Direct Phase Correction of D ifferential FT-IR Spectra

Step -scan transient F ourier tran sform infrared (FT-IR ) d ifference sp ectra are often m easured in an ac-coup led con ® guration . The resu lting differential intensity sp ectra con tain both posit ive and negative bands. This condition poses problem s for d irect phase correction by the standard M ertz and Form an methods. Restricting th e calculated phase angle to the range [ 2 p /2, p /2] was p reviously sh own to ® x som e of these p roblem s, but we show that the u se of a reduced-resolu tion phase spectrum can produce oth er artifacts. The effect of reduced reso lution is analyzed for a simu lated noise-free sp ectrum and for a m easured tran sient spectrum of a rea l pho tochem ica l system , bacteriorhodopsin . Exam ination of these resu lts revea ls th at the M ertz and M ertz Sign ed methods can produce sp ectral bands of reduced m agn itude and unu su al band shape, w ith considerable am ounts of in tensity rema ining along the im aginary axis after phase correct ion . However, these errors can be elim inated by self-convolution of the m easu red interferogram , wh ich doubles all phase angles, prior to sm oothing . This procedure rem oves the poten tia l d iscon tinuities in the phase angle due to sign changes in the d ifferential spectrum . W ith bacteriorhodopsin , th is doubled-angle m ethod for d irect phase correction is able to produce a transient sp ectrum wh ich closely m atches that p roduced by using a separately m easured dc interferogram to calculate th e phase ang le.