Photoacoustic Spectroscopy Technique Research Articles

Analysis of Carbon Materials with Infrared Photoacoustic Spectroscopy.

Measurement of infrared spectroscopy has emerged as a significant challenge for carbon materials due to the sampling problem. To overcome this issue, in this work, we performed measurements of IR spectra for carbon materials including C60, C70, diamond powders, graphene, and carbon nanotubes (CNTs) using the photoacoustic spectroscopy (PAS) technique; for comparison, the vibrational patterns of these materials were also studied with a conventional transmission method, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, or Raman spectroscopy. We found that the IR photoacoustic spectroscopy (IR-PAS) scheme worked successfully for these carbon materials, offering advantages in sampling. Interestingly, the profiles of IR-PAS spectra for graphene and CNTs exhibit negative bands using carbon black as the reference; the negative spectral information may provide valuable knowledge about the storage energy, production, structure, defect, or impurity of graphene and CNTs. Thus, this approach may open a new avenue for analyzing carbon materials.

Quartz-enhanced multiheterodyne resonant photoacoustic spectroscopy

The extension of dual-comb spectroscopy (DCS) to all wavelengths of light along with its ability to provide ultra-large dynamic range and ultra-high spectral resolution, renders it extremely useful for a diverse array of applications in physics, chemistry, atmospheric science, space science, as well as medical applications. In this work, we report on an innovative technique of quartz-enhanced multiheterodyne resonant photoacoustic spectroscopy (QEMR-PAS), in which the beat frequency response from a dual comb is frequency down-converted into the audio frequency domain. In this way, gas molecules act as an optical-acoustic converter through the photoacoustic effect, generating heterodyne sound waves. Unlike conventional DCS, where the light wave is detected by a wavelength-dependent photoreceiver, QEMR-PAS employs a quartz tuning fork (QTF) as a high-Q sound transducer and works in conjunction with a phase-sensitive detector to extract the resonant sound component from the multiple heterodyne acoustic tones, resulting in a straightforward and low-cost hardware configuration. This novel QEMR-PAS technique enables wavelength-independent DCS detection for gas sensing, providing an unprecedented dynamic range of 63 dB, a remarkable spectral resolution of 43 MHz (or ~0.3 pm), and a prominent noise equivalent absorption of 5.99 × 10-6 cm-1·Hz-1/2.

Rare earth elements and radioactive elements in black sands from the Konkan and Malabar coasts of India

ABSTRACT The objective of the present work is to demonstrate the ability of the laser-induced breakdown spectroscopy (LIBS) technique for quick detection and quantification of rare earth elements (REEs) and radioactive elements in the black sands at selected locations of the Konkan coast (Goa) and Malabar coast (Kerala) in India. Several REEs such as Ce, Nd, Sm, Dy, Ho, Tm, Gd, and Th have been detected in the LIBS spectra. Calibration-free LIBS method has been used to calculate the concentration of REEs and radioactive elements. The experimental result shows that the concentration of REEs such as Sm, Dy, Ho, and radioactive element Th is maximum in the sands collected from Thiruvananthapuram (Kerala) beach and minimum in the Baga (Goa) beach. The reliability of the results (qualitative and quantitative) obtained by the LIBS method has been examined by other spectroscopic techniques like XRF and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and found to be in close agreement. The multivariate approach (principal component analysis) has been applied to discern the samples based on their spectral data. Photoacoustic spectroscopy and XRD technique are used to detect compounds/minerals of REEs. The relative hardness, degree of crystallinity (DOC), and crystallite size have been calculated.

Multigas nonresonant photoacoustic spectroscopy sensor based on a broadband radiation source

AbstractMultigas detection is in demand for a wide range of environmental and industrial applications. In this work, a sensitive and selective multigas photoacoustic spectroscopy (MG‐PAS) sensor was developed by employing a nonresonant photoacoustic spectroscopy technique combining a broadband radiation source. Six gases including CO, CO2, CH4, C2H2, C2H4, and C2H6 were selected as target measurement molecules. Selectivity of the MG‐PAS was realized by using six mid‐infrared narrow‐band filters which integrated in a homemade filter wheel. A minimum concentration detection limits of 0.2 ppm for CO, 0.05 ppm for CO2, 0.12 ppm for CH4, 0.1 ppm for C2H2, 0.81 ppm for C2H4, and 0.05 ppm for C2H6 were obtained, respectively, with a 1‐s integration time. Measurements of six gas mixtures showed that the average measurement error of the MG‐PAS sensor was within 2%.

