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Dual-comb spectroscopy can provide broad spectral bandwidth and high spectral resolution in a short acquisition time, enabling time-resolved measurements. Specifically, spectroscopy in the mid-infrared wavelength range is of particular interest, since most of the molecules have their strongest rotational-vibrational transitions in this "fingerprint" region. Here we report time-resolved mid-infrared dual-comb spectroscopy for the first time, covering ~300 nm bandwidth around 3.3 {\mu}m with 6 GHz spectral resolution and 20 {\mu}s temporal resolution. As a demonstration, we study a CH4/He gas mixture in an electric discharge, while the discharge is modulated between dark and glow regimes. We simultaneously monitor the production of C2H6 and the vibrational excitation of CH4 molecules, observing the dynamics of both processes. This approach to broadband, high-resolution, and time-resolved mid-infrared spectroscopy provides a new tool for monitoring the kinetics of fast chemical reactions, with potential applications in various fields such as physical chemistry and plasma/combustion analysis., Comment: 21 page, 6 figures

Trace gas detection has many applications in research and industry; such as breath analysis, environmental monitoring, plasma diagnostics, and fruit quality monitoring [1] . Amongst various methods used for trace gas detection, molecular absorption spectroscopy is a powerful technique, which provides fast, selective, sensitive, and non-destructive detection. The fundamental rotational-vibrational transitions of molecular species are mostly in the mid-infrared (MIR) "fingerprint" (2–20 μm) region, which are significantly stronger than the overtone or combination bands in the near-infrared region. Fourier transform spectroscopy (FTS) is one of the well-known optical techniques, which is capable of broadband detection in the MIR region. However, incoherent thermal light sources, traditionally used in FTS, are omnidirectional and have low spectral brightness. Thus, the maximum achievable interaction length with a gas sample is very limited, resulting in low detection sensitivity. On the other hand, recently developed MIR supercontinuum (SC) light sources can deliver very high spectral brightness compared to the thermal sources or even a synchrotron radiation source [2] . Here, we present a multi-species trace gas sensor based on a MIR SC source, a multi-pass absorption cell, and a compact home-built FTS. For enhancement of the detection sensitivity and reducing the averaging time, we utilized a balanced detection scheme to reduce the relative intensity noise. For this, we measured the two out-of-phase outputs of the interferometer by two detectors. The two interferograms are 180 degrees out-of-phase, but they have an in-phase intensity noise. By subtracting the two outputs, the interferograms are doubled, while the noise is significantly reduced.

Nitrate and ammonia deferentially modulate primary metabolism during the hypersensitive response in tobacco. In this study, tobacco RNAi lines with low nitrite reductase (NiRr) levels were used to investigate the roles of nitrite and nitric oxide (NO) in this process. The lines accumulate NO2–, with increased NO generation, but allow sufficient reduction to NH4+ to maintain plant viability. For wild-type (WT) and NiRr plants grown with NO3–, inoculation with the non-host biotrophic pathogen Pseudomonas syringae pv. phaseolicola induced an accumulation of nitrite and NO, together with a hypersensitive response (HR) that resulted in decreased bacterial growth, increased electrolyte leakage, and enhanced pathogen resistance gene expression. These responses were greater with increases in NO or NO2– levels in NiRr plants than in the WT under NO3– nutrition. In contrast, WT and NiRr plants grown with NH4+ exhibited compromised resistance. A metabolomic analysis detected 141 metabolites whose abundance was differentially changed as a result of exposure to the pathogen and in response to accumulation of NO or NO2–. Of these, 13 were involved in primary metabolism and most were linked to amino acid and energy metabolism. HR-associated changes in metabolism that are often linked with primary nitrate assimilation may therefore be influenced by nitrite and NO production.

