A system, including methods and apparatus, for microfluidic analysis of a nucleic acid target in a nucleic acid mixture. The system includes a method to preselect the target from the mixture before amplification. Preselection enriches the mixture for the target by retaining the target on a target-selective receptor and then removing unretained non-target nucleic acids. The preselected target then may be amplified from the enriched mixture and assayed. Devices configured to carry out the method are also disclosed.