RADIAL MASS SENSITIVITY OF THE QUARTZ CRYSTAL MICROBALANCE IN LIQUID-MEDIA

The radial sensitivity function of the quartz crystal microbalance (QCM), employing an AT-cut quartz resonator in aqueous media, has been determined by simultaneous in situ measurement of frequency and charge during copper electrodeposition in holes etched in a photoresist polymer on the QCM. Frequency-charge correlations are determined by copper deposition either in small (0.025 in.) holes with different angular and radial locations or in concentric circular holes with different radii. The data indicate a Gaussian mass sensitivity distribution, with the greatest differential sensitivity in the center of the resonator, decreasing monotonically with increasing radius. Narrower sensitivity distributions and less mass sensitivity beyond the electrode edges were observed with plano-convex crystals, consistent with a greater degree of energy trapping that results from the larger mass in the center of contoured crystals. The results demonstrate, as with applications of the QCM in the gas phase, that coating uniformity is important for accurate measurements of mass with the QCM when used in liquid media. The significant sensitivity observed beyond the electrode boundary in plano-plano resonators indicates that sensors for liquid-phase applications must be properly calibrated to account for this property.