Quartz Crystal Microbalabnce (QCM)
A quartz crystal microbalance (QCM) measures a mass per unit area by measuring the change in frequency of a quartz crystal resonator. The resonance is disturbed by the addition or removal of a small mass due to oxide growth/decay or film deposition at the surface of the acoustic resonator. The QCM can be used under vacuum, in gas phase ("gas sensor", first use described by ) and more recently in liquid environments. It is useful for monitoring the rate of deposition in thin film deposition systems under vacuum. In liquid, it is highly effective at determining the affinity of molecules (proteins, in particular) to surfaces functionalized with recognition sites. Larger entities such as viruses or polymers are investigated, as well. Frequency measurements are easily made to high precision (discussed below); hence, it is easy to measure mass densities down to a level of below 1 μg/cm2. In addition to measuring the frequency, the dissipation is often measured to help analysis. The dissipation is a parameter quantifying the damping in the system, and is related to the sample's viscoelastic properties.
"Quartz crystal microbalance." Wikipedia, The Free Encyclopedia. 4 Feb 2008, 14:01 UTC. Wikimedia Foundation, Inc. 7 Feb 2008 <http://en.wikipedia.org/w/index.php?title=Quartz_crystal_microbalance&oldid=189022845>.
How it works
The Sauerbrey equation was developed by G. Sauerbrey in 1959 as a method for correlating changes in the oscillation frequency of a piezoelectric crystal with the mass deposited on it. He simultaneously developed a method for measuring the characteristic frequency and its changes by using the crystal as the frequency determining component of an oscillator circuit. His method continues to be used as the primary tool in quartz crystal microbalance experiments for conversion of frequency to mass and is valid in nearly all applications.
The equation is derived by treating the deposited mass as though it were an extension of the thickness of the underlying quartz , . Because of this, the mass to frequency correlation (As determined by Sauerbrey’s equation) is largely independent of electrode geometry. This has the benefit of allowing mass determination without calibration, making the set-up desirable from a cost and time investment standpoint.
"Sauerbrey equation." Wikipedia, The Free Encyclopedia. 16 Dec 2007, 22:04 UTC. Wikimedia Foundation, Inc. 7 Feb 2008 <http://en.wikipedia.org/w/index.php?title=Sauerbrey_equation&oldid=178375346>.