Differential Photoacoustic Spectroscopy Research Articles

TT-type resonator-based differential photoacoustic spectroscopy for trace gas detection.

A novel TT-type resonator was proposed for the first time, to our knowledge, to realize differential photoacoustic (PA) detection for trace gas measurement. The special design of the TT-type resonator allows us to install the microphone at the resonant center of the acoustic field to maximize the use of the absorption-induced PA signal. To meet the requirement of low gas consumption and easy integration, the TT-type resonator-based PA cell was fabricated as a fiber-coupled module with an inner volume of only 1.1 ml. For validation, the TT-type PA cell was integrated to a photoacoustic spectroscopy (PAS) system for acetylene detection. As a result, a linearity of 0.99999 was achieved in a concentration range from 0 to 5000 ppm with a noise equivalent sensitivity of 101 ppb. The proposed TT-type resonator contributes a new style of PA cell structure to the field of PAS gas detection, combining the advantages of easy integration, low gas consumption, differential detection, and photoacoustic enhancement together.

Signal Enhancement of a Differential Photoacoustic Cell by Connecting the Microphones via Capillaries.

Differential photoacoustic spectroscopy (DPAS) cells are usually excited on the first longitudinal ring mode, with a microphone situated in the middle of each of the two resonator tubes. However, it is known from other photoacoustic spectroscopy cell designs that connecting the microphones via a capillary can lead to signal enhancement. By means of finite element method (FEM) simulations, we compared such a photoacoustic spectroscopy (PAS) cell with a capillary to a DPAS cell with a capillary attached to each of the two resonators and showed that the behavior of both systems is qualitatively the same: In both the PAS and the DPAS cell, in-phase and anti-phase oscillations of the coupled system (resonator-capillary) can be excited. In the DPAS cell, capillaries of suitable length also increase the pressure signal at the microphones according to the FEM simulations. For different capillary diameters (1.2 mm/1.7 mm/2.2 mm), the respective optimal capillary length (36-37.5 mm) and signal amplification was determined (94%, 70%, 53%). According to the results of these FEM simulations, a significant increase in sensitivity can, therefore, also be achieved in DPAS cells by expanding them with thin tubes leading to the microphones.

Simultaneous detection of greenhouse gases CH4 and CO2 based on a dual differential photoacoustic spectroscopy system.

In addition to the atmospheric measurement, detection of dissolved carbon oxides and hydrocarbons in a water region is also an important aspect of greenhouse gas monitoring, such as CH4 and CO2. The first step of measuring dissolved gases is the separation process of water and gases. However, slow degassing efficiency is a big challenge which requires the gas detection technology itself with low gas consumption. Photoacoustic spectroscopy (PAS) is a good choice with advantages of high sensitivity, low gas consumption, and zero background, which has been rapidly developed in recent years and is expected to be applied in the field of dissolved gas detection. In this study, a miniaturized differential photoacoustic cell with a volume of 7.9 mL is designed for CH4 and CO2 detection, and a dual differential method with four microphones is proposed to enhance the photoacoustic signal. What we believe to be a new method increases photoacoustic signal by 4 times and improves the signal to noise ratio (SNR) over 10 times compared with the conventional single-microphone mode. Two distributed feedback (DFB) lasers at 1651 nm and 2004nm are employed to construct the PAS system for CH4 and CO2 detection respectively. Wavelength modulation spectroscopy (WMS) and 2nd harmonic demodulation techniques are applied to further improve the SNR. As a result, sensitivity of 0.44 ppm and 7.39 ppm for CH4 and CO2 are achieved respectively with an integration time of 10 s. Allan deviation analysis indicates that the sensitivity can be further improved to 42 ppb (NNEA=4.7×10-10cm-1WHz-1/2) for CH4 and 0.86 ppm (NNEA=5.3×10-10cm-1WHz-1/2) for CO2 when the integration time is extended to 1000 s.

Ppb-level methane detection sensitivity based on a homemade Raman fiber amplifier and differential photoacoustic technology.

