The present disclosure provides method and systems for improving nanopore-based analysis of polymers. The disclosure provides methods for selectively modifying one or more monomeric subunit(s) of a kind in a re-analyte polymer that results in a polymer analyte with a modified subunit. The polymer analyte produces a detectable signal in a nanopore-based system. The detectable signal, and/or its deviation from a reference signal, indicates the location of the modified subunit in the polymer analyte and, thus, permits the identification of the subunit at that location in the original pre-analyte polymer.