Showing papers on "Photoacoustic spectroscopy published in 2004"

Photoacoustic mammography laboratory prototype: imaging of breast tissue phantoms

Abstract: We present a laboratory version of a photoacoustic mammoscope, based on a parallel plate geometry. The instrument is built around a flat high-density ultrasound detector matrix. The light source is a Q-switched Nd:YAG laser with a pulse duration of 5 ns. To test the instrument, a novel photoacoustic phantom is developed using poly(vinyl alcohol) gel, prepared by a simple procedure that imparts optical scattering suggestive of breast tissue to it without the requirement for extraneous scattering particles. Tumor simulating poly(vinyl alcohol) gel spheres appropriately dyed at the time of preparation are characterized for optical absorption coefficients. These are then embedded in the phantom to serve as tumors with absorption contrasts ranging from 2 to 7, with respect to the background. Photoacoustic studies in transmission mode are performed, by acquiring the laser-induced ultrasound signals from regions of interest in the phantom. Image reconstruction is based on a delay-and-sum beamforming algorithm. The results of these studies provide an insight into the capabilities of the prototype. Various recommendations that will guide the evolving of our laboratory prototype into a clinical version are also discussed.

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

Abstract: 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-μm 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 × 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.

Formaldehyde sensor using interband cascade laser based quartz-enhanced photoacoustic spectroscopy

Abstract: A novel continuous-wave mid-infrared distributed feedback interband cascade laser was utilized to detect and quantify formaldehyde (H2CO) using quartz-enhanced photoacoustic spectroscopy. The laser was operated at liquid-nitrogen temperatures and provided single-mode output powers of up to 12 mW at 3.53 μm (2832.5 cm-1). The noise equivalent (1σ) detection sensitivity of the sensor was measured to be 2.2×10-8 cm-1 W (Hz)-1/2 for H2CO in ambient air, which corresponds to a detection limit of 0.6 parts in 106 by volume (ppmv) for a 10 s sensor time constant and 3.4 mW laser power delivered to the sensor module.

Application of a widely electrically tunable diode laser to chemical gas sensing with quartz-enhanced photoacoustic spectroscopy

Abstract: A near-infrared diode laser with sample-grating distributed Bragg reflectors was used as a widely tunable spectroscopic source for multispecies chemical sensing. Quartz-enhanced photoacoustic spectroscopy was utilized to obtain high absorption sensitivity in a compact gas cell. CO2, H2O, C2H2, and NH3 were monitored. A noise equivalent sensitivity of 8×10-9 cm-1 W-1 Hz-1/2 for NH3 detection was achieved, which corresponds to a NH3 mixing ratio of 4.4 parts in 106 by volume (ppmv) with a 1-s time constant and available 5.2-mW optical power in the gas cell.

Photoacoustic phase shift as a chemically selective spectroscopic parameter

Abstract: The phase information obtained in photoacoustic experiments can be used to separate the signals originating from chemical species with overlapping absorption spectra. This approach was applied to quantify parts per million CO levels in propylene using quartz-enhanced photoacoustic spectroscopy and a quantum cascade laser as an excitation source. The experimental data were used to evaluate V–T relaxation rates of CO and N2O in propylene.

Photoacoustic imaging of blood vessels with a double-ring sensor featuring a narrow angular aperture

Abstract: A photoacoustic double-ring sensor, featuring a narrow angular aperture, is developed for laser-induced photoacoustic imaging of blood vessels. An integrated optical fiber enables reflection-mode detection of ultrasonic waves. By using the cross-correlation between the signals detected by the two rings, the angular aperture of the sensor is reduced by a factor of 1.9, from 1.5 to 0.8 deg. Consequently, photoacoustic images could be obtained in a manner analogous to the ultrasound B-scan mode. Next, the cross section of artificial blood vessels is visualized by reconstruction of the absorbed energy distribution. Finally, in vivo imaging and the subsequent reconstruction of the absorbed energy distribution is demonstrated for superficial blood vessels in the human wrist.

