Dual-feedback mid-infrared cavity-enhanced absorption spectroscopy for H2CO detection using a radio-frequency electrically-modulated interband cascade laser

A mid-infrared cavity-enhanced sensor system was demonstrated for the detection of formaldehyde (H2CO) using a continuous-wave (cw) interband cascade laser (ICL) centered at 3599 nm. A compact Fabry-Perot (F-P) cavity with a physical size of 38 × 52 × 76 mm3 was developed consisting of two concave mirrors with a radius of curvature of 80 mm and a reflectivity of 99.8% at 3.6 μm. Different from the widely reported electro-optical (EO) external modulation based Pound-Drever-Hall (PDH) locking technique, a radio-frequency electrical internal modulation based PDH technique was used for locking the laser mode to the cavity mode. A dual-feedback control on the laser current and on the piezo transducer (PZT) displacement was utilized for further stabilizing mode locking. A 20 m effective optical path length was achieved with a cavity length of 2 cm and a finesse of 1572. The effectiveness and sensitivity of the sensor system were demonstrated by targeting an absorption line at 2778.5 cm−1 for H2CO measurements. A linear relation between the cavity transmitted signal amplitude and the H2CO concentration was obtained within the range of 0−5 ppm. A 1σ detection limit of 25 parts-per-billion (ppb) was achieved with an averaging time of 1 s based on Allan-Werle variance analysis. The reported dual-feedback RF modulation based PDH technique led to a method for gas detection using a similar experimental setup and measurement scheme

Repetitively Mode-Locked Cavity-Enhanced Absorption Spectroscopy (RML-CEAS) for Near-Infrared Gas Sensing

A Pound-Drever-Hall (PDH)-based mode-locked cavity-enhanced sensor system was developed using a distributed feedback diode laser centered at 1.53 µm as the laser source. Laser temperature scanning, bias control of the piezoelectric ceramic transducer (PZT) and proportional-integral-derivative (PID) feedback control of diode laser current were used to repetitively lock the laser modes to the cavity modes. A gas absorption spectrum was obtained by using a series of absorption data from the discrete mode-locked points. The 15 cm-long Fabry-Perot cavity was sealed using an enclosure with an inlet and outlet for gas pumping and a PZT for cavity length tuning. The performance of the sensor system was evaluated by conducting water vapor measurements. A linear relationship was observed between the measured absorption signal amplitude and the H2O concentration. A minimum detectable absorption coefficient of 1.5 × 10–8 cm–1 was achieved with an averaging time of 700 s. This technique can also be used for the detection of other trace gas species by targeting the corresponding gas absorption line

Interband cascade laser based mid-infrared methane sensor system using a novel electrical-domain self-adaptive direct laser absorption spectroscopy (SA-DLAS)

To suppress sensor noise with unknown statistical properties, a novel self-adaptive direct laser absorption spectroscopy (SA-DLAS) technique was proposed by incorporating a recursive, least square (RLS) self-adaptive denoising (SAD) algorithm and a 3291 nm interband cascade laser (ICL) for methane (CH4) detection. Background noise was suppressed by introducing an electrical-domain noise-channel and an expectation-known-based RLS SAD algorithm. Numerical simulations and measurements were carried out to validate the function of the SA-DLAS technique by imposing low-frequency, high-frequency, White-Gaussian and hybrid noise on the ICL scan signal. Sensor calibration, stability test and dynamic response measurement were performed for the SA-DLAS sensor using standard or diluted CH4ﾠsamples. With the intrinsic sensor noise considered only, an Allan deviation of ~43.9 ppbv with a ~6 s averaging time was obtained and it was further decreased to 6.3 ppbv with a ~240 s averaging time, through the use of self-adaptive filtering (SAF). The reported SA-DLAS technique shows enhanced sensitivity compared to a DLAS sensor using a traditional sensing architecture and filtering method. Indoor and outdoor atmospheric CH4measurements were conducted to validate the normal operation of the reported SA-DLAS technique

Isolation of Endogenously Assembled RNA-Protein Complexes Using Affinity Purification Based on Streptavidin Aptamer S1

Efficient isolation of endogenously assembled viral RNA-protein complexes is essential for understanding virus replication mechanisms. We have developed an affinity purification strategy based on an RNA affinity tag that allows large-scale preparation of native viral RNA-binding proteins (RBPs). The streptavidin-binding aptamer S1 sequence was inserted into the 3′ end of dengue virus (DENV) 5′–3′ UTR RNA, and the DENV RNA UTR fused to the S1 RNA aptamer was expressed in living mammalian cells. This allowed endogenous viral ribonucleoprotein (RNP) assembly and isolation of RNPs from whole cell extract, through binding the S1 aptamer to streptavidin magnetic beads. Several novel host DENV RBPs were subsequently identified by liquid chromatography with tandem mass spectrometry (LC-MS/MS), including RPS8, which we further implicate in DENV replication. We proposed efficient S1 aptamer-based isolation of viral assembled RNPs from living mammalian cells will be generally applicable to the purification of high- and low-affinity RBPs and RNPs under endogenous conditions

