The Development of an Ion Mobility Sensor
Dr. Dale Russell
The object of this project is the development and validation of a compact ion mobility spectrometer (IMS). This instrument is expected to be capable of detecting and quantifying volatile organic compounds (VOCs). It is designed for field deployment, for analysis of VOCs in the shallow geologic subsurface or other venues where they pose a threat to health or the environment. Target analyte species include chlorinated organic solvents such as perchloroethylene (PCE) and trichloroethylene (TCE). A laboratory based bench top system is used to optimize flow systems and analysis protocols. The optimum conditions determined in this set of experiments would then be used in testing a field portable version of the instrument. The theory of operation is similar to more familiar chromatography. Analyte species are separated from a mixture based on differential migration in a flow chamber. A carrier gas is pumped through the chamber in one direction, and a drift gas is flowed in an opposite direction. The drift gas opposes flow and causes some molecules to traverse the chamber more slowly than others. The velocity of various types of molecules through the chamber is a function of their size and their interaction with the two gases. Analyte molecules are introduced though a sampling port into a charging chamber where they receive electron charge from a Ni63 source. They are then blocked from entering the chamber until an electrostatic port is opened. The time of flight through the chamber is detected at the far end by an ion detector and identifies the chemical species involved. The current at this detector is proportional to the number of molecules of that type in the sample. In this way, both chemical identity and concentration of various VOC can be determined in any gas phase mixture. The chamber can be "tuned" for selective detection of certain species relative to other species.
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