Photoacoustic detection of ozone using a quantum cascade laser

Abstract: A 9.5-μm pulsed quantum cascade laser (QCL) and a differential photoacoustic (PA) detector were used to measure trace concentrations of ∼100 ppbv ozone at ambient pressure with high selectivity. The QCL was tuned by temperature variation between -41 °C and 30.6 °C and the corresponding wavelengths were determined by the PA spectrum of CO2. Good agreement was found between the measured PA spectrum and the simulated HITRAN spectrum of ozone. The PA signal showed a linear dependence on the ozone concentration in the investigated 4300–100 ppbv range. In comparison with recently published results, in which a similar QCL in combination with an optical absorption analysis technique was applied, an improvement in the ozone-detection sensitivity by a factor of about 200 was achieved.

O-H Stretch Overtone Excitation in Methyl and Ethyl Hydroperoxides

Abstract: We use laser photoacoustic spectroscopy to obtain vibrational overtone spectra in the regions of four and five quanta of O−H stretch (4νOH and 5νOH) for gas-phase methyl (MeOOH), ethyl (EtOOH), and...

A method and apparatus for analysis of semiconductor materials using photoacoustic spectroscopy techniques

Abstract: A photoacoustic spectrometer apparatus adapted to enable an observation and characterisation of non-radiative sub bandgap defects in narrow and large bandgap materials using photoacoustic spectroscopy techniques, the apparatus providing for an irradiation of a sample material provided within a photoacoustic cell and the subsequent detection and processing of an acoustic signal emitted by the sample, the apparatus comprising a light source having a polychromatic output substantially in the photonic energy range 0.5 eV to 6.2 eV, focusing means adapted to couple the output from the light source onto the sample material, the focusing means providing for an alignment and focusing of the light emitted from the light source so as to provide a substantially parallel incident light onto the sample material, and means for detecting and acquiring the acoustic signal emitted by the sample in response to the irradiation. A method of providing an acoustic signal spectrum emitted by a sample material provided within a photoacoustic cell following irradiation of the sample by an incident light beam is also provided.

Diode Laser Based Photoacoustic Water Vapor Detection System for Atmospheric Research

Abstract: 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 × 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.

Non-resonant multiphoton photoacoustic spectroscopy for noninvasive subsurface chemical diagnostics.

Abstract: This paper describes the development and validation of a novel noninvasive spectroscopic subsurface chemical detection technique, non-resonant multiphoton photoacoustic spectroscopy (NMPPAS). In this technique, non-resonant multiphoton excitation is used to provide subsurface excitation of chemical constituents in a sample followed by the subsequent detection of an acoustic signal using a piezoelectric transducer. Because NMPPAS relies on non-radiative relaxation of the absorbing species, it is capable of monitoring both fluorescent and non-fluorescent species. Moreover, since the majority of the energy imparted to most molecules upon the absorption of light is released through non-radiative pathways, sensitive measurements of even fluorescent molecules can be performed. In this paper, demonstration of proof-of-principle of this novel technique has been shown using test samples of common fluorescent dyes and biomarkers including rhodamine 6G, tryptophan, and NADH in solution and gelatin tissue phantoms. From these studies, it was found that detection limits of these chromophores are in the subnanomolar concentration regime. In addition, preliminary results on excised tumor and healthy tissue samples have demonstrated significant differences between the tumorous and non-tumorous tissues at 740 nm and 950 nm wavelengths. From this work, it was found that NMPPAS has a great deal of potential for subsurface chemical diagnostics in the field of biomedical research.

Sensitive wavelength-modulated photoacoustic spectroscopy with a pulsed optical parametric oscillator

Abstract: With a laser-excited acoustic wave as the carrier wave and by modulation of the light wavelength of a multikilohertz-repetition-rate optical parametric oscillator at a lower frequency than the acoustic frequency, we demonstrate a wavelength-amplitude double-modulation technique and achieve an enhancement factor of 35 in sensitivity in photoacoustic trace gas detection with the technique.

