh environment. Sources with different radiation patterns with respect to the optical axis of the fiber, such as normal, sideways, as well as focused, have been devised. We present a proof-of-concept experiment using these sources in combination with fiber-optic ultrasonic receivers.

On the application of the photoacoustic methods for the determination of thermo-optical properties of polymers

Abstract: In this paper, the application of photoacoustic methods to study thermo-optical and spectroscopic properties of polymers is described. The Photoacoustic Spectroscopy, the Two-Beam Phase-Lag and also the so-called Open Photoacoustic Cell methods will be presented. The theoretical basis for quantitative measurements is discussed together with the advantages and limitations of the methods as compared with conventional measurements. Applications for spectroscopic and depth profile analysis and also for thermal properties measurements in several polymers samples are discussed.

Differential Helmholtz resonant photoacoustic cell for spectroscopy and gas analysis with room-temperature diode lasers

Abstract: The results of theoretical and experimental studies and the design of a multi-purpose differential Helmholtz resonant photoacoustic detector (DHRD) and its applications to high-resolution spectroscopy of molecular gases and gas analysis with a room-temperature diode laser in the near-IR region are summarized. The series of experiments and numerical analysis of the DHRD sensitivity were performed for both types (single-pass and multi-pass) of DHRDs within a wide pressure range 0.1–101 kPa, including the regime of a gas flowing through a DHRD cell. The hardware and electronic arrangement of DHRDs for diode laser spectrometers and gas analyzers providing a limiting absorption sensitivity better than 10-7 Wm-1 are described. The results of measurements of spectral line parameters of H2O near 800 and 1390 nm and CH4 near 1650 nm (intensities, line broadening and shifting by atomic and molecular gases) are presented and discussed. The problems and the ways of perfection of the methodology and accuracy of DHRD techniques with tunable diode lasers of near-IR and visible spectral ranges are discussed.

Photoacoustic blood glucose and skin measurement based on optical scattering effect

Abstract: Non-invasive blood glucose determination has been investigated by more than 100 research groups in the world during the past fifteen years. The commonly optical methods are based on the capacity of near-IR light to penetrate a few hundreds micrometers or a few millimeters into human tissue where it interacts with glucose. A change of glucose concentration may modify the optical parameters in tissue, with the result that its glucose concentration can be extracted by analyzing the received optical signals. This paper demonstrates that glucose affects on the scattering coefficient of human blood, by applying the streak camera and pulsed photoacoustic techniques; and drinking water seems also affecting on PA signal from skin surface.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A new biophysics approach using photoacoustic spectroscopy to study the DNA-ethidium bromide interaction.

Abstract: We have examined the binding processes of ethidium bromide interacting with calf thymus DNA using photoacoustic spectroscopy These binding processes are generally investigated by a combination of absorption or fluorescence spectroscopies with hydrodynamic techniques The employment of photoacoustic spectroscopy for the DNA-ethidium bromide system identified two binding manners for the dye The presence of two isosbestic points (522 and 498 nm) during DNA titration was evidence of these binding modes Analysis of the photoacoustic amplitude signal data was performed using the McGhee-von Hippel excluded site model The binding constant obtained was 34×108 M(bp)–1, and the number of base pairs excluded to another dye molecule by each bound dye molecule (n) was 2 A DNA drug dissociation process was applied using sodium dodecyl sulfate to elucidate the existence of a second and weaker binding mode The dissociation constant determined was 043 mM, whose inverse value was less than the previously obtained binding constant, demonstrating the existence of the weaker binding mode The calculated binding constant was adjusted by considering the dissociation constant and its new value was 12×109 M(bp)–1 and the number of excluded sites was 26 Using the photoacoustic technique it is also possible to obtain results regarding the dependence of the quantum yield of the dye on its binding mode While intercalated between two adjacent base pairs the quantum yield found was 087 and when associated with an external site it was 004 These results reinforce the presence of these two binding processes and show that photoacoustic spectroscopy is more extensive than commonly applied spectroscopies

Detection of N2O by photoacoustic spectroscopy with a compact, pulsed optical parametric oscillator

Abstract: A pulsed optical parametric oscillator (OPO) operated in an optical cavity with a grazing-incidence grating configuration (GIOPO) was used for sensitive photoacoustic detection of trace quantities of dinitrogen oxide (N2O). The (ν1+ν3) combination vibration band of N2O was excited with the idler beam of the GIOPO at 2.86 μm using an optical cavity optimized for the idler beam. The linewidth of the GIOPO could be reduced to 0.4 cm-1, allowing the rotational structure of the absorption spectrum to be resolved. A concentration sensitivity (signal-to-noise ratio=3) of 60 parts in 109 by volume (60 ppb V) N2O in synthetic air was obtained. This may be sufficient for continuous monitoring of N2O in the atmosphere.

