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

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

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

The Crystal Structure of Quartz Quartz is crystalline silica (SiO2) at temperatures below 870°C Figure 2 – The -quartz crystal lattice structure Figure 1 – The Natural Form of Quartz

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

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

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

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

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

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

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

References Bottom, Virgil E. Introduction to Quartz Crystal Unit Design. Van Nostrand Reinhold Company. New York. 1982. Harris, Daniel C. Quantitative Chemical Analysis. 5th Edition. W.H.Freeman and Company. New York. 1999. Heising, Raymond A. Quartz Crystals for Electrical Circuits: Their Design and Manufacture. D.Van Nostrand Company, Inc. New York. 1946. Ikeda, Takuro. Fundamentals of Piezoelectricity. Oxford University Press. Oxford. 1990. Miessler, Gary L.; D.A. Tarr. Inorganic Chemistry. 2nd Edition. Prentice-Hall, Inc. New Jersey. 1999. Skoog, Douglas A., F.J. Holler; T.A. Nieman. Principles of Instrumental Analysis. 5th Edition. Saunders College Publishing. Philadelphia, 1998.