A quartz crystal microbalance or QCM measures a mass per unit area by measuring the change in frequency of a quartz crystal resonator. The sensor, which is the crystal, oscillates at a constant frequency and as the mass on the crystal changes, so does the resonance frequency. The addition or abstraction of the mass is due to oxide growth or film deposition at the surface of the acoustic resonator. The QCM works under vacuum, in gas and even in liquid environments. Under vacuum, it is useful for monitoring the rate of deposition in thin film deposition system and in liquid; it is effective at determining the affinity of molecules to surfaces functionalised with recognition sites. Simply put, QCM is the mass measurement standard, just as laser diffraction is essential for the measurement of particle size.
A basic QCM includes a source of alternating current — the oscillator, a quartz crystal, two metal electrodes on opposite sides of the thin crystal wafer and a frequency counter. According to most experts, choosing a QCM is a matter of finding the right match for the analytical objective and sample conditions. There are however, three important things that play critical roles in the equipment’s functionality that you should consider.
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