In Situ Combinations of Electrochemistry with Other Techniques

2 In Situ Combinations of Electrochemistry with Other Techniques 

The electrochemieal quartz crystal microbalance (EQCM) is the traditional name of this technique however, the term electrochemical quartz crystal nanobalaiKe (EQCM) is also used and is more accurate, since nanogram changes are usually measured by it, and even 1 ng variations in the surface mass hm/A) can be detected. 

The crystal cut determines the mode of oscillations. AT-cut quartz crystals, vibrating in a thickness shear mode, are almost exclusively used in EQCM devices however, it should be mentioned that attempts have been made to exploit other modes of oscillation. 

The essential point is that the resonant frequency decreases when the crystal is loaded with mass, and this change can be determined very acciuntely. Changes of 1 Hz, or even 0.1 Hz, can be measiued when a crystal with a resonance frequency of 10 MHz is used. The relationship between mass change (A/w) and frequency change (A/) was derived by Sauerbrey [150], and is called the Sauerbrey equation  

It follows that the measurement is more sensitive when fo is higher (note the quadratic relationship) however, the fundamental frequency of vibrations is inversely proportional to the quartz wafer thickness. Usually crystals with fundamental frequencies of 5 MHz and 10 MHz are used, although 20 MHz crystals have also been applied on rare occasions, e.g., in [151], The thickness of the quartz plate is 0.13 mm at 10 MHz, meaning that crystals working at substantially higher fundamental frequencies are too thin to be handled safely. 

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