Showing papers on "Photoacoustic spectroscopy published in 2001"

Application of acoustic resonators in photoacoustic trace gas analysis and metrology

Abstract: The application of different types of acoustic resonators such as pipes, cylinders, and spheres in photoacoustics is considered. This includes a discussion of the fundamental properties of these resonant cavities. Modulated and pulsed laser excitation of acoustic modes is discussed. The theoretical and practical aspects of high-Q and low-Q resonators and their integration into complete photoacoustic detection systems for trace gas monitoring and metrology are covered in detail. The characteristics of the available laser sources and the performance of the photoacoustic resonators, such as signal amplification, are discussed. Setup properties and noise features are considered in detail. This review is intended to give newcomers the information needed to design and construct state-of-the-art photoacoustic detectors for specific purposes such as trace gas analysis, spectroscopy, and metrology.

Photoacoustic spectroscopy with quantum cascade distributed-feedback lasers.

Abstract: We present photoacoustic (PA) spectroscopy measurements of carbon dioxide, methanol, and ammonia. The light source for the excitation was a single-mode quantum cascade distributed-feedback laser, which was operated in pulsed mode at moderate duty cycle and slightly below room temperature. Temperature tuning resulted in a typical wavelength range of 3 cm-1 at a linewidth of 0.2 cm-1. The setup was based on a Herriott multipass arrangement around the PA cell; the cell was equipped with a radial 16-microphone array to increase sensitivity. Despite the relatively small average laser power, the ammonia detection limit was 300 parts in 109 by volume.

Photoacoustic material analysis

Abstract: Disclosed is a method and apparatus for determining the concentration of a compound in a medium (12) by resonant photoacoustic spectroscopy (RPAS) with a light pulse-train comprising equidistant short pulses having variable duration, frequency, number, and power. The light wavelength is selected so as to be absorbed by the component of interest. Upon irradiation, acoustic oscillations (28) are generated by the absorbed light (26) in a relatively thin layer of the medium (12), characterized by a heat-diffusing length. The frequency repetition of the light short pulses in the pulse-train is chosen to equal the natural acoustic oscillation frequency of the thin layer of the medium (12). So, the acoustic oscillation (28) becomes resonant. Measuring of the amplitude and the frequency of the resonant oscillations (28) determine the concentration of interest component. The method and apparatus are suitable monitoring of blood components, especially glucose.

Detection of trace levels of water in oil by photoacoustic spectroscopy

Abstract: We have demonstrated the feasibility of non-destructively analyzing trace levels of water in petroleum-based transmission and hydraulic oils and in a synthetic polyol ester engine-lubricating oil using photoacoustic spectroscopy (PAS) at a wavelength of 2.93 μm. Using the method of standard additions, detection limits of 45, 60, and 515 ppm were obtained for water in the transmission, hydraulic, and synthetic engine oils, respectively. Lower sensitivity for water in the synthetic polyol ester oil was due to a 0.16 μm shift in the water absorption maximum from the wavelength used for the other oils. A detection limit of 62 ppm was also observed for analysis of water in standard reference material (SRM) 8507 from the National Institute of Standards and Technology (NIST), which is a petroleum oil having a nominal water content of 77 ppm. The results for the petroleum-based oils exceed the current best detection limits of approximately 100 ppm obtained using Fourier transform infrared spectroscopy (FTIR) and compare favorably with the desired detection limits for water in oil of 10–50 ppm. Factors involved in implementing this technology in on-line sensors are discussed.

Miniaturization and integration of photoacoustic detection with a microfabricated chemical reactor system

Abstract: Photoacoustic spectroscopy is a useful technique for monitoring chemical composition in mesoscale analysis systems because the detection limit scales favorably with miniaturization The key element of a photoacoustic spectrometry system is the detector This work focuses on the miniaturization of photoacoustic detection In particular, we are using 34 /spl mu/m light to detect propane in a carbon dioxide background-a system that is useful for monitoring combustion reactions Two systems have been developed In the first, a miniature photoacoustic cell has been machined into the mounting block of a microfabricated chemical reactor, demonstrating the integration of a photoacoustic detector with a microsystem The cell used a hearing aid microphone and an infrared diode that was modulated at the first acoustic resonance of the cell As the gas composition of the cell changed from carbon dioxide to propane the resonance peak was observed to shift and increase, as was expected from theory This work also presents the first demonstration of a microfabricated photoacoustic detection cell The cell used an optical microphone and laser excitation brought into the cell via an optical fiber The light was modulated at a frequency far below the first acoustic resonance, and a signal of 005 Pa was observed in the presence of propane

Time-resolved characterization of diesel particulate emissions. 2: instruments for elemental and organic carbon measurements

