Quartz Crystal Microbalance
A technology which has been
known for some time, but has been applied to chemical problems only recently is
the quartz crystal microbalance
("QCM"). It is interesting
because of its ability to accurately weigh in the femto (10-15) gram
range!
The QCM depends upon the
piezoelectric effect. When electricity
is applied to a thin quartz disk, the disk vibrates at a very predictable
frequency which is a function of the mass of the disk. Frequency can be measured very accurately, so
it's possible to observe very small changes in the mass of the disk. The Sauerbrey equation quantifies this
effect:
,
where: 
is the resonant frequency of the fundamental mode of the
crystal,
is
the area of the disk (for thin film applications),
r is the density of the
crystal (2.684 g/cm3), and
m is the shear modulus of
quartz (2.947 x 1010 g/cm s2).
Fortunately, quartz crystals
are readily available due to their extensive use in the electronics industry as
time standards ("clocks"). (Every
TV has a "colorburst" crystal operating at 3.579545 MHz which can be
purchased at Radio Shack for $1.18.)
However, it is more
convenient to buy crystals that are specifically designed for QCM use. Our initial QCM design (Figure 1, 2) utilized
two 10 MHz crystals which cost $10 apiece.
Why two? There are several
advantages to employing an identical crystal as a standard and measuring the difference
between the two frequencies as the mass of the probe crystal changes: 1) There
is automatic temperature and pressure compensation and 2) the difference
frequency is much easier to measure because it is in the audio range (Hz or kHz
rather than MHz) [Hwang, 1996]. The
interface to a PC is an adaptation of a low-cost data acquisition device
[Fisher, 2000].
Figure 1
Electronic
schematic of the initial QCM design. The
two crystals are labeled "10 MHz".
A PIC microcontroller translates the difference frequency to PC keyboard
codes.
Figure 2 Initial QCM design.
The left module is the sensing unit consisting of an exposed and an
enclosed 10 MHz quartz crystal. The
right module interfaces the QCM to a PC via
the keyboard port.
In general, the sensor
crystal is chemically activated by reacting a compound
containing a sulfhydryl with the gold film electrodes of the quartz
crystal. However, this requires cleaning
the gold with "piranha" solution (30% hydrogen peroxide:98% sulfuric acid, 1:3) and handling very smelly sulfhydryl
reagents. In order to quickly demonstrate
"proof-of-concept" we sidestepped this procedure and instead followed
a method which produces a benzene vapor detection system based on a pyrrolidine
derivative merely smeared onto the gold electrode [Finlea, 1998].
This device was placed in a
chamber where known concentrations of benzene vapor could be introduced. Our initial results are shown (Figure 3, 4)
[Gilman, 2001]:
Notice that response is
linear and reversible. A portable
version of this QCM could be deployed to check for vapor leaks from gas station
reservoirs since most non-leaded gasoline contains benzene.
We are currently redesigning
the oscillator circuitry to yield more stable (less harmonics)
operation and are learning to deal with piranha solution and evaluating
non-volatile sulfhydryls.
Acknowledgement: Darell Brehm of International
Crystal Manufacturing Co. Inc. suggested the circuit design and provided other
valuable advice.
References
Finlea, H.O., M.A.
Phillippi, and E. Lompert (1998) Highly sorbent films derived from Ni(SCN)2(4-picoline)4 for the
detection of chlorinated and aromatic hydrocarbons with quartz crystal
microbalance sensor. Anal Chem 70:1268-76
Gilman,
Charles P. (2001) A practical device for the real-time monitoring of air-borne
aromatic hydrocarbons with a quartz crystal microbalance. Juniata College Research Symposium 7 Apr 01 poster
Hwang, Euijin
and Youngran Lim (1996)
Construction of a low noise electrochemical quartz crystal microbalance. Bull Korean Chem Soc 17(1):39-42