INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 41)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
CRYSTALS, HOLDERS AND FLOW CELL 4-3
4.1.3 CRYSTAL SURFACE FINISH
Studies have shown that electrode surface roughness can cause large apparent mass loadings due
to the liquid that is trapped within pores at the crystal surface 1 . INFICON crystals are optically
polished to 50 Å average surface roughness to minimize this effect. Polished crystals are required
to obtain good agreement between theory and measurement during liquid immersion experiments.
Polished crystals are also required to obtain measurements reproducibility from crystal to crystal 2 .
Non-polished crystals (R
a
=1.8 microns) are also available at reduced costs for applications that do
not require the accuracy and reproducibility of the polished crystals.
4.1.4 CRYSTAL ELECTRODE MATERIALS
INFICON
’s crystals are available in a variety of electrode materials including Gold, Platinum,
Aluminum, Silver, Titanium, etc. INFICON also offers Gold electrode crystals with an additional
SiO2 outer layer to create a hydrophilic surface needed for some biological applications.
4.1.5 CRYSTAL THICKNESS
INFICON
AT cut, 1-inch diameter crystals are plano-plano. Their physical thickness is determined
by a frequency constant and their final frequency. The frequency constant for an AT cut crystal is
1.668E5 Hz × cm or 65.5 kHz × in. Therefore, the crystal thicknesses for various frequencies are
as follows.
5 MHz AT cut thickness = 333 microns (0.013 inch)
6 MHz AT cut thickness = 227 microns (0.0109 inch)
9 MHz AT cut thickness = 185 microns (0.007 inch)
4.1.6 MASS SENSITIVITY
The quartz crystal microbalance is an extremely sensitive sensor capable of measuring mass
changes in the nanogram/cm 2 range with a wide dynamic range extending into the 100 µg/cm 2
range.
Sauerbrey was the first to recognize the potential usefulness of the technology and demonstrate
the extremely sensitive nature of these piezoelectric devices towards mass changes at the surface
of the QCM electrodes 3 . The results of his work are embodied in the Sauerbrey equation, which
relates the mass change per unit area at the QCM electrode surface to the observed change in
oscillation frequency of the crystal:
∆ f= - C
f
× ∆m
where
∆
f = the observed frequency change in Hz,
C
f
= the sensitivity factor of the crystal in Hz/ng/cm 2
(0.056 Hz/ng/cm 2 for a 5 MHz crystal @ 20° C)
(0.081 Hz/ng/cm 2 for a 6 MHz crystal @ 20° C)
(0.181 Hz/ng/cm 2 for a 9 MHz crystal @ 20° C)
∆ m = the change in mass per unit area, in g/cm 2