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A quartz crystal microbalance is basically a very sensistive scale, able to weigh as little as a few billionths of a gram. A QCM measures a mass per unit area by measuring the change in frequency of a quartz crystal oscillator. The frequency increases or decreases by the addition or removal of a small amount of mass deposited on the surface of the quartz sensing crystal. The QCM is generally used under vacuum, in gas phase to detect molecular contamination. Frequency measurements can be made to very high precision. Using 15MHz base frequency crystals allows mass measurements in the nanogram range to be made. Quartz naturally exhibits the piezoelectric effect. This effect causes a thin wafer of quartz, plated with metallic electrodes, to mechanically deform when a voltage is applied across it. An AC voltage applied across the crystal will induce it to oscillate at its resonant frequency with great precision. Any molecules landing and sticking to the surface of the oscillating crystal will cause its frequency to decrease. By measuring this frequency change, the amount of mass deposited on the surface can be determined. The Sauerbrey equation can be used to determine the mass change per unit change in frequency: Where fo is the fundamental crystal frequency, ρq is the density of quartz, μq is the shear modulus, and A is the active crystal area between the electrodes. The CrystalTek QCM sensor
has an aperture which results in a mass sensitivity of
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