Attenuated total reflectance infrared (ATR-IR) spectroscopy was employed to quantitatively evaluate the dissociation states (di-cationic, cationic. zwitterionic, and anionic) of lysine adsorbed on amorphous silica. To determine the relationship between the ATR-IR spectra and each dissociation state, we first measured pH-induced Spectral changes of dissolved lysine and correlated these changes with the thermodynamically calculated dissociation states of lysine. This procedure yielded calibration curves with good linearity: we used these curves for the quantitative analysis of adsorbed lysine. our analysis revealed that 81 +/- 5% of the lysine adsorbed on amorphous silica was present in a cationic state and 19 5% was in a zwitterionic state; these percentages remained mostly Unchanged over the whole range of pH values tested (pH = 7.1-9.8). We interpret the values obtained to indicate that lysine adsorption is mainly driven by electrostatic interaction with the negatively charged Silica surface (Si-O-center dot center dot center dot Lys(+), Si-O-center dot center dot center dot Lys(+/-) (C) 2008 Elsevier Inc. All rights reserved