The Quartz Crystal Microbalance

The Quartz Crystal Microbalance
The Quartz Crystal Microbalance (QCM) is an extremely sensitive detector of
mass. It can detect a single layer of molecules deposited on its surface. It is
also a very versatile instrument, useful in a diverse range of applications.
Although commercial instruments can be expensive it is possible to build a
low cost device without any special equipment and still achieve full accuracy.
Basically a QCM is a piezoelectric quartz crystal whose resonance frequency
changes when a mass is added to its surface. The resonance frequency can
be easily observed by using a simple oscillator circuit.
Quartz Crystals
In 1880 the brothers Pierre and Jacques Curie discovered that certain crystals
produce an electric field when subjected to a mechanical stress. This is called
the piezoelectric effect. The following year they demonstrated the converse
piezoelectric effect: applying an electric field to a piezoelectric crystal
produces a mechanical deformation. The properties of quartz make it a
particularly useful piezoelectric material.
Oscillators
In the early part of the 20th century it was found that quartz crystals could be
used to make extremely stable oscillators. Accuracies of a few tens of parts
per million are common. This makes crystal oscillators accurate enough to be
used for regulating digital clocks and generating references for radio circuits.
In the early days of crystal oscillators it was found that the frequency could be
slightly altered by putting a pencil mark on the surface of the crystal or by
taking a little of the electrode surface off with an eraser.
Microbalances
This effect of mass on the surface of a crystal was quantified by a researcher
called Gunter Sauerbrey in 1959. Put simply the Sauerbrey Equation states
that:Change in Mass
Original Mass
=
Change in Frequency
Original Frequency
Just how sensitive does that make the QCM? Let’s do the math: a crystal with
a frequency of 9.8304 MHz will be 0.170 mm thick. If the crystal has a
diameter of 8 mm it will have a mass of 22.58 milligrams. If we can detect a
change in frequency of one Hz then we can detect a 1/9830400th of this mass.
A change in frequency of 1 Hz indicates a change in mass of 2.30 nanograms
(a nanograms is a one billionth of a gram).
This rule applies when the mass is small, evenly spread across the surface of
the crystal and rigidly bound to the surface. If any of these conditions are not
met then there are other rules that must be taken into account.
The Quartz Crystal Microbalance
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This basic microbalance has been used in a vacuum to accurately measure
the thickness of vacuum deposited metal films. It has been used in air for
everything from gas detection to investigations of friction and lubrication. With
the right oscillator circuit the QCM can operate with one face of the crystal
under liquid. Under liquid sensors can be used to measure everything from
electroplating to bio-film formation.
The Quartz Crystal Microbalance
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