1. A method for improving regression-based spectroscopic analysis, comprising: a. computing a residual error between an estimated spectrum and a measured spectrum taken of a sample, wherein the estimated spectrum is derived from mixing coefficients for members of a library of spectra that are produced by computing a regression on the measured spectrum with the library; b. identifying peaks in the residual error that extend in a direction opposite to that of peaks in the measured spectrum; c. performing a regression on the peaks to produce retro-regression coefficients; d. computing corrected mixing coefficients based on the retro-regression coefficients; and e. repeating (a) through (d) with the corrected mixing coefficients computed in (d) being used to generate a new estimated spectrum for computing the residual error in (a) at the next iteration.