Assessment of vibrational-translational relaxation dynamics of methane isotopologues in a wet-nitrogen matrix through QEPAS

Here we report on a study of the non-radiative relaxation dynamic of 12CH4 and 13CH4 in wet nitrogen-based matrixes by using the quartz-enhanced photoacoustic spectroscopy (QEPAS) technique. The dependence of the QEPAS signal on pressure at fixed matrix composition and on H2O concentration at fixed pressure was investigated. We demonstrated that QEPAS measurements can be used to retrieve both the effective relaxation rate in the matrix, and the V-T relaxation rate associated to collisions with nitrogen and water vapor. No significant differences in measured relaxation rates were observed between the two isotopologues.

A Miniaturized 3D-Printed Quartz-Enhanced Photoacoustic Spectroscopy Sensor for Methane Detection with a High-Power Diode Laser.

In this invited paper, a highly sensitive methane (CH4) trace gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) technique using a high-power diode laser and a miniaturized 3D-printed acoustic detection unit (ADU) is demonstrated for the first time. A high-power diode laser emitting at 6057.10 cm-1 (1650.96 nm), with the optical power up to 38 mW, was selected as the excitation source to provide a strong excitation. A 3D-printed ADU, including the optical and photoacoustic detection elements, had a dimension of 42 mm, 27 mm, and 8 mm in length, width, and height, respectively. The total weight of this 3D-printed ADU, including all elements, was 6 g. A quartz tuning fork (QTF) with a resonant frequency and Q factor of 32.749 kHz and 10,598, respectively, was used as an acoustic transducer. The performance of the high-power diode laser-based CH4-QEPAS sensor, with 3D-printed ADU, was investigated in detail. The optimum laser wavelength modulation depth was found to be 0.302 cm-1. The concentration response of this CH4-QEPAS sensor was researched when the CH4 gas sample, with different concentration samples, was adopted. The obtained results showed that this CH4-QEPAS sensor had an outstanding linear concentration response. The minimum detection limit (MDL) was found to be 14.93 ppm. The normalized noise equivalent absorption (NNEA) coefficient was obtained as 2.20 × 10-7 cm-1W/Hz-1/2. A highly sensitive CH4-QEPAS sensor, with a small volume and light weight of ADU, is advantageous for the real applications. It can be portable and carried on some platforms, such as an unmanned aerial vehicle (UAV) and a balloon.

Dual Absorption Broadband Photoacoustic Technique to Eliminate Interference in Gas Mixtures

This article presents a dual-wavelength band broadband photoacoustic spectroscopy (BPAS) technique ideally suited to low-energy broadband sources for industrial and environmental multigas sensing applications. The broad spectral range of the supercontinuum source is used to cover a wide range of absorption bands of various gases. The major limitation in most of the gas sensors with broadband sources is the effect of interference between gases. This article proposes a dual-wavelength technique to eliminate the impact of cross sensitivity among gases. This sensor exhibits a comprehensive concentration measurement from parts per million (ppm) to % levels. The proposed technique showed a ±1.2% error between the calculated and calibrated concentration values. We achieved a minimum detection limit (MDL) of 0.4 and 0.7 ppm for methane (CH4) with an integration time of 142 and 200 s using 2315- and 1650-nm band, respectively, 0.3 and 1.8 ppm of ammonia (NH3) with an integration time of 100 and 48 s using 2315- and 2017-nm band, respectively, and also 2.6 ppm of carbon monoxide (CO) with an integration time of 71 s using the 2315-nm band.