Abstract: The measurement of elemental carbon (EC) and organic carbon (OC) mass for particles emitted by diesel vehicles is currently accomplished using particle collection on filters, followed by analysis using the thermal/optical reflectance carbon analysis method (TOR) or one of its variations. Such filter methods limit time resolution to a minimum of several minutes, making it impossible to study emissions during transient operating conditions. Testing of five different measurement methods has demonstrated that fast response measurement of diesel exhaust particulate EC and OC concentrations, consistent with TOR filter measurements, is feasible using existing technology. EC mass concentrations are best measured through determination of particulate light absorption with a photoacoustic instrument or determination of light extinction with a smoke meter. The photoacoustic instrument has the better dynamic range and sensitivity, whereas the smoke meter is a simpler instrument. Fast response OC measurements cannot be made with any single instrument tested. However, a combination of real time weighing as implemented in the tapered element oscillating microbalance with the photoacoustic instrument has been shown to be capable of determining OC concentrations with good time response. The addition of a nephelometer to the OC measurement could potentially improve time resolution, freedom from interferences, and sensitivity.

Preparation of Cd(1-x)Zn(x)Se using microwave-assisted polyol synthesis.

Abstract: A new method for the preparation of a nanophased ternary selenide is described. The synthesis employs microwave heating for the preparation of Cd1-xZnxSe. The reaction was carried out in basic ethylene glycol, which serves both as a solvent and reducing agent. The resulting compound was composed of monodispersed nanoparticles with an average diameter of 6 nm. The particles were characterized using TEM, HR-TEM, XRD, and photoacoustic spectroscopy (PAS).

Design and testing of an endoscopic photoacoustic probe for determination of treatment depth after photodynamic therapy

Abstract: An endoscopic photoacoustic probe is designed and tested for use in PDT treatment of esophageal cancer. The probe, measuring less than 2.5 mm in diameter, was designed to fit within the lumen of an endoscope that will be inserted into an esophagus after PDT. PDT treatment results in a blanched, necrotic layer of cancerous tissue over a healthy, deeper layer of perfused tissue. The photoacoustic probe was designed to use acoustic propagation time to determine the thickness of the blanched surface of the esophagus, which corresponds to treatment depth. A side-firing 600 micrometers fiber delivered 532 nm laser light to induce acoustic waves in the perfused layer of the esophagus beneath the blanched (treated) layer. A PVDF transducer detected the induced acoustic waves and transmitted the signal to an oscilloscope. The probe was tested on clear and turbid tissue phantom layers over an optically absorbing dye solution.

Pulsed laser photoacoustic detection of SO 2 near 225.7 nm

Abstract: A trace concentration of SO2 near 225.7 nm has been detected with a master-oscillator power-oscillator laser system for the first time, to our knowledge. A photoacoustic absorption spectrum of SO2 has been recorded on the 1A2–1B2 (π–π*) transition. Parametric dependence of the photoacoustic signal has been investigated. A detection limit (signal-to-noise ratio of 1) of 1.3 parts in 109 [1.3 ppbv (parts per billion by volume)] for SO2 have been determined at 1 atmospheric pressure inside a photoacoustic cell.

Acoustic resonance frequency locked photoacoustic spectrometer

Abstract: A photoacoustic spectroscopy method and apparatus for maintaining an acoustic source frequency on a sample cell resonance frequency comprising: providing an acoustic source to the sample cell, the acoustic source having a source frequency; repeatedly and continuously sweeping the source frequency across the resonance frequency at a sweep rate; and employing an odd-harmonic of the source frequency sweep rate to maintain the source frequency sweep centered on the resonance frequency.

Photoacoustic spectroscopy apparatus and method

Abstract: The invention encompasses photoacoustic apparatuses and photoacoustic spectrometry methods. The invention also encompasses sample cells for photoacoustic spectrometry, and sample cell/transducer constructions. In one aspect, the invention encompasses a photoacoustic spectroscopy apparatus, comprising: a) a sample reservoir and an acoustically-stimulable transducer acoustically coupled with the sample reservoir, the transducer comprising a detector surface having a substantially planar portion; and b) a beam of light configured to be directed through the sample at an angle oblique relative to the substantially planar portion of the detector surface to generate sound waves in the sample. In another aspect, the invention encompasses a photoacoustic spectroscopy sample cell, comprising: a) a first block of material having opposing front and back surfaces, the front surface comprising a substantially planar portion configured to be against a sample and the back surface comprising a substantially planar portion configured to be joined to a transducer, the back surface being parallel to the front surface; and b) a pair of opposing side surfaces joined to opposite ends of the front and back surfaces, one of the opposing side surfaces being configured for passage of light therethrough and extending at a first oblique angle relative to a plane containing the substantially planar portion of the front surface.