Performance Enhancement of Methane Detection Using a Novel Self-Adaptive Mid-Infrared Absorption Spectroscopy Technique

An electrical-domain self-adaptive mid-infrared absorption spectroscopy for methane detection based on an interband cascade laser was demonstrated. By adding noise into the laser drive signal, denoising and sensing performances were evaluated for the technique. Experiments were made to study the effects of noise level/type on sensor stability, characterized by Allan deviation. High- and low-frequency noise levels have the same functional variation trend on Allan deviation, which differs from white Gaussian noise. Within a noise level range of 0-125 mV for low- and high-frequency noise and 0-62.5 mV for white Gaussian noise in the mercury-cadmium-telluride detector's output (with a pure signal amplitude of ~300 mV), the sensor stability using self-adaptive denoising was enhanced by a factor of 1.05-20, 1.32-6.25, and 1.15-3.33 times compared to that using no filtering, for the three kinds of noise, respectively. The reported self-adaptive methane sensor system shows enhanced stability compared to the direct laser absorption spectroscopy sensor using traditional sensing architecture and classic filtering method. The sensor was further evaluated through outdoor atmospheric methane measurements using such technique. A second-order self-adaptive direct laser absorption spectroscopy technique was also proposed for noise suppression in both optical and electrical domain as an outlook of the concept of this paper

Exploratory study of atmospheric methane enhancements derived from natural gas use in the Houston urban area

The extensive use of natural gas (NG) in urban areas for heating and cooking and as a vehicular fuel is associated with potentially significant emissions of methane (CH4) to the atmosphere. Methane, a potent greenhouse gas that influences the chemistry of the atmosphere, can be emitted from different sources including leakage from NG infrastructure, transportation activities, end-use uncombusted NG, landfills and livestock. Although significant CH4 leakage associated with aging local NG distribution systems in the U.S. has been reported, further investigation is required to study the role of this infrastructure component and other NG-related sources in atmospheric CH4 enhancements in urban centers. In this study, neighborhood-scale mobile-based monitoring of potential CH4emissions associated with NG in the Greater Houston area (GHA) is reported. A novel dual-gas 3.337 μm interband cascade laser-based sensor system was developed and mobile-mode deployed for simultaneous CH4 and ethane (C2H6) monitoring during a period of over 14 days, corresponding to ∼ 90 h of effective data collection during summer 2016. The sampling campaign covered ∼250 exclusive road miles and was primarily concentrated on eight residential zones with distinct infrastructure age and NG usage levels. A moderate number of elevated CH4 concentration events (37 episodes) with mixing ratios not exceeding 3.60 ppmv and associated with atmospheric background enhancements below 1.21 ppmv were observed during the field campaign. Source discrimination analyses based on the covariance between CH4 and C2H6 levels indicated the predominance of thermogenic sources (e.g., NG) in the elevated CH4 concentration episodes. The volumetric fraction of C2H6 in the sources associated with the thermogenic CH4 spikes varied between 2.7 and 5.9%, concurring with the C2H6 content in NG distributed in the GHA. Isolated CH4 peak events with significantly higher C2H6 enhancements (∼11%) were observed at industrial areas and locations with high density of petroleum and gas pipelines in the GHA, indicating potential variability in Houston's thermogenic CH4 sources

Double-range near-infrared acetylene detection using a dual spot-ring Herriott cell (DSR-HC)

Design and fabrication of a dual spot-ring Herriott cell (DSR-HC) were proposed. The sealed Herriott cell with a dimensional size of 5.5 cm × 9.2 cm × 32.1 cm, possessed two input/output coupling holes leading to two absorption path lengths of ~20 m and ~6 m, respectively. An acetylene (C2H2) sensor system with a double-range was developed using the DSR-HC and wavelength modulation spectroscopy (WMS) technique. A near-infrared distributed feedback (DFB) laser was employed for targeting a C2H2 absorption line at 6521.2 cm-1. C2H2concentration measurements were carried out by modulating the laser at a 5 kHz frequency and demodulating the detector signal with LabVIEW software. An Allan-Werle deviation analysis indicated that the limit of detection (LoD) for the two absorption path lengths of 20 m and 6 m are 7.9 parts-per-million in volume (ppmv) and 4.0 ppmv, respectively. The DSR-HC concept can be used to fabricate similar cells for single-gas detection requiring two different detection ranges as well as for dual-gas detection requiring different absorption path lengths