Abstract: A method of analysis of a heterogeneous product, for example heparin or heparin derivatives, to define whether said heterogeneous product is consistent with a library of verified heterogeneous samples (Library 1) by analysing the variation, natural or alien. The acceptable variation of the heterogeneous product is determined by comparing Library 1 with a second set of verified spectra (Library 2), by use of comparative two-dimensional correlation spectroscopic filtering (comparative 2D-COS-f). The method comprises obtaining a one-dimensional complex spectrum, for example 1H-NMR spectra, of a heterogeneous product and testing if it has features that are greater than features found testing a spectrum from Library 2 against Library 1. In a second embodiment comparative 2D-COS-f with iterative random sampling (2D-COS-firs) is applied, which provides a more accurate and stable extraction of aliens/unnatural features.