Liquid Chromatography Detectors - The Electrochemical Detector > Page 90

The response of the detector or current (i) is described by the following equation,

 

              

 

where (n) is the number of electrons per molecule involved in the reaction, 
(F) is the Faraday Constant,
(A) is the area of the working electrode,
(KT) is the limiting Mass Transfer Coefficient,
(c) is the solute concentration,
(u) is the linear velocity of the mobile phase over the surface of the electrode,
and (a) is a constant usually taking a value between 1/3 and 1/2.

It is seen that the current (i) (and, thus, the sensitivity), can be raised by either increasing the electrode area, increasing the transfer coefficient or increasing the velocity of the mobile phase past the electrodes. It would appear that increasing the electrode area would be the easiest, however, increasing the electrode surface area while maintaining an amperometric response also increases the noise, often to such an extent that there is an overall reduction in detector sensitivity. Weber and Purdy (50) and Hanekamp et al. (51) showed that under certain conditions a reduction in the sensor size produces a significant increase in signal-to-noise and, thus an increase in sensitivity. Higher flow rates will also increase the rate of solute transfer and the sensitivity which would be an added advantage to miniaturization. However, the sensor will be very flow sensitive and thus the flow rate must be kept very constant and the detector would not be amenable to flow programming development. Miniaturization would also reduce peak dispersion in the sensor.