Analysis of Recombination Process Resonantly Excited with Free Exciton Energy on CuInS2 Using Photoacoustic Spectroscopy

Abstract: Photoacoustic (PA) spectra have been investigated using a piezoelectric device and a tunable laser for CuInS2 crystals grown by the traveling heater method. The PA spectrum at 9 K shows two dips located at 1.536 and 1.556 eV corresponding to "A" and "B" free excitons, respectively. The dips of the PA spectrum indicate an increase in the probability of radiative recombination for resonantly excited electron-hole pairs with free exciton energies.

A new cylindrical photoacoustic cell with improved performance

Abstract: We report a new design for an acoustically resonant photoacoustic cell with high performance. A cylindrical cell with suitable size was selected so that the resonant frequency of the first radial mode was equal to that of a longitudinal higher mode. The ends of the cell have a pair of thin coaxial tubes which lengths are 1/2 and 1/4 of acoustic wavelength λ, respectively. The λ/4 acoustic resonance makes a coupling between the first radial resonance and the higher longitudinal resonance, then leads to an acoustic energy concentrating in the middle of the cell. Thus, the surface loss was decreased, the acoustic quality factor and pressure amplitude increased obviously as compared with conventional radial resonant cylindrical cell. The ability to measure absorption coefficients as low as 2.43×10−9 cm−1 (1:1 signal to noise level) has been achieved.

Ultrasensitive gas detection using diode lasers and resonant photoacoustic spectroscopy

Abstract: A novel trace-gas sensor system has been developed based on resonant photoacoustics, wavelength modulation spectroscopy, near-infrared diode lasers and optical fiber amplifiers that can achieve parts-per-billion sensitivity with a ten centimeter long sample cell and standard commercially-available optical components. An optical fiber amplifier with 500 mW output power is used to increase the photoacoustic signal by a factor of 25, and wavelength modulation spectroscopy is used to minimize the interfering background signal from window absorption in the sample cell, thereby improving the overall detection limit. This sensor is demonstrated with a diode laser operating near 1532 nm for detection of ammonia that achieves an ultimate sensitivity of less than 6 parts-per-billion. The minimum detectable fractional optical density, αminl, is 1.8x10-8, the minimum detectable absorption coefficient, αmin, is 9.5x10-10 cm-1, and the minimum detectable absorption coefficient normalized by power and bandwidth is 1.5x10-9 Wcm-1/s Hz. These measurements represent the first use of fiber amplifiers to enhance photoacoustic spectroscopy, and this technique is applicable to all other species that fall within the gain curves of optical fiber amplifiers.

Simultaneous online detection of isoprene and isoprene-d(2) using infrared photoacoustic spectroscopy

Abstract: A photoacoustic spectrometer for the simultaneous detection of isoprene and the deuterated species [4,4-2H]-2-methyl-1,3-butadiene (isoprene-d2) is presented. Using a sealed-off 13CO2 laser a single-component detection limit of 400 ppt (isoprene) and 600 ppt (isoprene-d2) was achieved. Simultaneous monitoring of both compounds allowed the detection of labelling levels down to 6% (isoprene-d2 in total isoprene) with a time resolution of 3 min.

Quantitative depth profile analysis of micrometer-thick multilayered thin coatings using step-scan FT-IR photoacoustic spectroscopy.

