The detection of volatile vapors of unknown species in a complex field environment is required in many different applications. Mass spectroscopic techniques require subsystems including an ionization unit and sample transport mechanism. All of these subsystems must have low mass, small volume, low power, and be rugged. A volatile molecular detector, an ambient pressure pyroelectric ion source (APPIS) that met these requirements, was recently reported by Caltech researchers to be used in in situ environments. APPIS creates ions through temperature changes of the crystal. A change in temperature of the pyroelectric crystal creates a potential difference between the +z and –z surface. With a sufficient voltage buildup, electrical discharge occurs at the surface of the crystal and either positive or negative ions are produced depending on the crystal face. This discharge ionizes compounds on and near the surface of the crystal. If thermal cycling is applied to the pyroelectric crystal (i.e., heated and cooled repeatedly), one can obtain negative and positive ions through a thermal cycle. This process, however, creates ions at random times throughout operations and makes the source difficult to use with several detection techniques.
The improved APPIS system employs an active cooling system equipped with a feedback capability using a Peltier module to achieve better temperature control, and a heat rejection system to force-cool the crystal in order to increase the ionization efficiency of the pyroelectric crystal, on which the original APPIS source is based. In addition, the APPIS crystal housing system was completely redesigned to take advantage of the full ionization capability of the crystal source. The temperature is monitored using a thermocouple, and a custom LabVIEW program is used to control the temperature gradient of the crystal.
A Peltier module utilizes the thermo-electric-Seebeck effect to create a temperature difference between the front and back sides of the module. If the temperature of one side of the Peltier module is heated, the other side can be cooled, depending on the polarity of the applied voltage. The system is able to provide a thermal cycling improvement compared to a conventional system by a factor of three, and can produce more uniform cooling of the crystal.
To apply thermal cycling to the crystal, two Peltier modules, one for heating and the other for cooling, are thermally bonded to the pyroelectric crystal. A chilled water pipe serves as a heat sink to prevent heating of the APPIS system. The crystal temperature is monitored with a thermocouple. The polarity of the applied voltage is set by a control circuit operated by LabVIEW. When the crystal temperature heats to a threshold by one Peltier module, the other Peltier module starts cooling, and vice versa. For testing, the APPIS was mated to an ion mobility spectrometer (IMS). These two separate but fully compatible technologies are very simple to mate since both function at ambient pressures.
This APPIS-IMS system is a rugged, field-portable, low-power, and lightweight system. It has the potential for ultra-sensitive detection of molecules in both terrestrial and extraterrestrial applications such as chemical and biological warfare agents, explosives, and other harmful volatile chemical compounds, as well as organic molecule detection on Mars without use of radioactive material, complicated vacuum systems, and any consumables.