J. Reuss

Sensitive intracavity photoacoustic measurements with a CO2 waveguide laser

A photoacoustic intracavity configuration is presented; a resonant photoacoustic cell excited in its first longitudinal mode is placed inside the cavity of a CO2 waveguide laser. Due to the high laser power and the sharp intracavity focus, saturation effects occur in the excitation and relaxation process of absorbing C2H4 molecules. A more optimal configuration is applied to measure the C2H4 emission of several Rumex species. A detection sensitivity of 6 ppt (parts per trillion) C2H4 is reached, equivalent to a minimal detectable absorption of 1.8×10−10 cm−1.

The role of ethylene in shoot elongation with respect to survival and seed output of flooded Rumex maritimus L. plants

Rumex maritimus L. occurs in frequently flooded sites of lowland flood plains. Upon submergence this species exhibited rapid elongation of shoots, but the response depended upon the developmental stage when flooding was initiated. In the rosette stage, petioles showed a fast and large response; during early stem elongation the response of petioles was less, but the extension of the lower internodes considerable; during flowering stem development, high internodes extended and the contribution to final stem length diminished. Ethylene production by an intact plant before, during and after submergence was measured with a laser-driven photoacoustic technique. Internal ethylene concentrations increased within 12 h of submergence. Ethylene accumulated in the submerged plant due to increased synthesis as well as a reduced diffusion from the plant to the water (instead of air). The elongation response could in part be mimicked by exogenous ethylene. Directly after submergence a further increase of ethylene synthesis was observed, which may be of vital importance in causing shoots to continue their rapid elongation even after the water surface is reached. The responses of shoots were related to fitness in experimental field plots. Survival of submerged R. maritimus depended on its ability to emerge above the water surface, while seed production was positively correlated with shoot height above the

Ethylene and CO2 emission rates and pathways in harvested fruits investigated, in situ, by laser photothermal deflection and photoacoustic techniques

Abstract CO 2 laser photothermal deflection (limit 1 nl l −1 ) and photoacoustic detection (limit 6 pl l −1 ) systems were used to measure ethylene emission rates from point sources of a wide range of mature nonclimacteric and climacteric fruits. In addition, an infrared gas analyzer was used to measure CO 2 from the same samples. As well as total rates, the percentages emanating from the skin, tissue at the distal end (floral scar) and the pedicel were established. Possible links with fruit behaviour during abscission are discussed.

Non-intrusive, fast and sensitive ammonia detection by laser photothermal deflection

Abstract A recently developed non-intrusive photothermal deflection (PTD) instrument allows sensitive, rapid and quantitative detection of local ammonia concentrations in the air. Ammonia is vibrationally excited by an infrared CO 2 laser in an intracavity configuration. A HeNe beam passing over the CO 2 laser beam (multipass arrangement) is deflected by the induced refractive index gradient. The detection limit for ammonia in ambient air is 0.5 ppbv with a spatial resolution of a few mm 3 . The time resolution is 0.1 s (single line) or 15 s (multi line). The system is fully automated and suited for non-stop measuring periods of at least one week. Results were compared to those obtained with a continuous-flow denuder (CFD).

A REAL-TIME, NONINTRUSIVE TRACE GAS DETECTOR BASED ON LASER PHOTOTHERMAL DEFLECTION

We present the layout and technical details of a trace gas monitor based on photothermal deflection. The operating principle of this instrument, i.e., the deflection of a (weak) probe laser beam by the thermal refractive index gradient induced by trace gas absorption of an intense pump laser beam, allows nonintrusive measurements with good space and time resolution. An intra‐cavity CO2 laser is used as the pump beam and a red HeNe laser as the probe. The latter runs perpendicular to the pump beam to optimize spatial resolution. To increase sensitivity, the probe laser is incorporated in a multipass setup. The instrument is demonstrated by the localization of ethylene emission sites on a cherry tomato and by monitoring ammonia production due to nitrogen fixation by cyanobacteria. Both C2H4 and NH3 can be detected at the 1–3 ppb level, at a spatial resolution of 2 mm (along the pump laser)×0.6 mm (perpendicular to it), and a response time of 0.1 s (without background correction) or 15 s (including backgroun...

In situ, real-time monitoring of wound-induced ethylene in cherry tomatoes by two infrared laser-driven systems

Abstract Two laser-based detection setups, one involving photothermal deflection and the other photoacoustics, have been used to follow ethylene release when ripening cherry tomatoes (cv. Favorita) were mechanically wounded. Removing the calyx caused a double peak in ethylene release, the first peak over 1–2 h and the second over the next 4–7 h. Wounding the stem-scar yields similar results. However, wounding part of the fruit skin led to modest and variable releases of ethylene, much less than given on calyx-removal or stem-scar wounding. This higher emission does not originate from ethylene accumulated in the tomato already before wounding. Laser systems are shown to be useful in quantifying ethylene-releasing systems.

IR laser photothermal trace gas detection applied to environmental and biological problems

The photothermal effect is utilized for the detection of molecular trace gases in the open air, with high spatial and temporal resolution. An IR CO2 laser is used to excite specific molecules to higher vibrational levels. The subsequent change in temperature and refractive index is monitored by the deflection of a HeNe laser beam which crosses the CO2 laser beam. To improve the detection limit of the system the CO2 laser has an intracavity focus near to which the HeNe laser passes 31 times in a multipass mirror setup. A detection sensitivity of 1 ppbv (C2H4) and 0.5 ppbv (NH3) is achieved under practical conditions.

Ethylene Exhalation of a Single Flower Detected by Photoacoustic Methods

The combination of a powerful CO2 waveguide laser and a resonant photoacoustic cell provides sufficient sensitivity to investigate the emission of ethylene from a single orchid flower (Cymbidium Mary Pinchess “Del Rey”) in the interval up to 70 hours after emasculation of the flower. Ethylene plays a special role in agriculture. As an air pollutant it inhibits the growth of plants, causes yellowing and abscission of leaves and wilting of flowers /1/. As a gaseous hormone produced by the plant itself, it causes the same effect in unpolluted plants under closed storage.

Combination of photoacoustic detector with gas diffusion probes for the measurement of methane concentration gradients in submerged paddy soil

Dissolved methane was monitored by means of a diffusion probe in combination with a photoacoustic (PA) detector cell placed in the cavity of a liquid nitrogen-cooled CO laser. The detection limit of the photoacoustic detector was 1 ppbv methane (= 2 uM in aqueous solution), the time response was 60 s, the spatial resolution was 1.36 mm. These limits were determined by the acoustic noise and the configuration of the diffusion probe. The combination of PA detector with gas diffusion probes was found to be useful for monitoring gaseous compounds. However, the membrane material of the diffusion probe was critical. Silicone as membrane material was useful only for measurement of CH4. Goretex as membrane material was applicable to measurement of dimethylsulfide (DMS), but did not give a stable signal for trimethylamine (TMA). Vertical concentration profiles of CH4 in anoxic paddy soil agreed well with earlier results obtained with a gas chromatograph as detector. Methane was produced in anoxic soil layers below 8-l 0 mm depth and diffused upwards to the surface through a layer of CH4-consuming bacteria situated at about 2 mm depth. In the oxic upper 2 mm soil layer the concentration of CH4 decreased below the detection limit of our system. Methane-containing gas bubbles that were embedded in the soil were detected by a steep increase of the CH4 signal. The combination of PA detector and gas diffusion probe was found to be a useful tool to measure CH4 gradients in submerged soil or sediment with high temporal and spatial resolution, thus allowing the localization and quantification of CH4 production and CH4 oxidation rates within the soil profile. Copyright