Characterization of Beef and Pork using Fourier-Transform Infrared Photoacoustic Spectroscopy

Abstract: The potential of Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) to examine successive layers of meat (beef and pork) was studied. Sample preparation for FTIR-PAS measurement was simple and nondestructive. Spectra of meat samples using the more conventional Attenuated Total Reflectance (ATR) accessory were also obtained for validation of the photoacoustic spectroscopy. Comparison of the photoacoustic and attenuated total reflectance spectra indicated that the PAS spectra were better resolved and more sensitive to moisture variation with respect to sample depth.

Acoustic resonance phase locked photoacoustic spectrometer

Abstract: A photoacoustic spectroscopy method and apparatus for maintaining an acoustic source frequency on a sample cell resonance frequency comprising: providing an acoustic source to the sample cell to generate a photoacoustic signal, the acoustic source having a source frequency; continuously measuring detection phase of the photoacoustic signal with respect to source frequency or a harmonic thereof; and employing the measured detection phase to provide magnitude and direction for correcting the source frequency to the resonance frequency.

Monitoring Chemical Changes in Some Foods Using Fourier Transform Photoacoustic Spectroscopy

Abstract: The potential of Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) to characterize some common foods was studied. Lard, peanut butter, mayonnaise, and whipped topping were heated to 60 and 90 8C separately, and the spectra were obtained periodically for a period of up to 32 and 16 d, respectively. Key bands in the mid-infrared spectral region were examined to monitor changes presumably due to lipid oxidation. Spectral data were analyzed, using principal component analysis (PCA) correlation, and linear discriminate analysis (LDA) techniques with Mahalanobis distances to estimate the extent of deterioration attributable to oxidation. The PAS was found to be simple, rapid, and nondestructive. It required limited sample preparation, and proved highly desirable for analyzing low-moisture samples.

Photoacoustic spectroscopy sample array vessel and photoacoustic spectroscopy method for using the same

Abstract: Methods and apparatus for analysis of multiple samples by photoacoustic spectroscopy are disclosed. Particularly, a photoacoustic spectroscopy sample array vessel including a vessel body having multiple sample cells connected thereto is disclosed. At least one acoustic detector is acoustically coupled with the vessel body. Methods for analyzing the multiple samples in the sample array vessels using photoacoustic spectroscopy are provided.

Resonance photoacoustic spectroscopy and gas analysis of gaseous flow at reduced pressure

Abstract: The results of theoretical and experimental studies of sensitivity of a resonant photoacoustic Helmholtz resonator detector for gas flowing through a photoacoustic cell under reduced pressure are presented. The measurements of the sensitivity and ultimate sensitivity of the differential photoacoustic cell were performed with a near-IR room-temperature diode laser using the well-known H2O absorption line (12496.1056 cm-1) as a reference. The measured value of the sensitivity (6–17 Pa W m-1) is in satisfactory agreement with the calculated one, which equals 6–35 Pa W m-1. The obtained value of the ultimate sensitivity ((3–5)×10-7 W m-1 Hz-1/2) provides measurements of the concentration of molecules at the ppb–ppm level.

Experimental comparison between optical spectroscopy and laser-ultrasound generation in polymer-matrix composites

Abstract: Laser ultrasound is a technique based on lasers to generate and detect ultrasound. The generation mechanism involves several parameters among which one of the most important is the optical penetration depth. This letter presents a comparison between the amplitude of ultrasonic waves generated by an optical parametric oscillator and optical penetration depth spectra measured by photoacoustic spectroscopy in the 3.0 to 3.5 μm wavelength range for three different composite samples. The laser-ultrasound amplitude spectra closely track the photoacoustic spectra. The results presented in this letter experimentally demonstrate why the 3.0–3.5 μm wavelength range generates more efficiently generates ultrasonic waves in the ultrasonic frequency range of interest for the inspection of polymer-matrix composites than the 10.6 μm wavelength of the CO2 laser.

Detection of trace levels of water

Abstract: Methods and apparatuses for detecting low levels of a substance of interest in a sample using photoacoustic spectroscopy are provided. This method comprises exciting the sample with light having a wavelength that is absorbed by the substance of interest; generating an acoustic wave within the sample, detecting the acoustic wave, and determining the amount of the substance of interest present in the sample. The method can be used to detect the amount of water in an oil sample at detection limits lower than currently available.