A mid-infrared trace gas detection system based on off-axis integrated cavity output spectroscopy (OA-ICOS) is demonstrated for accurate and sensitive detection of N2O in combination with a continuous wave external-cavity quantum cascade laser (EC-QCL) working around 7.7 µm. A 13-times improvement in signal-to-noise ratio is achieved using a re-injection mirror and a minimum detection limit of 70 ppbv in less than 10 s averaging time is achieved, which yields a noise-equivalent absorption sensitivity (NEAS) of 6×10−9 cm−1 Hz−1/2. For comparison, a compact multipass cell is deployed to measure the same absorption line of N2O using wavelength modulation spectroscopy with second harmonic detection (WMS-2f). An enhancement factor of 20 in comparison to direct absorption spectroscopy (DAS) is achieved, yielding a minimum detection limit of 15 ppbv in less than 10 s averaging and a NEAS of 1×10−9 cm−1 Hz−1/2. A comprehensive comparison between the two systems is carried out in terms of residual amplitude noise (RAM), linearity, long-term stability, detection limit, spectral fitting, reproducibility, and background variations. The proposed sensor based on OA-ICOS is potentially advantageous for trace gas sensing in outdoor applications and harsh environments due to its robustness and flexibility of alignment.

Collision-induced absorption between O2 and CO2 molecules associated with the a1Δg (v = 1) ← X3Σ−g (v = 0) band of oxygen around 1060 nm was measured using cavity ring-down spectroscopy. The lineshape for this transition is measured for the first time, and the integrated cross-section is found to be smaller than the only previous report. For pure oxygen, we find an integrated absorption value of (2.10 ± 0.31) × 10−4 cm−2 amg−2 which is in good agreement with the previous reported values. For O2–CO2 collisions we report an integrated value of (6.37 ± 1.09) × 10−5 cm−2 amg−2 which is small but still significant and not accounted for by theory.

A Fourier transform spectrometer (FTS), equipped with a broadband MIR supercontinuum (SC) source, is developed for analyzing CH4/N2 discharge plasma and the kinetics of plasma products (typically C2H6 and HCN) under different discharge currents.

We present a multi-species trace gas sensor based on a fast, compact home-built Fourier transform spectrometer (FTS) combined with a broadband mid-infrared supercontinuum (SC) source. The spectrometer covers the spectral bandwidth of the SC source (2 - 4 µm) and provides a best spectral resolution of 1 GHz in 6 seconds. It has a detection sensitivity of a few hundred of ppbv Hz-1/2 for different gas species. We study the performance of the developed spectrometer in terms of precision, linearity, long-term stability, and multi-species detection. We use the spectrometer for measuring fruit-produced volatiles under different atmospheric conditions and compare the performance with a previously developed scanning grating-based spectrometer.

A major challenge for breath research is the lack of standardization in sampling and analysis. To address this, a test that utilizes a standardized intervention and a defined study protocol has been proposed to explore disparities in breath research across different analytical platforms and to provide benchmark values for comparison. Specifically, the Peppermint Experiment involves the targeted analysis in exhaled breath of volatile constituents of peppermint oil after ingestion of the encapsulated oil. Data from the Peppermint Experiment performed by proton transfer reaction mass spectrometry (PTR-MS) and selected ion flow tube mass spectrometry (SIFT-MS) are presented and discussed herein, including the product ions associated with the key peppermint volatiles, namely limonene, alpha- and beta-pinene, 1,8-cineole, menthol, menthone and menthofuran. The breath washout profiles of these compounds from 65 individuals were collected, comprising datasets from five PTR-MS and two SIFT-MS instruments. The washout profiles of these volatiles were evaluated by comparing the log-fold change over time of the product ion intensities associated with each volatile. Benchmark values were calculated from the lower 95% confidence interval of the linear time-to-washout regression analysis for all datasets combined. Benchmark washout values from PTR-MS analysis were 353 min for the sum of monoterpenes and 1,8-cineole (identical product ions), 173 min for menthol, 330 min for menthofuran, and 218 min for menthone; from SIFT-MS analysis values were 228 min for the sum of monoterpenes, 281 min for the sum of monoterpenes and 1,8-cineole, and 370 min for menthone plus 1,8-cineole. Large inter- and intra-dataset variations were observed, whereby the latter suggests that biological variability plays a key role in how the compounds are absorbed, metabolized and excreted from the body via breath. This variability seems large compared to the influence of sampling and analytical procedures, but further investigations are recommended to clarify the effects of these factors. Marie Skodowska-Curie Actions Innovative Training Network: Ion-Molecule Processes for Analytical Chemistry Technologies (IMPACT) through the European Commission's HORIZON 2020 Programme [674911]; UK's National Physical Laboratory

A compact and lightweight mid-infrared supercontinuum gas spectrometer has been developed and successfully tested in airborne campaigns to simultaneously monitor the methane and water vapor concentration in the exhaust fumes of ships.

