Methods and apparatus providing improved fidelity and specificity when separating nucleic acids from a sample, but without need for amplification. In particular, using the disclosed methods, it is possible to isolate a variant nucleic acid (i.e., a mutation) from a non-target nucleic acid (i.e., a wild-type) when the variant is present in the original sample at a much lower concentration than the non-target, e.g., 1:10,000, without substantial loss of the variant.