Abstract: This paper demonstrates the results of the comparison of step-scan FT-IR photoacoustic spectroscopy with other established spectroscopic and microscopic techniques in the quantitative depth profile determination of micrometer- and submicrometer-thick multilayered thin coatings. The power of the phase rotation and phase spectrum analytical methods to clearly distinguish the infrared signature of submicrometer-thick coatings is demonstrated. The thickness determined by the step-scan FT-IR photoacoustic method is in very reasonable agreement with optical microtomy/microscopy measurements performed at-line during the coating process. The former technique described in detailed here offers substantial benefits in terms of measurement time and operator dependency, while not sacrificing the accuracy of the measurement. The problem of saturation and its effect on “real-life” samples is also discussed.

Quantitative Depth Profiling Using Saturation-Equalized Photoacoustic Spectra

Abstract: Depth profiling using photoacoustic spectra taken at multiple scanning speeds or modulation frequencies is normally impaired by the increase in spectral saturation that occurs with decreasing speed or frequency. Photothermal depth profiling in general is also impeded by the ill conditioned nature of the mathematical problem of determining a depth profile from photothermal data. This paper describes a method for reducing the saturation level in low-speed or low-frequency spectra to the level at high speed or frequency so that all spectra have the same saturation. The conversion method requires only magnitude spectra, so it is applicable to both conventional and phase-modulation photoacoustic spectra. This paper also demonstrates a method for quantitative depth profiling with these converted spectra that makes use of prior knowledge about the type of profile existing in a sample to reduce the instabilities associated with the mathematically ill conditioned task.

Characterization of thermal properties of porous silicon film/silicon using photoacoustic technique

Abstract: We have applied photoacoustic (PA) technique to study the thermal properties of porous silicon (PS) films formed on p-type Si substrates by electrochemical anodic etching. Four PS samples with close thicknesses but greatly different porosities (from 20 to 60%) were examined. From the dependences of the PA signals on the modulation frequency of excitation light measured under a transmission detection configuration (TDC), effective thermal diffusivities for the two-layered PS/Si samples were determined and found to decrease greatly from 0.095 to 0.020 cm2 s-1 as the porosity increased from 20 to 60%.

Preparation of nanocrystalline ceria in CCl4

Abstract: Nanocrystalline ceria was prepared by a hydrothermal technique using CCl4 as the solvent at 130 °C. X-ray diffraction (XRD) analysis confirmed the cubic phase of ceria with fluorite structure. Transmission electron microscopy analysis showed spherical particles having a narrow size distribution. X-ray photoelectron spectroscopy (XPS) analysis indicated no detectable absorbed Cl− or NO3− ion, demonstrating that CCl4 acting as medium instead of water would introduce no extra impurity ions. Photoacoustic (PA) spectroscopy analysis showed that the PA absorption band shifted blue as well as broadeded accompanying with a blue shift of the absorption edge with the decrease of the grain sizes and this phenomenon was explained by the quantum size effect.

A versatile photoacoustic spectrometer for sensitive trace-gas analysis in the mid-infrared wavelength region (5.1-8.0 and 2.8-4.1 μm)

Abstract: A versatile CO laser-based photoacoustic spectrometer is presented equipped with three photoacoustic cells placed inside the laser cavity. The newly designed CO laser can operate both in the Δv=1 and the Δv=2 modes (5.1–8.0 μm and 2.8–4.1 μm) on 400 laser lines. Improved laser operation originating from a better cooling of the gas discharge was evidenced by a shift of the laser output power to lower J-values. Due to the wide emission range of the source, many molecules of biological and atmospheric interest, including methane and ethane, can be detected with sensitivities typically at the (sub)ppb level. Measurement of the respiration of a cockroach showed that the spectrometer is not only sensitive, but also has a good time response (8 s at a flow rate of 10 l/h).

Photoacoustic studies on n-type InP

Abstract: We discuss an open photoacoustic cell study on sulfer-doped n-type InP wafer. The thermal diffusivity of the sample is evaluated from the phase data associated with the photoacoustic signal as a function of the modulation frequency under heat transmission configuration. Analy- sis is made on the basis of the Rosencwaig-Gersho theory and the re- sults are compared with those from earlier reported photoacoustic stud- ies of semiconductors. Our investigation clearly indicates that the instantaneous thermalization process is the major heat diffusion mecha- nism responsible for the photoacoustic signal generation in an InP sample. © 2002 Society of Photo-Optical Instrumentation Engineers.