The Quartz Crystal Microbalance and its Applications

By: Monica Melo March 25, 2004

What is a Quartz Crystal Microbalance?

• A quartz crystal microbalance is a sensor i.e.. a class of analytical devices that are capable of monitoring specific chemical species continuously and reversibly • A device that is based on the piezoelectric characteristics of the quartz crystal • The piezoelectric effect forms the basis for the quartz crystal microbalance 2

The Piezoelectric Effect

• The appearance of an electric potential across certain faces of a crystal when it is subjected to mechanical pressure • The effect has a converse i.e. when an electric field is applied on certain faces of the crystal, the crystal undergoes mechanical distortion • The effect is explained by the displacement of ions in crystals that have a nonsymmetrical unit cell 3

The Piezoelectric Effect

• The compression causes a displacement of the ions of the unit cell, causing an electric polarization of the unit cell • These effects are accumulative and an electric potential difference appears across certain faces of the crystal • When an external electric field is applied to the crystal, the ions of each unit cell is displaced by electrostatic forces • The result is the mechanical deformation of the whole crystal 4

The Crystal Structure of Quartz

• Quartz is crystalline silica (SiO 2 ) at temperatures below 870°C Figure 2 – The  -quartz crystal lattice structure Figure 1 – The Natural Form of Quartz 5

The Quartz Crystal Resonator

• A quartz crystal resonator is a precisely cut slab from a natural or synthetic single crystal of quartz • A resonator can have many modes of resonance, or standing wave patterns at the resonant frequencies • The quartz crystal resonator must be cut at a specific crystallographic orientation and have the proper shape • This allows for selection of a specific mode of resonance and for suppression of all unwanted modes 6

The Quartz Crystal Resonator

• Commonly, quartz crystal resonators are cut in one of two types: AT-cut or BT-cut • The angle is measured relative to the z-axis of rotation and the thickness is in the y-direction in a rectangular, square or disc shape Figure 3 – The Ideal Cuts of the Quartz Crystal 7

The Quartz Crystal Resonator

These cuts are ideal because: 1. They oscillate in the thickness shear mode – most sensitive to the addition or removal of mass (perfect for microweighing!) 2. They are insensitive to temperature change near room temperature (at the conditions of an analytical laboratory!) 8

The Operation of a Quartz Crystal Microbalance

• Electrodes are affixed to either side of the quartz resonator and connected to a voltage source • The quartz crystal is made to vibrate at the frequency of the exciting voltage 9

Mass Determination

• The crystal in most quartz crystal microbalances in an essential part of an oscillator circuit • The material to be weighed is deposited onto the quartz crystal plate (resonator) as a thin film • A quartz crystal microbalance does not actually measure the mass • It measures the areal density or mass thickness of the deposited material 10

Mass Determination & Sources of Error

• The mass is calculated from a frequency change on the quartz due to the deposited material • A complicated formula is used and the display shows only the mass of the deposited material • Errors occur because of the instrument’s inability to distinguish between a frequency change due to the deposited mass or other disturbances such as stress changes or temperature 11

Applications

• Microweighing • Detection of toxic gases such as sulfur dioxide, ammonia, hydrogen sulfide, carbon monoxide, and aromatic hydrocarbons • Detection of biomolecules by antigen/antibody attachment to quartz resonators 12

References

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Bottom, Virgil E.

Introduction to Quartz Crystal Unit Design

. Van Nostrand Reinhold Company. New York. 1982.

Harris, Daniel C.

Quantitative Chemical Analysis

. 5 th Company. New York. 1999.

Edition. W.H.Freeman and Heising, Raymond A.

Quartz Crystals for Electrical Circuits: Their Design and Manufacture

. D.Van Nostrand Company, Inc. New York. 1946.

Ikeda, Takuro.

1990.

Fundamentals of Piezoelectricity

. Oxford University Press. Oxford.

Miessler, Gary L.; D.A. Tarr.

New Jersey. 1999.

Inorganic Chemistry

. 2 nd Edition. Prentice-Hall, Inc.

Skoog, Douglas A., F.J. Holler; T.A. Nieman.

Principles of Instrumental Analysis

. 5 th Edition. Saunders College Publishing. Philadelphia, 1998.

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