H.S.M. de Vries

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.

The Photothermal Deflection Technique (PDT): Fast Trace Gas Detection in the Atmosphere

Photothermal deflection is a sensitive technique for detecting very low concentrations of trace gases, such as NH3, C2H4 and CO2. These gases can have an important influence on the ecosystem of plants. To get a better understanding of this influence it is desirable to measure local concentrations in the surroundings of plants. These local measurements make it possible to determine the activity of the different parts of the plants. For example, C2H4 exhaust in leafs and stems [1], CO2 and H2O emission through stomata, and NH3 emission in barleyplants [2] can be measured.

Photoacoustic measurements of Lorentz broadening in CO2 between 25° C and 450° C

A variable temperature photoacoustic cell has been constructed and tested by studying the interplay of CO2, H2O, and NH3 in synthetic smoke. Saturation effects for CO2 and NH3 have been modeled and compared with experiments, and results are obtained for the vibrational relaxation rate associated with NH3-H2O collisions. The cell has been used for studying the temperature dependence of self-broadening and N2 broadening of CO2 lines. The temperature dependence of the scattering rate is well described by a T−n law with n=0.77 in both cases. This result agrees with previous results obtained by tunable diode laser spectroscopy, but disagrees with results obtained by indirect methods.

Real-time and non-intrusive detection of ambient ammonia using the photothermal deflection technique

The recent development of a non-intrusive photothermal deflection (PD) instrument allows the quantitative detection of local ammonia concentrations in air to be measured continuously, sensitively and fast. Ammonia is vibrationally excited by an infrared CO2 laser in an intracavity configuration. A HeNe beam passes over the CO2 laser beam in a multipass arrangement and is deflected by a thermal, CO2 laser induced, refractive index gradient. This deflection, due to the mirage effect, is a measure for the local ammonia concentration. In ambient air the detection limit is 0.5 ppb and the spatial resolution is 2.5 cm times π (0.282 mm2). The time resolution is 0.1 s for single line operation and 15 s for multi-line operation. The system is fully automated and suited for measuring periods of at least one week. Results are compared with those of a continuous-flow denuder system.