A small, lightweight, portable apparatus based on an electrochemical sensing principle has been developed for monitoring low concentrations of ethylene in air. Ethylene has long been known to be produced by plants and to stimulate the growth and other aspects of the development of plants (including, notably, ripening of fruits and vegetables), even at concentrations as low as tens of parts per billion (ppb). The effects are magnified in plant-growth and -storage chambers wherein ethylene can accumulate. There is increasing recognition in agriculture and related industries that it is desirable to monitor and control ethylene concentrations in order to optimize the growth, storage, and ripening of plant products. Hence, there are numerous potential uses for the present apparatus in conjunction with equipment for controlling ethylene concentrations.
The ethylene sensor is of a thick-film type with a design optimized for a low detection limit. The sensor includes a noble metal sensing electrode on a chip and a hydrated solid-electrolyte membrane that is held in contact with the chip. Also located on the sensor chip are a counter electrode and a reference electrode. The sensing electrode is held at a fixed potential versus the reference electrode. Detection takes place at active-triple-point areas where the sensing electrode, electrolyte, and sample gas meet. These areas are formed by cutting openings in the electrolyte membrane. The electrode current generated from electrochemical oxidation of ethylene at the active triple points is proportional to the concentration of ethylene. An additional film of the solid-electrolyte membrane material is deposited on the sensing electrode to increase the effective triple-point areas and thereby enhance the detection signal.
The sensor chip is placed in a holder that is part of a polycarbonate housing. When fully assembled, the housing holds the solid-electrolyte membrane in contact with the chip (see figure). The housing includes a water reservoir for keeping the solid-electrolyte membrane hydrated. The housing also includes flow channels for circulating a sample stream of air over the chip: ethylene is brought to the sensing surface predominately by convection in this sample stream. The sample stream is generated by a built-in sampling pump. The forced circulation of sample air contributes to the attainment of a low detection limit.
In addition to the sensor and the sampling pump, the apparatus includes electronic circuitry for regulating the sensor potentials, measuring the sensing-electrode current, and displaying the ethylene-concentration reading. The electronic circuitry includes a data logger for digital collection via a serial port with an optional analog output.
Overall, the apparatus is capable of measuring ethylene concentrations from 5 to 5,000 ppb with a response time of less than 30 seconds. The magnitude of response of the sensor current is in the range of 5 to 50 picoamperes/ ppb. The signal-to-noise ratio is greater than 3 at the low detection limit of 5 ppb.
The sensor is fairly selective for ethylene. It is not subject to interference by O2 or CO2. It does respond to NO, NO2, and H2S, but these gases are generally not expected to be present at significant concentrations in controlled plant growth environments. The sensor also responds to some volatile compounds present in some soil samples. Further research will be necessary to reduce these interferences.
This work was done by Mourad Manoukian, Linda A. Tempelman, and John Forchione of Giner, Inc. and W. Michael Krebs and Edwin W. Schmitt of Giner Electrochemical Systems, LLC for Kennedy Space Center. For further information, contact:
Linda A. Tempelman, Ph.D.
89 Rumford Ave.
Newton, MA 02466
Phone No.: (781) 529-0514
Refer to KSC-12825.