Application of doubled-angle phase correction method to time-resolved step-scan FT-IR spectra

To increase the sensitivity with which time-resolved Fourier transform infr ared (FT-IR) difference spectra are measured, the detector is often AC-coup led. Thus, the measured interferograms correspond to spectra with both posi tive and negative intensities. The presence of signed intensities presents problems for the standard Mertz and Forman phase correction methods. The Me rtz Signed phase correction method was designed to handle signed intensitie s, but the smoothing inherent in calculating the phase angles at reduced re solution introduces other errors in AC-coupled spectra produced with this a lgorithm. These errors are evident as signal remaining along the imaginary axis after phase correction. A new approach to phase correction, the Double d-Angle method, can directly correct the phases of transient AC-coupled spe ctra without the need for a DC interferogram [M.S. Hutson, M.S. Braiman, Ap pl. Spectrosc. 52 (1998) 974]. When this method was applied to the transien t AC interferograms measured after photolysis of bacteriorhodopsin, the sig nal was fully rotated onto the real axis following phase correction. Here, we show that the Doubled-Angle method can be applied to time-resolved diffe rence FT-IR spectra of halorhodopsin, a more demanding biological system du e to its intrinsically small differential absorption signals. (C) 1999 Else vier Science B.V. All rights reserved.