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.
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