Photoacoustic phase-controlled Fourier-transform infrared spectroscopy

We demonstrate a 13-fold speed improvement in broadband cantilever-enhanced photoacoustic spectroscopy (CEPAS) by combining it with phase-controlled Fourier-transform spectroscopy (PC-FTS) instead of traditional Fourier-transform infrared spectroscopy (FTIR). PC-FTS is a modification of FTIR and capable of fundamentally faster interferogram acquisitions. The speed-improvement is beneficial for CEPAS, which is an especially sensitive version of the background-free photoacoustic spectroscopy technique. We used the PC-FTS-CEPAS technique to measure the absorption spectrum of methane in the mid-infrared region (3.3–3.5 µm) with an optical frequency comb as the light source.

Evaluation of the nutrients in milk supplements suitable for children using LIBS and PAS technique

The presence of macro and micro nutrients essential for human health in four milk supplements is quickly identified using Laser-induced Breakdown Spectroscopy (LIBS) and Photoacoustic Spectroscopy (PAS). In addition to the minerals reported by the manufacturers, some additional spectral signatures of the elements like Na and K are also observed in the LIBS spectra which are not listed by some manufacturers. The ratio of various minerals like Ca/Mg, Na/Mg, etc. has been evaluated, and found a good correlation with the nutritional value listed on the product packaging. The organic elements C, H, N, and O together with the CN molecular band observed in the LIBS spectra of milk supplement indicate the presence of organic molecules. PAS is used for the identification of organic compounds such as Vitamins, Proteins, Fat, Taurine, and Sucrose in milk supplements. The results of the present study help in selecting milk supplements suitable for children suffering from chronic kidney diseases and diabetes. Based on the presence of minerals and their relative concentration, milk supplements are classified into three groups by applying Principal Component Analysis (PCA). Therefore, the results from the present work extend the application of LIBS and PAS together with PCA for the identification of the nutrients in milk supplements.

Parts-per-billion-level detection of hydrogen sulfide based on doubly resonant photoacoustic spectroscopy with line-locking

We report on the development of a highly sensitive hydrogen sulfide (H2S) gas sensor exploiting the doubly resonant photoacoustic spectroscopy technique and using a near-infrared laser emitting at 1578.128 nm. By targeting the R(4) transition of H2S, we achieved a minimum detection limit of 10 part per billion in concentration and a normalized noise equivalent absorption coefficient of 8.9 × 10−12 W cm−1 Hz−1/2. A laser-cavity-molecule locking strategy is proposed to enhance the sensor stability for fast measurement when dealing with external disturbances. A comparison among the state-of-the-art H2S sensors using various spectroscopic techniques confirmed the record sensitivity achieved in this work.

Domainal Investigation of a Quartz-Fluorite Composite Using Spectroscopic Techniques

The analysis of geological samples that have several chemically diffused zones which formed under certain physico-chemical condition is difficult to achieve. The quantitative estimations of the minerals in such samples are tedious. The present work demonstrates the application of LIBS for qualitative and quantitative analyses of a quartz-fluorite composite which was procured from an amygdaloidal basalt from Deccan Traps, India. The presence of weak emission lines of F in the spectral range of 200–900 nm makes it challenging to quantify the fluorine. This study has addressed a promising alternative to quantify the fluorine using electronic bands of CaF molecules observed in the Laser-induced Breakdown Spectroscopy (LIBS) spectrum. In addition to this spectroscopic technique, the authors also have used Photoacoustic Spectroscopy (PAS) and UV-VIS spectroscopy technique to obtain molecular information from the geological sample. Principal Component Analysis (PCA) was applied to a truncated spectral region of the CaF molecule, and it showed 99% variance. Further, the obtained results with these spectroscopic techniques were compared with the results that were obtained from X-ray diffraction and Electron Probe Micro Analyzer, and they show good agreement. Thus, the LIBS technique can be promising for in situ profile section (varies from few microns to centimeters size) studies without the sample’s destruction using the point detection capability of LIBS.

Ex vivo UV-vis and FTIR photoacoustic spectroscopy of natural nanoemulsions from cellulose nanocrystals and saponins topically applied into the skin: Diffusion rates and physicochemical evaluation.

