IMPROVED CURVE RESOLUTION OF HIGHLY OVERLAPPING BANDS BY COMPARISON OF 4TH-DERIVATIVE CURVES

Curve fitting of highly overlapping bands can be improved by compariso n of the fourth derivative of the experimental composite band profile with that of the sum of the curve-fitted component bands. Optimization is achieved with fixed values of one band parameter, aiming thereby f or optimal correspondence between the fourth-derivative curves. FT-IR spectra of nitrate's v(1) band region in approximate to 1 M glassy Ca( NO3)(2) solution, and of its v(2) band region in 10 M LiNO3 solution, in approximate to 0.5 M glassy Mg(NO3)(2) solution and in 5 M Ca(NO3)( 2) solution, all with D2O as solvent, were used. As a general rule, re liable curve fitting of two overlapping bands of unknown band shape is possible only when separation of their peak maxima is larger than the ir average full width at half-height (FWHH). We show here that by comp arison of fourth-derivative curves many more overlapped composite band s can be reliably resolved into their component bands, and, for the fa vorable case of 10 M LiNO3 solution, separation of two bands by only a bout a third of their average FWHH is sufficient for reliable curve fi tting. This method requires data with very high Signal-to-noise ratios . Improved curve fitting of highly overlapping bands is demonstrated f or both a sum and a product of Gaussian and Lorentzian peak shapes, an d a sort of recipe is given for both types of curve fitting. Synthetic spectra of two overlapping bands, where the degree of overlap and the ir relative intensity were varied in a systematic way, are given toget her with their fourth-derivative curves. This information will be of h elp for interpreting fourth-derivative curves of experimental highly o verlapping two-band systems.