Heat diffusion and thermolastic vibration influence on the signal of an open photoacoustic cell for two layer systems

Abstract: Thermal diffusion and thermoelastic vibration in two layer systems is studied using photoacoustic spectroscopy. Three different two layer systems, copper/lead–tin (Cu/Pb–Sn), aluminum/paint (Al/paint) and aluminum/polyvinyl-chloride (Al/PVC) are studied. A model is developed considering that both layers present thermoelastic vibration and heat diffusion. It is shown that our equations can be used to obtain the theoretical photoacoustic signal independent of the mechanical properties when the thermal and thermomechanical properties of the composing layers are similar. It is shown that this approximation is much better suited for the study of metal/metal systems where the coupling of the thermoelastic bending is smooth. The restrictions of this approach and the possible extensions to study systems with different thermal expansion and mechanical behavior are discussed.

Pulsed near-infrared photoacoustic spectroscopy of blood

Abstract: The aim of this study was to use pulsed near infrared photoacoustic spectroscopy to determine the oxygen saturation (SO2) of a saline suspension of red blood cells in vitro. The photoacoustic measurements were made in a cuvette which formed part of a larger circuit through which the red blood cell suspension was circulated. Oxygen saturation of the red blood cell suspension was altered between 2-3% to 100% in step increments using a membrane oxygenator and at each increment an independent measurement of oxygen saturation was made using a co-oximeter. An optical parametric oscillator laser system provided nanosecond excitation pulses at a number of wavelengths in the near-infrared spectrum (740-1040nm) which were incident on the cuvette. The resulting acoustic signals were detected using a broadband (15MHz) Fabry-Perot polymer film transducer. The optical transport coefficient and amplitude were determined from the acoustic signals as a function of wavelength. These data were then used to calculate the relative concentrations of oxy- and deoxyhaemoglobin, using their known specific absorption coefficients and an empirically determined wavelength dependence of optical scattering over the wavelength range investigated. From this, the oxygen saturation of the suspension was derived with an accuracy of ±5% compared to the co-oximeter SO2 measurements.

Miniature photoacoustic chemical sensor using microelectromechanical structures

Abstract: Photoacoustic spectroscopy is a useful monitoring technique that is well suited for trace gas detection. The technique also possesses favorable detection characteristics when the system dimensions are scaled to a micro-system design. The objective of present work is to incorporate two strengths of the Army Research Laboratory (ARL), piezoelectric microelectromechanical systems (MEMS) and chemical and biological sensing into a monolithic MEMS photoacoustic trace gas sensor. A miniaturized macro-cell design was studied as a means to examine performance and design issues as the photoacoustics is scaled to a dimension approaching the MEMS level. Performance of the macro-cell was tested using standard organo-phosphate nerve gas simulants, Dimethyl methyl phosphonate (DMMP) and Diisoprpyl methyl phosphonate (DIMP). Current MEMS work centered on fabrication of a multi-layer cell subsystem to be incorporated in the full photoacoustic device. Preliminary results were very positive for the macro-cell sensitivity (ppb levels) and specificity indicating that the scaled cell maintains sensitivity. Several bonding schemes for a three-dimension MEMS photoacoustic cavity were investigated with initial results of a low temperature AuSn bond proving most feasible.

Photoacoustic Spectra from Co Doped ZnO with Different Grain or Cluster Sizes

Abstract: Photoacoustic (PA) spectra for Zn1-xCoxO mixed-crystal powders with various Co concentrations and sintering temperatures were measured. The PA spectra showed three peaks of Co2+ absorption between 560 nm and 660 nm. On the other hand, the PA spectra between 800 nm and 1000 nm were almost flat, and these are expected to be dominated by light scattering effects. The PA spectra were normalized by using the Co2+ absorption peaks in order to compare the sample size and the PA signals intensity, and the sintering temperature dependence of the PA spectra is discussed. The PA signal intensity decreased with the increase of the sintering temperature. The decrease of the PA spectra seems to be caused by small light scattering effects for the large clusters under higher temperature sintering, where the grains and clusters size increase. We could evaluate the grain growth in the sintering processes by PA spectroscopy in a noncontact mode.