Nanoemulsions are increasingly gaining importance in the development of topically applied medicine and cosmetic products because their small droplets favor the penetration rates of active compounds into the body. In this scenario, the measurements of their diffusion rates as well as eventual physicochemical changes in the target tissues are of utmost importance. It is also recognized that the use of natural surfactants can avoid allergic reactions as frequently observed for synthetic products. The natural saponins extracted from Sapindus Saponaria have the property of forming foam and are exploited as biocompatible and biodegradable, while cellulose nanocrystals are known to increase the stability of a formulation avoiding the coalescence of drops at the interface. Therefore, nanoemulsions combining natural saponins and cellulose nanocrystals are promising systems that may facilitate greater diffusion rates of molecules into the skin, being candidates to substitute synthetic formulations. This study applied the Photoacoustic Spectroscopy technique to measure the diffusion rates and the physicochemical properties of nanoemulsified formulations containing saponins and cellulose nanocrystals topically applied to the skin. The ex vivo study combined the first-time photoacoustic measurements performed in both ultraviolet-visible and mid-infrared spectral regions. The toxicity of these formulations in L929 cells was also evaluated. The results showed that the formulations were able to propagate throughout the skin to a depth of approximately 756μm, reaching the dermal side. The non-observation of absorbing band shifting or new bands in the FTIR spectra suggests that there were no structural changes in the skin as well as in the formulations after the nanoemulsions administration. The cytotoxicity results showed that the increase of cellulose nanocrystals concentration decreased cellular toxicity. In conclusion, the results demonstrated the advantage of combining photoacoustic methods in the ultraviolet-visible and mid-infrared spectral regions to analyze drug diffusion and interaction with the skin tissues. Both methods complement each other, allowing the confirmation of the nanoemulsion diffusion through the skin and also suggesting there were no detectable physicochemical changes in the tissues. Formulations stabilized with saponins and cellulose nanocrystals showed great potential for the development of topically administered cosmetics and drugs.

Infrared dual-gas CH4/C2H2 sensor system based on dual-channel off-beam quartz-enhanced photoacoustic spectroscopy and time-division multiplexing technique

Highly sensitive and stable measurement of methane (CH4) and acetylene (C2H2) based on a novel dual-channel off-beam quartz-enhanced photoacoustic spectroscopy and time-division multiplexing technique was realized by a compact 3D-printed gas cell with a size of 3 × 2 × 1 cm3. Two near-infrared distributed feedback diode lasers were employed to target the CH4 absorption line at 6046.9 cm−1 and the C2H2 absorption line at 6521.2 cm−1, respectively. Second-harmonic wavelength modulation spectroscopy method was used for photoacoustic signal recovery. A minimum detection level of ∼ 7.63 parts-per-million in volume (ppmv) for CH4 and a level of ∼ 17.47 ppmv for C2H2 were achieved with a 1 s lock-in integration time, leading to a normalized noise equivalent absorption (NNEA) coefficient of 7.24 × 10-8 cm−1·W·Hz−1 and 3.73 × 10-8 cm−1·W·Hz−1 for CH4 and C2H2, respectively. Allan-Werle deviation analysis was employed to evaluate the stability and the minimum detection limit (MDL) of the developed photoacoustic CH4/C2H2 dual-gas photoacoustic sensor. Owing to the high stability of the developed sensor system, an MDL of ∼ 0.73 ppmv and an MDL of ∼ 1.60 ppmv with a 100 s averaging time were achieved for CH4 and C2H2, respectively.

Simultaneous measurement of NO and NO2 by dual-channel cavity photoacoustic spectroscopy technique

