A novel measurement technique of pure out-of-plane vibrational modes of thin films on a nonmetallic substrate has recently been proposed, which is named multiple-angle incidence resolution spectrometry (MAIRS). Since this technique could not be replaced by other conventional techniques, MAIRS was expected to be a promising tool for analysis of thin soft materials and surface adsorbates. Nevertheless, some experimental conditions have been found to be inappropriate for MAIRS, which yields incorrect results. In the present study, therefore, the problems in the technique have been investigated in terms of optics to improve the accomplishments of MAIRS. The problems have been found to have a strong relationship with optics in FT-IR, which is influenced by refractive index of the sample material and angle of incidence. In particular, optimization of the size matching of the detector surface and the infrared spot at the detector was a key to having MAIRS perform properly. It has been concluded that reliable MAIRS measurements require overfilling of the detector and a substrate with a high-refractive index.

Conformational changes of R-and keto-mycolic acid monolayers, which respond to monolayer compression, have been compared by use of the Langmuir-Blodgett (LB) technique and infrared reflection-absorption (RA) spectroscopy. Mycolic acids (MA) are major chemical components specifically found in cell envelopes of mycobacteria, and they are considered to largely influence the cell properties such as drug permeability.In our previous study, the long hydrocarbon chain in R-MA was found to have at least two distinct molecular conformations during the monolayer compression: fully extended and folded forms. In the present study, a surface pressure-area isotherm of keto-MA has suggested that keto-MA molecules have a different response of conformational changes to the monolayer compression from that of R-MA. The infrared RA spectra of the monolayer LB films were readily collected to monitor the conformational changes. The second-derivative analysis of the RA spectra clearly suggested that the long hydrocarbon chain in the keto-MA molecule remains folded even when a high surface pressure is applied to the monolayer, while the long chain in R-MA is unfolded. These results suggest that the small substituted keto group greatly influences the change of molecular conformation against the external pressure.