Phase References and Cell Effects in Photoacoustic Spectroscopy

Abstract: In Fourier transform infrared (FT-IR) photoacoustic spectroscopy, advanced scanning and data-handling techniques have placed increasing emphasis on the phase of the photoacoustic signal. Unfortunately, there is no agreement on the best material to use as a phase reference. We have examined the frequency dependence of the signal from several candidate phase references and found that cell effects dominate the absolute phases and magnitudes observed. The absolute phase is exceptionally fast at low frequencies and exceptionally slow at high frequencies because of the cell effects. Accordingly, details such as sample position must be scrupulously controlled to achieve accurate, reproducible results. Because of the cell effects, no candidate material behaves like an ideal phase reference. If relative phases are used, however, glassy carbon comes closest to the ideal, differing from theory by no more than 8° at any frequency examined.

High-precision pulsed photoacoustic spectroscopy in NO2-N2

Abstract: A method for high-precision pulsed photoacoustic spectroscopy applied to a simple system for detection of NO2 traces in nitrogen is presented. The acoustic signal from a closed cell containing NO2/N2 samples irradiated by a pulsed visible laser is analyzed in the frequency domain. A signal-processing method to obtain a high-resolution Fourier spectrum of the signal was developed. An accurate fitting of the resonance peaks with Lorentzian profiles gives high-precision determination of the amplitude and width of the resonance peaks. The resonance maximum is proportional to the absorbed energy; therefore, the choice of the laser wavelength, linewidth and frequency stability are critical for a precise calibration due to the fine structure of the NO2 optical spectrum. The method also allows high-accuracy measurement of the Q of the acoustic cavity. The dependence of Q on the buffer gas pressure is characteristic of an acoustic cavity where energy losses near the walls predominate. Consequently, an important enhancement of sensitivity takes place at high N2 pressure.

Array-based photoacoustic spectroscopy

Abstract: Methods and apparatus for simultaneous or sequential, rapid analysis of multiple samples by photoacoustic spectroscopy are disclosed. A photoacoustic spectroscopy sample array including a body having at least three recesses or affinity masses connected thereto is used in conjunction with a photoacoustic spectroscopy system. At least one acoustic detector is positioned near the recesses or affinity masses for detection of acoustic waves emitted from species of interest within the recesses or affinity masses.

Photoacoustic spectroscopy sample array vessels and photoacoustic spectroscopy methods for using the same

Abstract: Methods and apparatus for simultaneous or sequential, rapid analysis of multiple samples by photoacoustic spectroscopy are disclosed. Particularly, a photoacoustic spectroscopy sample array vessel including a vessel body having multiple sample cells connected thereto is disclosed. At least one acoustic detector is acoustically positioned near the sample cells. Methods for analyzing the multiple samples in the sample array vessels using photoacoustic spectroscopy are provided.

A study on the detection of weak photoacoustic signals by stochastic resonance.

Abstract: On the basis of stochastic resonance (SR) theory a study of the detection of photoacoustic (PA) signals without a lock-in amplifier is presented. The applicability of the SR method is investigated both with simulated signals calculated by use of a Gaussian model and with experimental PA signals. Signal-to-noise ratio (SNR) is improved substantially by adjusting the properties of a non-linear system to realize SR. By use of this method the lock-in amplifier can be removed from a PA spectrometer.

Color toner for flash fixing

Abstract: A color toner to which flash light is applied and is fixed on a recording medium at least includes a binding resin, a colorant, and a infrared absorbent. PAS intensity thereof obtained from integration of infrared PAS spectrum obtained based on photoacoustic spectroscopy (PAS) analytical measurement through a range between 800 and 2000 nm is in a range between 0.01 and 0.2 assuming that the value of carbon black is 1.

Photoacoustic Spectra of ZnO-CoO Alloy Semiconductors.

Abstract: Photoacoustic spectra (PA) of Zn1-xCoxO alloy semiconductors sintered as powdered samples were measured to estimate the optical absorption. Photoacoustic spectroscopy (PAS) is insensitive to the light scattering effects on the samples. CoO molecular percentages in the alloy were changed from 0 to 20 mol%. The samples were sintered at 600 or 950°C for 24 h in a quartz glass tube. From the PA spectra in the short wavelength region, we plotted (ρh ν)2 vs h ν and estimated the band-gap energy (Eg) of the alloy semiconductors, where ρ is the PA signal intensity and h ν is the excitation light photon energy. Eg of ZnO-CoO alloy decreases as the CoO content increase and reaches to 2.19 eV in the 20 mol% CoO sample. Although the band-gap variation in the lower-CoO-content samples (CoO content below 10 mol%) is large, it becomes small in the high CoO concentration samples. This result concurred with the result of dissolving CoO in ZnO as obtained by X-ray diffraction measurement.