Traditional Fourier transform spectrometers (FTSs) in an asymmetric configuration can provide both absorption and dispersion spectra of the molecules simultaneously. However, they require a long measurement time, mainly due to the limitations of the signal-to-noise-ration (SNR) as well as maximum speed and travel distance of the retro-reflector mirrors in the FTS. These constraints make FTS inefficient for time-dependent multispecies detection with high SNR in certain specific applications, like plasma and combustion systems. These limitations can be resolved by mid-infrared dual-comb spectroscoy (DCS), which provides unique features of broad spectral bandwidth, high spectral resolution, high detection sensitivity, and simultaneous measurement of absorption and dispersion spectra in the fingerprint region within a short acquisition time. Here, we report time-resolved absorption and dispersion spectroscopy of methane and ethane diluted in helium under static and dynamic electrical discharge plasma, covering a spectral bandwidth of 2850-3150 cm -1 , with a spectral resolution of 0.16 cm -1 [1] . We used an asymmetric configuration for the DCS to measure the absorption and dispersion spectra in a 50-cm-long discharge tube. In this configuration, the gas sample is placed in the beam path of the signal mid-infrared comb, and the transmitted beam interferes with the local mid-infrared comb on a beam splitter. Thus, the molecular absorption and dispersion information is probed by the signal comb, and the local comb is used to retrieve the molecular information through multi-heterodyne down-conversion. The absorption and dispersion spectra retrieved from DCS portray a complete picture of electromagnetic filed interaction with gas phase molecules, which can provide the broadband complex refractive index of the gas samples under discharge.

Air pollution is one of the largest risk factors for disease or premature death globally, yet current portable monitoring technology cannot provide adequate protection at a local community level. Within the TRIAGE project, a smart, compact and cost-effective air quality sensor network will be developed for the hyperspectral detection of gases which are relevant for atmospheric pollution monitoring or dangerous for human health. The sensor is based on a mid-infrared supercontinuum source, providing ultra-bright emission across the 2-10 mu m wavelength region. Within this spectral range, harmful gaseous species can be detected with high sensitivity and selectivity. The spectroscopic sensor, which includes a novel multi-pass cell and detector, enables a smart robust photonic sensing system for real-time detection. With built-in chemometric analysis and cloud connection, the sensor will feed advanced deep-learning algorithms for various analyses, ranging from long-term continental trends in air pollution to urgent local warnings and alerts. Community-based distributed pollution sensing tests will be verified on municipal building rooftops and local transport platforms. Funding Agencies|Horizon 2020 [101015825, 732968]; European Unions Framework Programme for Research and Innovation

We present a fast-scanning Fourier transform spectrometer (FTS) in combination with high-repetition-rate mid-infrared supercontinuum sources, covering a wavelength range of 2–10.5 µm. We demonstrate the performance of the spectrometer for trace gas detection and compare various detection methods: baseband detection with a single photodetector, baseband balanced detection, and synchronous demodulation at the repetition rate of the supercontinuum source. The FTS uses off-the-shelf optical components and provides a minimum spectral resolution of 750 MHz. It achieves a noise equivalent absorption sensitivity of ∼10−6 cm−1 Hz−1/2 per spectral element, by using a 31.2 m multipass absorption cell.

An asymmetric time-resolved mid-infrared dual-comb spectrometer with 5 GHz spectral resolution is used to monitor the absorption and dispersion dynamics of different species and their rotational distributions within a non-thermal plasma.

We present a mid-infrared supercontinuum-based trace gas sensor. A lock-in technique is applied to enhance the signal-to-noise ratio by over an order of magnitude, achieving a sensitivity of ~10 ppbv Hz-1/2 for the detection of methane.

We present a multi-species trace gas sensor based on a high-repetition-rate mid-infrared supercontinuum source, in combination with a 30 m multipass absorption cell, and a scanning grating spectrometer. The output of the spectrometer is demodulated by a digital lock-in amplifier, referenced to the repetition rate of the supercontinuum source. This improved the detection sensitivity of the system by a factor 5, as compared to direct baseband operation. The spectrometer provides a spectral coverage of 950 cm−1 (between 2.85-3.90 µm) with a resolution of 2.5 cm−1 in 100 ms. It can achieve noise equivalent detection limits in the order of 100 ppbv Hz−1/2 for various hydrocarbons, alcohols, and aldehydes.

