Application of Harmonic Analysis and Principal Component Analysis for Discrimination of Adsorbates in Gas-Sensitive ITO/Nanostructured TiO2 Heterojunction.

Fast Fourier transform (FFT) and principal component analysis (PCA) were successfully applied to realize a selective technique of adsorbate discrimination by spectral analysis of the waveforms of current through a gas-sensitive ITO/nanostructured TiO₂ heterojunction when a sinusoidal alternating voltage is applied to it. The novel technique shows good differentiation of sensor responses for adsorption of water, ammonia, ethanol and isopropanol molecules with sequential injection of analytes and fair differentiation of methanol, ethanol and isopropanol adsorption even for random injection of analytes. Comparison of the score plots and current epures obtained at various frequencies of the probing signal and various sets of harmonics used as source data for PCA allowed to find the optimal measurement conditions in terms of selectivity and reveal the basic factor, which predetermines substance-dependent variations in the harmonic spectrum of current waveforms. It is considered that substance-dependent changes in the shape of current waveforms are due to the processes of recharging of electron and/or hole traps caused by the interaction of adsorbed molecules with the surface of the structure and their diffusion deep into the porous TiO₂ layer. [ABSTRACT FROM AUTHOR]