A high-power near-infrared wavelength-modulated differential photoacoustic spectroscopy sensor for parts-per-billion (ppb) level methane detection is reported by using a homemade Raman fiber optical amplifier. A commercial 1653.7nm continuous wave distributed feedback laser is employed as a seed source to excite a high light power of ∼550m W, which greatly improves sensor performance. Wavelength modulation spectroscopy and differential techniques are applied to further improve the signal-to-noise ratio of the photoacoustic signal. A 1σ minimum detection limit of ∼10p p b for methane detection is achieved with an integration time of 10s.

Wavelength-modulated photoacoustic spectroscopy sensor for multi-gas measurement of acetone, methane, and water vapor based on a differential acoustic resonator

Precise assessment of breath acetone and methane is significant to the medical diagnosis process. A multi-gas sensing system based on wavelength-modulated differential photoacoustic spectroscopy was developed for simultaneous measurement of acetone and methane. A distributed feedback diode laser emitting in the range from 3363 to 3371 nm was employed to scan the absorption lines of acetone and methane. The cross sensitivities in terms of spectral interference among acetone, methane, and water vapor are effectively eliminated by using a linear combination method of reference spectra for accurately determining the concentration of acetone and methane. The positive effect of water vapor on photoacoustic signal resulting from the light absorption of acetone and methane was precisely evaluated. To improve the instrument performance, a differential PA cell companying with a differential amplifier circuit is experimentally demonstrated in providing a better performance of noise suppression compared with a single acoustic resonator. With a low detection limit down to 0.43 ppm and 12 ppb (integration time of 10 s) for acetone and methane, the sensor shows a great potential for medical diagnosis in simultaneous measurement of acetone and methane.

Advances in differential photoacoustic spectroscopy for trace gas detection

AbstractPhotoacoustic spectroscopy (PAS), as a nondestructive method, permits high sensitivity and selectivity in trace gas detection fields. Advances in differential PAS configurations have dramatically suppressed the background noise and triggered the detectivity improvement. A detailed review on the development of differential photoacoustic gas sensors is presented: using two identical cavity differential cells and optical path types provides good performance for coherent noise suppression, and differential mode excitation technique, which evaluates the signal ratios with two frequency excitations, suppresses the common mode noise and signal drifts generated by the light power or the responsivity of the PA system. An overview of the latest developments of the differential PAS sensors is reported. Distinguished results in terms of sensitivity, selectivity, and miniaturization are presented. The advantages and limitations of differential sensors with different photoacoustic sensing technologies are discussed.

Non-Invasive Monitoring of Human Health by Photoacoustic Spectroscopy.

For certain diseases, the continuous long-term monitoring of the physiological condition is crucial. Therefore, non-invasive monitoring methods have attracted widespread attention in health care. This review aims to discuss the non-invasive monitoring technologies for human health based on photoacoustic spectroscopy. First, the theoretical basis of photoacoustic spectroscopy and related devices are reported. Furthermore, this article introduces the monitoring methods for blood glucose, blood oxygen, lipid, and tumors, including differential continuous-wave photoacoustic spectroscopy, microscopic photoacoustic spectroscopy, mid-infrared photoacoustic detection, wavelength-modulated differential photoacoustic spectroscopy, and others. Finally, we present the limitations and prospects of photoacoustic spectroscopy.

Fourier-Transform Infrared Differential Photoacoustic Spectroscopy (FTIR-DPAS) for Simultaneous Monitoring of Multiple Air Contaminants/Trace Gases

Air pollutants have severe impact on the global environment and the health of human beings. There is an urgent need for cost-effective devices for trace gas monitoring in ambient conditions. However, water vapor in ambient air is still an obstacle in the trace gas absorption detection field due to its complex and strong infrared absorbing characteristics. In this work, a step-scan Fourier-transform infrared differential photoacoustic spectroscopy (FTIR-DPAS) methodology developed in our laboratory through the introduction of two identical T-resonators for enhancing and resolving the DPA signal from two potentially pollutant gases is extended to multiple ambient gas components: carbon dioxide (CO2) and acetylene (C2H2). A key feature of this technique is the ability to resolve hidden target spectral components in ambient air: Despite the fact that the acetylene absorption peaks lie within the strong water vapor absorption band, the infrared PA absorption spectra of acetylene and carbon dioxide are detected with high sensitivity and selectivity in the presence of significant interference of water vapor in the laboratory ambient air, thereby confirming the superiority and capability of step-scan FTIR-DPAS configuration to effectively and totally suppress often dominant background water signals and simultaneously detect multiple trace gases.