We present a transportable, multi-species, trace gas sensor using a high repetition rate mid-infrared supercontinuum source in combination with a lock-in amplifier, achieving tens of ppbv·Hz-1/2 detection sensitivity for multiple gas species.

We present a time-resolved mid-infrared dual-comb spectrometer based on a femtosecond two-crystal singly resonant optical parametric oscillator, suitable for monitoring the dynamics of fast chemical reactions in millisecond and microsecond time-scales.

We present a multi-species trace gas sensor based on broadband mid-infrared absorption spectroscopy as well as its application to measure fruit-produced volatiles under different atmospheric conditions, demonstrating its potential for monitoring fruit storage rooms.

Strong ro-vibrational transitions of molecules in the mid-infrared wavelength region, coupled with fast data acquisition of dual-comb spectroscopy (DCS), yields an effective tool for time-resolved simultaneous monitoring of the concentrations of different species involved in a gas-phase chemical reaction [1]. Here we used the broadband and high resolution properties of mid-infrared DCS, to monitor the static and dynamic concentrations of methane and the reaction products, in an electrical discharge. The mid-infrared dual comb source is based on an optical parametric oscillator (OPO) in a singly resonant cavity configuration, containing two temperature stabilized, 5 mm long, MgO: PPLN crystals. The crystals are pumped by two mode-locked Yb:fiber lasers, counter propagating and crossed polarized, emitting around 1040 nm. The two Yb:fiber lasers have a slightly different repetition rate (Δf rep ≈ 250 Hz) near 90 MHz [2]. The two coherent mid-infrared output combs, emitted from the OPO, are combined and sent through a discharge tube filled with methane. The discharge tube is 50 cm long, with an internal diameter of 3 mm, and is water cooled. The cathodes are at the two ends of the tube and the anode as at the center. A stabilized high-voltage (HV) power supply (25 kV, 50 mA) is used for making a DC discharge in the tube. To monitor the dynamic events in the discharge, the current of HV power supply is modulated (at 2 Hz) by an external signal generator. After the discharge tube, the two combs are focused on a single TE cooled HgCdTe photodetector (bandwidth 50 MHz). The time domain interferogram is recorded by a field-programmable gate array (FPGA) and post-processed to yield the spectrum after a Fourier transform. The voltage and current of the HV power supply, as well as the flow rate and the pressure of the sample gas are adjusted depending on the discharge operation.

An overview will be given on the new coherent sources that became available and can fulfil the needs for analytical spectroscopy in the mid infrared wavelength region. Recent highly-visible applications will be presented.

In the field of laser-based absorption spectroscopy, off-axis integrated cavity output spectroscopy is considered to be a sensitive and robust method, employing a simple optical design. However, one of the major drawbacks of non-mode-matched cavities combined with highly reflective mirrors (>99.98%) is its low output intensity. Here, we systematically investigate the increase in cavity output intensity, using a third re-injection mirror before the absorption cavity. The presented design not only enables high transmission power but also retains a long effective path length. To investigate the intensity enhancement, we used a CO2 absorption line in the near-IR wavelength region at 6240.10 cm−1. In agreement with our simulation model, we achieved an intensity enhancement factor of 38. We achieved a noise equivalent absorption sensitivity to 1.6×10−8 cm−1 Hz−1/2, which is no longer limited by the detectivity of the detector.

The detection of gas-phase molecular species at low concentration levels is an attractive research field, supporting a wide variety of industrial, environmental, and biological applications. Among various types of detection methods, mid-infrared (MIR) laser absorption spectroscopy provides a unique opportunity for selective and sensitive detection by targeting the characteristic spectral patterns in the so-called MIR fingerprint region.

We present a fully integrated and transportable multi-species trace gas sensor based on a mid-infrared (MIR) supercontinuum light source. The high brightness (surpassing synchrotron) and ultra-broad spectral bandwidth (2&ndash, 4 &mu, m) of this light source allows simultaneous detection of multiple broadband absorbing gas species. High sensitivity in the sub-ppmv level has been achieved by utilizing an astigmatic multipass cell. A grating-based spectrometer at a scanning rate of 20 Hz is developed employing a balanced detection scheme. A multi-component global fitting algorithm is implemented into a central LabVIEW program to perform real-time data analysis. The obtained concentration values are validated by the standard gas chromatography mass spectrometry (GC-MS) method. Field application of the sensor for quality control of stored fruits at a small scale is demonstrated, involving the detection of ethylene, ethanol, ethyl acetate, acetaldehyde, methanol, acetone, and water simultaneously. The sensor also shows promising potentials for other applications, such as environmental monitoring and biomedical research.

