Peak sharpening limits of solvent-assisted post-column refocusing to enhance detection limits in liquid chromatography

We report on a numerical and experimental study of the limits of peak refocusing and concentration enhancement that can be obtained with solvent front-assisted peak remobilization in a trap column receiving peaks eluting from a preceding analytical column. It is shown that the upper limit of peak refocusing can best be pursued by injecting a sufficiently large volume in a sufficiently narrow capillary and elute it with a sufficiently steep (ballistic) gradient. Corresponding equations offering a quantitative description have been derived and verified experimentally. For the latter purpose, peak volumes of the order of 0.5 to 2.0 μL were refocused in a dedicated set-up capable of trapping μL-sized peaks in a 75 μm i.d. capillary and remobilized using a nano-LC pump propelling an acetonitrile/isopropyl alcohol mixture with a viscosity matching that of the trapping solvent. Injecting 2.0 μL peaks, a peak refocusing factor of 17.3 could be achieved.