Cover Picture: Wavelength‐Modulated Differential Photoacoustic Spectroscopy (WM‐DPAS) for noninvasive early cancer detection and tissue hypoxia monitoring (J. Biophotonics 4/2016)

Cover Picture: Wavelength‐Modulated Differential Photoacoustic Spectroscopy (WM‐DPAS) for noninvasive early cancer detection and tissue hypoxia monitoring (J. Biophotonics 4/2016)

Wavelength-Modulated Differential Photoacoustic Spectroscopy (WM-DPAS) for noninvasive early cancer detection and tissue hypoxia monitoring.

This study introduces a novel noninvasive differential photoacoustic method, Wavelength Modulated Differential Photoacoustic Spectroscopy (WM-DPAS), for noninvasive early cancer detection and continuous hypoxia monitoring through ultrasensitive measurements of hemoglobin oxygenation levels (StO2 ). Unlike conventional photoacoustic spectroscopy, WM-DPAS measures simultaneously two signals induced from square-wave modulated laser beams at two different wavelengths where the absorption difference between maximum deoxy- and oxy-hemoglobin is 680 nm, and minimum (zero) 808 nm (the isosbestic point). The two-wavelength measurement efficiently suppresses background, greatly enhances the signal to noise ratio and thus enables WM-DPAS to detect very small changes in total hemoglobin concentration (CHb ) and oxygenation levels, thereby identifying pre-malignant tumors before they are anatomically apparent. The non-invasive nature also makes WM-DPAS the best candidate for ICU bedside hypoxia monitoring in stroke patients. Sensitivity tunability is another special feature of the technology: WM-DPAS can be tuned for different applications such as quick cancer screening and accurate StO2 quantification by selecting a pair of parameters, signal amplitude ratio and phase shift. The WM-DPAS theory has been validated with sheep blood phantom measurements. Sensitivity comparison between conventional single-ended signal and differential signal.

Characteristic absorption peak of the human blood measured with differential photoacoustic spectroscopy

A new highly sensitive spectroscopy technique-differential photoacoustic spectroscopy (PAS) is presented in this paper. The blood samples from 3 healthy persons, patients with leukemia, patients with pregnancy-induced hypertension (PIH), and 40 patients with nasopharyngeal carcinoma were measured by the PAS technique. The normalized, the first order, and the second order differential photoacoustic spectroscopy of the blood were gained. The results show that (i) weak absorption peaks or shoulder peaks, which could not be found using conventional photoacoustic spectroscopy, were determined by the first order and the second order differential photoacoustic spectroscopy which significantly improve the sensitivity of detection; and (ii) that two characteristic absorption peaks were found at the wave-length of 637 and 664 nm in all persons' blood samples by the differential photoacoustic spectroscopy technique. This experiment concludes that the differential photoacoustic spectroscopy technique is superior to the conventional photoacoustic spectroscopy technique in detecting photoacoustic spectroscopy of biological samples.

Simple setup for single and differential photoacoustic spectroscopy

Simple cells are described for normal and differential measurements in photoacoustic spectroscopy. The differential cell allows for easy background signal correction and for comparison of related samples. The arrangement allows great flexibility in cell design for adaptation to special sample forms. The normal cell can be used for very small volumes, liquids as well as solids, and is constructed in such a way as to allow the possibility of Helmholtz resonance to occur over a range of frequencies. The two cells are compared in terms of background and maximal signal strength and examples of spectra obtained with each of them are given. The general spectrometer setup is outlined as well.