Recent advancement in compact, stable, and broadband mid-infrared supercontinuum (SC) sources has provided attractive scientific and industrial applications in metrology, sensing, and communications [1]. The wide spectral coverage in the so-called molecular fingerprint region is the main advantage of such SC sources, opening up new opportunities for efficient multi-species trace gas detection at short time scale. Presently, there is a lack of sensitive, low-cost, uncooled MIR photodetectors. The alternative approach involves up-converting mid-infrared light into the near-infrared region via non-linear conversion, leveraging the benefits of sensitive and cost-effective CCD detectors [2]. Here, we present the development of a multi-species trace gas sensor utilizing a mid-infrared SC source in combination with a multipass absorption cell and an up-conversion-based spectrometer. The customized SC source (NKT Photonics) has a total power of 450 mW covering the 1.5–4.2 μm spectral window. The astigmatic Herriott multipass cell (AMAC-76, Aerodyne Research) provides an effective optical pathlength of 76 meters improving the detection sensitivity. Details of the up-conversion-based spectrometer (NLIR) have been recently published. [3]. Note that in the present upconversion system the intracavity laser at 1064 nm is generated by pumping the Nd:YVO 4 gain medium at 808 nm instead of 880 nm. In order to enhance the long term stability for single species detection, the amplitude drift of the SC source is minimized by counterbalancing the SC intensity with a non-absorbing reference.

Accurate measurements of line strengths and air-broadening coefficients for 10 lines of 12CH4 mostly from the band (2ν2 + ν3) and 2 lines of 13CH4 from the band (2ν3) around 1.66 µm were performed using cavity ring down spectroscopy. Spectra of mixtures of 12CH4 in air and 13CH4 in air with well-defined mole fractions of methane were recorded at pressures between 10 and 90 mbar at 296 K. The values of the line strength for 12CH4 and 13CH4 lines were obtained with the uncertainty 1% and 2%, respectively. The uncertainties of the values of the air-broadening coefficient varied from 2 to 5% depending on the line. The values of the line strength and air-broadening coefficient for two methane isotopes obtained in this work were compared with the corresponding values from different experimental studies, as well as with the data given in HITRAN 2016 database.

We present an UAV compatible spectrometer designed for sampling air pollutants in remote locations. Relying on a custom-made Multi-Pass Cell and a 3-5 µm broadband supercontinuum light source, it takes advantage of an uncooled MWIR camera to record 2D cross-dispersed molecular absorption spectra.

This study addressed the role of lipid-derived factors together with ethylene and ROS in programmed cell death (PCD) signaling in tobacco BY-2 suspension cultured cells. The cells were exposed to the chemical stress agents mastoparan (MP) and ethanol (EtOH). MP is an activator of membrane-associated heterotrimeric G-proteins and downstream phospholipids-dependent processes; EtOH is suggested to affect lipid-associated pathways. The effects of MP and EtOH were compared to cell death in response to CdSO4. All applied chemicals appeared potent cell death inducers. Ethylene and lipid signaling were found instrumental in chemical-induced cell death, presumably in conjunction with ROS. Cadmium and MP induced cell death of apoptotic-like phenotype. Lower EtOH concentrations (1–2%) induced vacuolar cell death associated with autophagy-associated formation of lysosome-like acidic organelles in part of the cells; in other cells in the same suspension apoptotic-like features were observed. At higher EtOH concentration (3%) the dead cells expressed exclusively apoptotic-like morphology. The results suggest that phospholipase D- and phospholipase C-derived phosphatidic acid triggers ROS generation that is responsible for the observed apoptotic-like PCD. Vacuolar cell death in EtOH-treated cells appeared dependent on phospholipase C - phosphatidylinositol-3-kinase-related pathway. The obtained results indicate that depending on the inducer and stress severity, similar or distinct regulatory pathways can be activated, and the signals may interact in the transmission of the cell death message.

