Methods and apparatus for separating, concentrating and/or detecting molecules based on differences in binding affinity to a probe are provided. The molecules may be differentially modified. The molecules may be differentially methylated nucleic acids. The methods can be used in fields such as epigenetics or oncology to selectively concentrate or detect the presence of specific biomolecules or differentially modified biomolecules, to provide diagnostics for disorders such as fetal genetic disorders, to detect biomarkers in cancer, organ failure, disease states, infection or the like.