Principal component analysis applied to admittance spectra of a quartz-crystal microbalance in contact with a liquid phase

Abstract

Principal component analysis (PCA) was applied to the admittance spectra of a single quartz-crystal microbalance (QCM) in contact with a liquid phase for the first time. Fifteen characteristic parameters on the admittance locus were selected as different measurement channels. Four eigenvectors were obtained, which represent directions and magnitudes of factors affecting the QCM responses, including mass loading, viscous coupling, liquid permittivity and conductivity changes. These effects were also successfully separated and displayed as different trends in eigenvector plots. Using copper deposition and glycerol solutions a reference system was built to distinguish contributions from mass loading and viscous coupling in a specific process. By observing the position and the direction of the unknown process in this reference system, much information concerning its nature was obtained. The proposed method was found to be more advantageous than qualitative comparisons of height and width of the conductance peak in normal methods. This new method was used to investigate different behaviours of Langmuir–Blodgett (LB) films depositing onto the QCM above and below the critical temperature (T_c). The adsorption of surfactant onto quartz crystal was also studied.