Recent advancements of mid-infrared (MIR) supercontinuum light sources have opened up new possibilities in laser-based trace gas sensing. While the supercontinuum sources inherently support wide spectral coverage, the detection of broadband absorption signals with high speed and low cost is traditionally limited by the MIR detector arrays. In this work, we demonstrate that this limitation can be circumvented by upconverting the MIR signal into the near-infrared (NIR) region, where cost-effective silicon-based detector arrays can be utilized to measure broadband absorption. We also show that, by combining a MIR supercontinuum source with a MIR-to-NIR upconverter and an astigmatic multipass cell, fast detection (~20 ms) of ethane with sub-ppmv sensitivity can be achieved at room temperature. For multi-species detection, a least-square global fitting method is presented, showing a promising potential for applications such as environmental monitoring and biomedical research.

We employed a single-mode, widely tunable (~300 cm&minus, 1) external-cavity quantum cascade laser operating around 8 &micro, m for broadband direct absorption spectroscopy and wavelength modulation spectroscopy where a modulation frequency of 50 kHz was employed with high modulation amplitudes of up to 10 GHz. Using a compact multipass cell, we measured the entire molecular absorption band of acetone at ~7.4 &micro, m with a spectral resolution of ~1 cm&minus, 1. In addition, to demonstrate the high modulation dynamic range of the laser, we performed direct absorption (DAS) and second harmonic wavelength modulation spectroscopy (WMS-2f) of the Q-branch peak of acetone molecular absorption band (HWHM ~10 GHz) near 1365 cm&minus, 1. With WMS-2f, a minimum detection limit of 15 ppbv in less than 10 s is achieved, which yields a noise equivalent absorption sensitivity of 1.9 &times, 10&minus, 8 cm&minus, 1 Hz&minus, 1/2.

Despite the established importance of nitric oxide (NO) in many physiological and molecular processes in plants, most methods for quantifying NO are open to criticism This reflects the differing methods either lacking specificity or sensitivity, or even from an undue dependence of results on experimental conditions (i.e., chemical concentrations, pH, etc.). In this chapter we describe a protocol to measure gaseous NO produced by a biological sample using quantum cascade laser (QCL)-based spectroscopy. This technique is based on absorption of the laser light by the NO molecules which have been passed from a biological sample into an optical s cell that is equipped with two mirrors placed at both ends. This design greatly increases the interaction path length with the NO molecules due to multiple reflections of the light coupled inside the cell. Thus, the method is able to provide online, in planta measurements of the dynamics of NO production, being highly selective and sensitive (down to ppbv levels;1 ppbv = part per billion by volume mixing ratio = 1:10

Detection and analysis of volatile compounds in exhaled breath represents an attractive tool for monitoring the metabolic status of a patient and disease diagnosis, since it is non-invasive and fast. Numerous studies have already demonstrated the benefit of breath analysis in clinical settings/applications and encouraged multidisciplinary research to reveal new insights regarding the origins, pathways, and pathophysiological roles of breath components. Many breath analysis methods are currently available to help explore these directions, ranging from mass spectrometry to laser-based spectroscopy and sensor arrays. This review presents an update of the current status of optical methods, using near and mid-infrared sources, for clinical breath gas analysis over the last decade and describes recent technological developments and their applications. The review includes: tunable diode laser absorption spectroscopy, cavity ring-down spectroscopy, integrated cavity output spectroscopy, cavity-enhanced absorption spectroscopy, photoacoustic spectroscopy, quartz-enhanced photoacoustic spectroscopy, and optical frequency comb spectroscopy. A SWOT analysis (strengths, weaknesses, opportunities, and threats) is presented that describes the laser-based techniques within the clinical framework of breath research and their appealing features for clinical use.

A novel approach to enhance the signal-to-noise ratio in off-axis integrated cavity output spectroscopy, centered around 8µm is demonstrated. This work exhibits cavity-enhanced spectroscopy using a compact size, with absorption sensitivity comparable to longer absorption cells.

A 3-D ray-tracing model is developed, to model reinjection in Off-Axis Cavity Enhanced Spectroscopy. Modelling and experiments demonstrate, at near-IR laser wavelengths, a 38-times enhancement, corresponding to a NEAS of 1.1 × 10−9 cm−1 Hz−1/2.

