title: Improving accuracy and reproducibility of vibrational spectra for diluted solutions.
abstract: In what appears to be a trivial operation in which the averaged spectrum of solvent is subtracted from the spectra of solutions, can be a misleading step in improving reproducibility of vibrational spectra. Near-infrared spectra of pure water and glycine solutions were used to quantify instrumental and spectral variations, and examine its influence on the reproducibility of difference spectra over a wide concentration range. Significant improvements were observed (fourfold), in comparison with the most commonly applied technique that uses an averaged spectrum of solvent. We propose a new technique, in which subtraction of the closest spectrum of solvent involves calculating the smallest area under the subtracted curve, to extract the optimal outcome. These results reveal that, contrary to common practice, reproducibility for spectra of diluted solutions can bypass even instrumental baseline shifts and render results that are limited only by the noise originating from the instrument's sensor.
authors.affiliation: Department of Pharmacology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo, 160-8582, Japan; Keio Advanced Research Center for Water Biology and Medicine, Keio University, Mita 2-15-45, Minato-ku, Tokyo, 108-8345, Japan. Electronic address: dusan.s.kojic@gmail.com.
authors.forename: Dušan
authors.initials: D
authors.lastname: Kojić
authors.affiliation: Keio Advanced Research Center for Water Biology and Medicine, Keio University, Mita 2-15-45, Minato-ku, Tokyo, 108-8345, Japan; Department of Agricultural Engineering and Socio-Economics, Kobe University, 1-1 Rokkodai, Nada-ku, Kobe, 657-8501, Japan.
authors.forename: Roumiana
authors.initials: R
authors.lastname: Tsenkova
authors.affiliation: Department of Pharmacology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo, 160-8582, Japan; Keio Advanced Research Center for Water Biology and Medicine, Keio University, Mita 2-15-45, Minato-ku, Tokyo, 108-8345, Japan.
authors.forename: Masato
authors.initials: M
authors.lastname: Yasui
keywords: Hydrogen bonding; Near infrared spectroscopy; Spectral pre-processing; Spectral subtraction; Water