Photothermal characterization of low density polyethylene food packages

Abstract: The present work discuss the applicability of photothermal techniques for determining diffusion coefficients of oxygen and carbon dioxide of commercial low-density polyethylene (LDPE). The methodology involves the monitoring of diffused gas by a photoacoustic analyzer. Diffusion coefficients measured for CO2 and O2 were 2.77 x 10-8 cm2/s and 1.68 x 10-7 cm2/s, respectively. To support the gas diffusion results, thermal properties were studied using photoacoustic spectroscopy and crystallinity was determined using X-ray diffraction. Values obtained for thermal diffusivity and specific heat capacity were 1.65 x 10-3cm2/s and 2.33 J.cm-3K-1, which are in good agreement with values available in the literature for pure LDPE and thus assure reliability of diffusion coefficients values.

Transparent Fabry-Perot polymer film ultrasound array for backward-mode photoacoustic imaging

Abstract: A novel optical ultrasound sensor has been developed for backward-mode photoacoustic imaging. The sensor is based on a Fabry Perot polymer film interferometer, the mirrors of which are transparent to 1064nm, but highly reflective at 850nm. When illuminated by a CW interrogating laser source at the latter wavelength, the system acts as a resonant Fabry Perot (FP) sensing cavity, the reflected intensity output of which is dependent upon acoustically-induced changes in the optical thickness of the polymer film. By optically addressing different regions of the sensor, a notional ultrasound array of arbitrary aperture and dimensionality can be synthesised. The system was demonstrated in backward mode by transmitting 1064nm excitation laser pulses through the sensor into an Intralipid scattering solution (mu(a) =0.03mm(-1),mu(s)'=1mm(-1)) containing various absorbing structures and detecting the resulting photoacoustic signals over a line. A ID depth profile of a 1.3mm thick absorbing polymer sheet (mu(a)=0.8mm(-1)) immersed to a depth of 12mm in the Intralipid solution was obtained by performing an 11mm linescan. In another experiment, a 3-layer structure consisting of 0.076mm thick line absorbers was immersed in Intralipid and a 21) image reconstructed from the detected photoacoustic signals using an inverse k-space reconstruction algorithm. Lateral resolution was 0.4mm and the vertical resolution 0.1mm. The ability of this system to map wideband photoacoustic signals with high sensitivity in backward mode may provide a useful tool for high resolution imaging of superficial tissue structures such as the skin microvasculature.

Phase transition in L-alaninium oxalate by photoacoustics

Abstract: Phase transition in L-alaninium oxalate is studied by using TG, DTA and photoacoustic spectroscopy A sharp transition at 378 K by photoacoustics is observed whereas at the same temperature the endothermic energy change observed by TG and DTA is not very sharp This is discussed in detail with reference to the other known data for the organic crystals

Laser photoacoustic spectroscopy: A powerful tool for measurement of trace gases of biological interest at sub-ppb level

Abstract: A high sensitive photoacoustic system has been developed to monitor trace gases released by biological samples at concentrations of sub-ppb level. We have applied this technique to measure the release of the plant hormone ethylene during seed germination and ripening of climacteric fruits and of the marker ethylene as a result of doping cultures of human cells with heavy metals, of damaging living organisms by ionising radiation(X-ray)and of unleashing lipid peroxidation in lung epithelium following the inhalation of cigarette smoke.