We present mid-infrared trace gas sensing based on a supercontinuum source combined with direct absorption spectroscopy and sum-frequency upconversion, showing broadband detection with a 10−4 Hz−1/2 noise equivalent absorbance sensitivity.

We developed an EC-QCL-based spectrometer around 8 μm for detection of nitrous oxide. Using wavelength modulation spectroscopy and 2nd harmonic detection, a minimum detection limit of 15 ppbv is achieved in less than 10 s.

Despite the established importance of nitric oxide (NO) in many physiological and molecular processes in plants, most methods for quantifying NO are open to criticism This reflects the differing methods either lacking specificity or sensitivity, or even from an undue dependence of results on experimental conditions (i.e., chemical concentrations, pH, etc.). In this chapter we describe a protocol to measure gaseous NO produced by a biological sample using quantum cascade laser (QCL)-based spectroscopy. This technique is based on absorption of the laser light by the NO molecules which have been passed from a biological sample into an optical s cell that is equipped with two mirrors placed at both ends. This design greatly increases the interaction path length with the NO molecules due to multiple reflections of the light coupled inside the cell. Thus, the method is able to provide online, in planta measurements of the dynamics of NO production, being highly selective and sensitive (down to ppbv levels;1 ppbv = part per billion by volume mixing ratio = 1:10-9).

Proton transfer reaction mass spectrometry (PTR-MS) is employed as a highly sensitive detection method for trace gas analysis of sulfur compounds. The effects of drift tube humidity and the reduced electric field ( E / N value) were evaluated to measure traces of methanethiol, dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) under optimal conditions. The three sulfur compounds showed a maximum intensity of the product ion signal at a low fraction of the protonated water clusters as compared to the total reagent ion signal. For methanethiol, DMS and DMDS the highest intensity was observed when the percentage branching ratio of H 3 O + ·H 2 O with total reagent ion signal was 3%, 5% and 3%, respectively. The methanethiol signal dropped sharply with an increase in H 3 O + ·H 2 O while DMS and DMDS showed a less strong dependence. This could be explained by their difference in proton affinity as compared to that of the water cluster (methanethiol 773 kJ mol −1 , DMS 830 kJ mol −1 , DMDS 815 kJ mol −1 , H 2 O·H 2 O >800 kJ mol −1 ). Taking into account the mean kinetic energy in the drift tube (19 kJ mol −1 ), the direct proton transfer reaction between DMS or DMDS and H 3 O + ·H 2 O is possible, in contrast to methanethiol. The PTR-MS was used to monitor trace gas emissions of methanethiol, DMS and DMDS emitted from Brassica rapa plants in real time. The concentrations of these sulfur compound emissions are in the part per billion range (ppbv) and optimization of the instrument for these compounds helps to measure the low concentrations emitted from control and infested plants. The plant roots were infested with Delia radicum larvae and all three sulfur compounds showed an immediate rise in the ion signal intensity after infestation, indicating that the plants were reacting after the roots were damaged by the insects. The sulfur compound emissions lasted for 30 h.

We demonstrate mid-infrared dual-comb spectroscopy with an optical parametric oscillator (OPO) toward real-time field measurement. A singly resonant OPO based on a MgO-doped periodically poled lithium niobate (PPLN) crystal is demonstrated. Chirped mirrors are used to compensate the dispersion caused by the optical cavity and the crystal. A low threshold of 17 mW has been achieved. The OPO source generates a tunable idler frequency comb between 2.7 and 4.7 μm. Dual-comb spectroscopy is achieved by coupling two identical Yb-fiber mode-locked lasers to this OPO with slightly different repetition frequencies. A measured absorption spectrum of methane is presented with a spectral bandwidth of $$300\,\hbox {cm}^{-1}$$ , giving an instrumental resolution of $$0.4\,\hbox {cm}^{-1}$$ . In addition, a second OPO containing two MgO-doped PPLN crystals in a singly resonant ring cavity is demonstrated. As such, this OPO generates two idler combs (average power up to 220 mW), covering a wavelength range between 2.7 and 4.2 μm, from which a mid-infrared dual-comb Fourier transform spectrometer is constructed. By detecting the heterodyned signal between the two idler combs, broadband spectra of molecular gases can be observed over a spectral bandwidth of more than $$350\,\hbox {cm}^{-1}$$ . This special cavity design allows the spectral resolution to be improved to $$0.2\,\hbox {cm}^{-1}$$ without locking the OPO cavity, indicating that this OPO represents an ideal high-power broadband mid-infrared source for real-time gas sensing.

