Diagram of the dual-column nanoLC system. The system interweaves two valves and employs two subsystems, each consisting of a sample loop, SPE column and analytical column. These subsystems share a sample collection tip, syringe, gradient LC pump and isocratic loading pump, allowing for SPE loading and column washing on one subsystem, while the other elutes peptides. Shown on the left is a commercial LC module containing both a gradient and a loading pump, which provides flow for all operations.