The levels of nitric oxide (NO) and nitrogen oxide (NO2) are trace gases in the order of ppb, which are the main products of fossil fuel, anthropogenic combustion, natural lightning and microbial emissions, and are important for further analysis of air quality. Photoacoustic spectroscopy (PAS) technique, which is based on absorption at the wavelength of 403.65 nm, has been used for NO2 measurements. In this study, a dual-channel PAS system for NO2 and NOx detection is reported. The PAS method is capable of measuring species with good sensitivity and accuracy. The calibrated slopes of photoacoustic cavity A and B are 0.040 μv/ppb and 0.039 μv/ppb, respectively. And the detection limits of the developed PAS system for NO2 and NOx measurements are estimated to be about 1.47 ppb (3σ, 60 s) and 1.50 ppb (3σ, 60 s), respectively. In order to reduce the influence of local noise, the photoacoustic cavity with multiple buffer partitions is designed and the corresponding photoacoustic frequency of the photoacoustic pool is analyzed. The sound pressure value and resonance frequency of the photoacoustic pool gradually decrease with the increase of the number of buffer partitions, but the amplitude is small. The relationship between the gas absorption cross section, volume fraction, laser power and photoacoustic excitation signal is derived. After deducting the influence of local noise, the photoacoustic response gradients of dual-channel cavities are obtained, the linear fitting of the response gradient of both cavities is good, the correlation R2 is greater than 0.99, and the fitting slope is 0.040 and 0.039 respectively. The influence of O3 generated by photolysis of the mercury lamp is considered under different NO level conditions. An intercomparison between the system and a cavity ring-down system (CRDS) independently developed by our research group showed a good agreement for measuring the ambient atmosphere NO2 gas. The results of NO2 showed a linear correction factor (R2) of 0.943 in a slope of 0.944 ± 0.002, with an offset of (0.636 ± 0.021) ppb. The accuracy of NO2 measurement by the photoacoustic spectroscopy system was verified. In addition, the simultaneous measurement of NO and NO2 by dual-channel cavity photoacoustic spectroscopy technique was deployed. These observations indicate that NO2 and NO concentrations can be effectively observed with the dual-channel PAS instrument.

Ammonia Concentration in Ambient Air in a Peri-Urban Area Using a Laser Photoacoustic Spectroscopy Detector.

Measuring ammonia from the environmental air is a sensitive and prioritized issue due to its harmful effects on humans, ecosystems, and climate. Ammonia is an environmental pollutant that has an important role in forming secondary inorganic aerosols, the main component of fine particulate matter concentrations in the urban atmosphere. Through this study, we present a gas analyzer that utilizes the technique of laser photoacoustic spectroscopy to measure ammonia concentration in three different sites located in Magurele, (44°20′58″ N 26°01′47″ E, 93 m altitude), Romania, from March to August 2021 at the breathing level of 1.5 m above ground. The ammonia concentrations from the ambient air were elevated in summer (mean of 46.03 ± 8.05 ppb (parts per billion)) compared to those measured in spring (18.62 ± 2.92 ppb), which means that atmospheric temperature affects ammonia concentrations. The highest mean ammonia concentrations occurred in August, with an ammonia concentration level of 100.68 ± 11.12 ppb, and the low mean ammonia concentrations occurred in March, with an ammonia level concentration of 0.161 ± 0.03 ppb. The results confirm that meteorological characteristics (i.e., temperature) and motor vehicles are major contributors to the elevated ammonia levels during the monitoring period.

Scattering enhanced photoacoustic gas sensor based on acetate membrane

AbstractIn this paper, a scattering enhanced photoacoustic spectroscopy (SEPAS) technique is introduced for trace gas detection. An acetate membrane is mounted at the inner wall of a miniaturized photoacoustic (PA) cell to improve the light‐gas interaction path. It has been found out from theoretical simulations that the light rays in the PA cell with the acetate material are obviously denser, leading to a large enhancement of the PA signal. A SEPAS system, mainly consisting of a PA sensor, a near‐infrared distributed feedback laser source, and a high‐speed demodulator has been developed for CH4 detection. The experimental results have successfully proved that the SEPAS technique based on the porous acetate material can achieve at least three times enhancement on the PA signal, compared with the PA cell without the acetate membrane. The minimum detection limit is calculated to be 57.5 parts per billion with an averaging time of 100 s.

A silicon micromechanical resonator with capacitive transduction for enhanced photoacoustic spectroscopy

This paper presents the design of a silicon micromechanical resonator with capacitive transduction for a gas sensor based on photoacoustic spectroscopy. It overcomes problems imposed by opposite physical trends originating from the sensor’s working principles. The capacitive transduction mechanism is considered incompatible with the photoacoustic gas detection as it reduces the mechanical displacement by a damping effect, thus decreasing the sensitivity. This occurs because the same part serves two functions: photoacoustic excitation and capacitive transduction. The suggested approach in this study focuses on the spatial separation of these two functions. We propose the first reported mechanical microresonator for photoacoustic gas sensing, which decouples photoacoustic excitation and capacitive transduction. The design has been modeled, fabricated, characterized, and compared with the on-beam Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) technique, a reference for compact and selective gas sensors. The performed photoacoustic measurement has been realized on calibrated concentrations of methane. For a 1 s integration time in first harmonic detection, we obtained a limit of detection (LOD) of 667 ppmv for silicon microresonator. In comparison to on-beam QEPAS technique, we obtained LOD of 163 ppmv, which constitutes a factor 4 between QEPAS and MEMS. The conditions for both experiments were the same.