Fruit and vegetables are regularly stored by consumers in the refrigerator at temperatures that may be well below the recommended storage temperatures. Apart from causing visible symptoms such as watery, sunken areas on the skin, chilling may also induce changes in fruit textural properties and flavour. The aim of this research was to investigate the effect of low temperature storage on tomato flavour and off-flavour production. To more closely mimic the real-consumer aroma perception while eating, in addition to the standard solid-phase microextraction gas chromatography-mass spectrometry (SPME/GC-MS) analysis, volatiles were also measured using a chewing device connected to a proton-transfer reaction-mass spectrometer (PTR-MS). Aroma volatiles were assessed in red ripe tomatoes of the cvs Cappricia RZ (round truss) and Amoroso RZ (cocktail truss) stored at refrigerator temperature (4 °C) and at higher temperatures (16 and 22 °C) for 20 days. The changes in aroma production were also monitored when the fruit was brought from room to refrigerator temperature and vice versa. After bringing the fruit from room to refrigerator temperature, the abundance of most volatiles was greatly reduced within 3 to 5 h, closely following the decrease in fruit temperature. When temperature was restored to room temperature following varying times of cold storage, the abundance of most volatiles increased again, but generally not to the original levels. Overall, the effects of low temperature storage on the decrease in volatile abundance were more pronounced in cv Cappricia RZ than in cv Amoroso RZ. On the contrary, the production of off flavours following prolonged cold storage was more pronounced in cv Amoroso RZ than in cv Cappricia RZ. Apart from changes in the overall abundance of the volatiles, marked changes in the volatile profile were observed in fruit stored for longer times in the cold and this may at least in part explain the negative effect of cold storage on overall tomato flavour.

Nitric oxide (NO) is a key messenger in plant stress responses but its exact role in drought response remains unclear. To investigate the role of NO in drought response we employed transgenic barley plants (UHb) overexpressing the barley non-symbiotic hemoglobin gene HvHb1 that oxidizes NO to NO3 −. Reduced NO production under drought conditions in UHb plants was associated with increased drought tolerance. Since NO biosynthesis has been related to polyamine metabolism, we investigated whether the observed drought-related NO changes could involve polyamine pathway. UHb plants showed increases in total polyamines and in particular polyamines such as spermidine. These increases correlated with the accumulation of the amino acid precursors of polyamines and with the expression of specific polyamine biosynthesis genes. This suggests a potential interplay between NO and polyamine biosynthesis during drought response. Since ethylene has been linked to NO signaling and it is also related to polyamine metabolism, we explored this connection. In vivo ethylene measurement showed that UHb plants significantly decrease ethylene production and expression of aminocyclopropane-1-carboxylic acid synthase gene, the first committed step in ethylene biosynthesis compared with wild type. These data suggest a NO-ethylene influenced regulatory node in polyamine biosynthesis linked to drought tolerance/susceptibility in barley., This work was supported by the Spanish Ministry of Economy and Competitiveness [AGL2013-48687-R and AGL2016-78965-R], the European Regional Development Funds, a JAE PreDoc fellowship from CSIC to GMB.

A radio frequency ion funnel reaction chamber is applied for use with a proton transfer reaction ion trap mass spectrometer (PIT-MS). The improvement in sensitivity over our dc-only drift tube ranged from 7.2 x for isoprene to 65 x for benzene. The ideal settings of the ion funnel are found to be a dc voltage of 60 V, RF voltage of 240 V and a pressure of 1.80 mbar. An experimental and theoretical investigation of the ion funnel is presented. Simulations using SIMION and experimental measurements of the m/z dependent transmission of the ion funnel are performed, as well as simulations of the trajectories of ions and their survival probability. These are used to deduce a greater understanding of the operation of the ion funnel and the sensitivity improvements. The funnel is shown to favor transmission of higher m/z, and to increase sensitivity by a combined effect of focusing and increased effective reaction time. The product ions of an example set of compounds are measured systematically and the reagent ion branching profiles studied and compared to ion trap fragmentations. This represents an important undertaking in the characterization studies of ion funnel technology for use in PTR-MS.