The invention provides improved methods and apparatus for fluid chromatography, and is particularly appropriate to high-pressure liquid chromatography carried out using eluent flow rates less than 1 μl/minute, for example on nanoflow columns. In both single- and multi-dimensional chromatography systems, especially those comprising trapping media to facilitate injection of relatively large volumes of sample on to nanoflow columns, the on-line addition of a diluting solvent enables stronger eluents and sample solvents to be employed without causing premature release of analytes from the trapping media or the degradation of the second dimension chromatographic separation. The invention may be advantageously used for two-dimension reverse phase/reverse phase separations, especially for the separation of complex mixtures of peptides.