Ultrasound irradiation effect on photosynthesis and transpiration of aquatic lirium plants

Purpose To find possible causes of the photobaric response decrease in photoacoustic measurements on Aquatic Lirium plants after ultrasonic irradiations reported elsewhere contributing to understanding the effect of ultrasonic irradiation on them and searching for environmentally friendly methodologies to control this weed. Materials and Methods The plants were extracted from their natural habitat in Xochimilco water canals, Mexico City. The irradiations on the plants were carried out to 2 hours exposure time, 17 kHz frequency, and 30 W x 4 output power. We used the photoacoustic spectroscopy technique at room temperature in the range of 400–750 nm to analyze the optical absorption response of photosynthetic pigments before and after ultrasonic irradiations. To monitor the leave transpiration rate, we used an LI-COR 6400XT portable system, expressed in units of mols H2O per second per unit area of the leaf surface. Results We obtained a significant decrease of the chlorophylls bands amplitude in the photoacoustic spectroscopy spectra and a drastic reduction in the leaves transpiration rate of irradiated plants that depends on the time elapsed after irradiation. Remarkable physical changes in the leaves and petioles of the irradiated plants were observed with the naked eye. Conclusions A significantly decreasing photosynthesis and transpiration in the leaves of the irradiated lirium plants were obtained. Together with the observed physical changes in the leaves and petioles, these results suggest an alteration in the water transport and the overall gas exchange mechanisms affecting the irradiated leaves' transpiration and photosynthesis processes that agree with the photobaric response decrease reported elsewhere. Due to the fundamental role of stomata in these mechanisms, it is suggested, as a possible cause, that the ultrasonic-induced disruption of stomata's mechanical operation by guard cells prevents them from performing their function normally. A hypothesis to be confirmed in future studies, for which a line of action is proposed.

Mid-Infrared Quartz-Enhanced Photoacoustic Sensor for ppb-Level CO Detection in a SF6 Gas Matrix Exploiting a T-Grooved Quartz Tuning Fork.

An optical sensor for highly sensitive detection of carbon monoxide (CO) in sulfur hexafluoride (SF6) was demonstrated by using the quartz-enhanced photoacoustic spectroscopy technique. A spectrophone composed of a custom 8 kHz T-shaped quartz tuning fork with grooved prongs and a pair of resonator tubes, to amplify the laser-induced acoustic waves, was designed aiming to maximize the CO photoacoustic response in SF6. A theoretical analysis and an experimental investigation of the influence of SF6 gas matrix on spectrophone resonance properties for CO detection have been provided, and the performances were compared with the standard air matrix. A mid-infrared quantum cascade laser with a central wavelength at 4.61 μm, resonant with the fundamental band of CO, and an optical power of 20 mW was employed as the light excitation source. A minimum detection limit of 10 ppb at 10 s of integration time was achieved, and a sensor response time of ∼3 min was measured.

An all-Optical Photoacoustic Sensor for the Detection of Trace Gas.

A highly sensitive Fabry–Perot based transduction method is proposed as an all-optical alternative for the detection of trace gas by the photoacoustic spectroscopy technique. A lumped element model is firstly devised to help design the whole system and is successfully compared to finite element method simulations. The fabricated Fabry–Perot microphone consists in a hinged cantilever based diaphragm, processed by laser cutting, and directly assembled at the tip of an optical fiber. We find a high acoustic sensitivity of 630 mV/Pa and a state-of-the-art noise equivalent pressure, as low as at resonance. For photoacoustic trace gas detection, the Fabry–Perot microphone is further embedded in a cylindrical multipass cell and shows an ultimate detection limit of 15 ppb of NO in nitrogen. The proposed optical trace gas sensor offers the advantages of high sensitivity and easy assembling, as well as the possibility of remote detection.