Quantitative analyses of absorption-sensitive surface plasmon resonance near-infrared spectra

Surface plasmon resonance near-infrared (SPR-NIR) spectroscopy provides 10-100 times absorption enhancement compared with the absorption in the corresponding attenuated total reflection (ATR) NIR spectra. However, analysis of the enhanced SPR-NIR spectra is not straightforward because of the substantial contribution from SPR. This paper proposes two analysis methods for concentration-dependent changes of SPR-NIR spectra from a viewpoint of change in absorption intensity. One is based on rapid scans of the SPR-NIR spectra with a fixed incident angle, and the other is based on multi-angle sequential scans. A concentration of methanol in water has successfully been determined by both methods. From the measurement of the light intensity within an absorption band of water (5230-5120 cm(-1)) at a fixed incident angle, the concentration was calibrated to an accuracy of 0.02 wt. %. In the latter multi-angle method, it has been proved that computed bottom ridges of the envelope curve of the SPR-NIR spectra are not only enhanced 30 times compared with the corresponding ATR-NIR spectra, but are also equivalent to the conventional transmittance NIR spectra in quality. The bottom ridges allow us to analyze SPR-NIR spectra in the same manner as conventional spectral analyses based on Beer's law.