INFICON PLO-10i Phase Lock Oscillator - Manual (Page 37)

PLO-10 PHASE LOCK OSCILLATOR
CRYSTALS, HOLDERS AND FLOW CELL 7-3
7.1.3 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’s
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.
7.1.4 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.
7.1.5 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)
7.1.6 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)