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Chemical Sensors
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The quartz crystal microbalance (QCM, right) is an extremely sensitive mass sensor, capable
of measuring mass changes in the nanogram range. QCM’s are piezoelectric devices fabricated of a thin plate of
quartz, with gold electrodes affixed to each side of the plate. A chemically selective film is applied to create
chemical detectors. Because of their size and sensitivity, QCMs are ideal for space flight scenarios.
We employ a dual crystal QCM setup, with the second crystal used as a reference. The outputs of the two crystals are mixed, and the difference frequency is monitored. The crystals are set in a gas flow cell for introduction of the analyte.
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QCM: Response of OV-1701 to Hexane
The decrease in frequency for a QCM on the addition of mass is given by the Sauerbrey equation:
A 10 MHz crystal has a theoretical sensitivity of 0.621 ng/Hz.
In preliminary testing, the dual QCM sensor was coated with a thin layer of OV-1701(a Carbowax used in GC). Hexane flowing over the crystal at a concentration of ~10E-5 produces a response of ~16 Hz, corresponding to a mass increase of 9.9 ng. Response time is approximately 5 s.
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