Prediction of Interlaminar Shear Strength of a Thermally Aged Carbon/Epoxy Composite Material by Fourier Transform Infrared Photoacoustic Spectroscopy

Abstract: Photoacoustic spectroscopy was used to predict the interlaminar shear strength of a carbon/epoxy composite. Samples were artificially aged by exposing the samples to elevated temperatures in an air environment. Short-beam shear tests were performed to determine mechanically the interlaminar shear strength of the samples. Photoacoustic spectra of the samples were also collected and compared to mechanical data. Chemometrics were performed on the spectral and mechanical data, and a good correlation was found between the near surface chemistry of the composite and overall mechanical integrity.

Development of a portable multiphoton photo-acoustic spectroscopy system for tumor diagnostics

Abstract: In this paper we describe the development of a novel fiber optic probe for subsurface tumor diagnostics, based on non-resonant multiphoton photoacoustic spectroscopy (NMPPAS). In this technique, endogenous biomarkers present in tissues are irradiated in the near infrared, using a tunable high-power laser. The resulting multiphoton excitation events are detected as an acoustic (i.e. ultrasonic) signal, using an ultrasonic piezoelectric transducer. The signal from the piezoelectric transducer is then corrected for laser power fluctuations by normalizing the NMPPAS signal at each wavelength with the laser intensity recorded, from an optical diode. By scanning the laser excitation over the appropriate wavelength range for the tissue of interest, absorption differences between normal and tumor tissues can be measured and analyzed. The fiber optic probe was characterized and optimized for transmission efficiency as well as its time dependent response to high power laser pulses. The focusing optics were optimized and a piezoelectric transducer film detector chosen based on its sensitivity in the ultrasonic frequency range of interest. Using this probe system NMPPAS measurements were performed on several common fluorescent dyes including rhodamine 6G as well as well-characterized biomarkers like tryptophan. Furthermore, the technique was further successfully applied to the differentiation of tumorous and healthy human brain tissues.

Concentration Profiling of a Molecular Sieve Membrane by Step-Scan Photoacoustic Spectroscopy

Abstract: We report the quantitative, nondestructive determination of the concentration profile of an organic molecule in a nanoporous polycrystalline zeolite molecular sieve membrane by step-scan IR photoacoustic experiments and analysis. A heterogeneous zeolite membrane model system was constructed by growing a zeolite MFI layer on a macroporous α-alumina substrate, followed by calcination to remove the organic tetrapropylammonium (TPA) structure-directing agent, and finally the growth of a second TPA-containing layer over the first. Step-scan photoacoustic spectroscopy is then used with a large range (10-500 Hz) of incident signal modulation frequencies to obtain a series of depth-dependent IR spectra. Deconvolution of these spectra and analysis of band intensities by the theory of photoacoustic signal generation allows for the determination of the TPA concentration profile. We briefly discuss the implications of this technique for understanding structure-property relationships in materials deposited as membranes for molecular sieving applications.

Interference effects in photoacoustic and reflectance spectroscopies on TiO2/Si structures and TiO2 band gap.

Abstract: Experimental results of photoacoustic (PAS) and reflectance (RS) spectroscopies of titanium dioxide thin films (TiO2), deposited on Si substrates, are compared in a wide optical range including transparent and absorbent regions of TiO2. Due to the fact that the light modulation frequency f used in the photoacoustic experiments was so low that the thermal diffusion length of the TiO2 (μ = 100 μm) is always larger than the thickness of the studied films, the PAS turns out to be complementary to RS over the entire range. The presence of multiple reflection interference effects makes difficult a direct evaluation of the TiO2 band gap from the PAS signal. However, by employing k(λ) values, obtained from transmission experiments on equivalent TiO2 films deposited on transparent fused quartz substrates, the PAS spectra for the films deposited on silicon are reconstructed by using those theoretical models that consider multiple reflections. The reasonable agreement of the simulated and experimental PAS spectra allows one to obtain reliable Eg values for the TiO2 films deposited on opaque silicon substrates.