An in-capillary sample preconcentration strategy based on solid phase extraction (SPE) technology coupled with capillary electrophoresis (CE) has been developed taking advantage of both techniques (SPE and CE). An in-line frit-free preconcentration device for capillary electrophoresis containing MCX beads, obtained from the corresponding Waters OASIS® cartridges, was prepared. The retention of the particles was based on the relative diameters of the particles, carefully selected, and the capillaries. An experimental preconcentration factor of 100 was found for the system. Conditions were optimised for 3-nitrotyrosine measurement in rat urine being 4.4 M spiked in the urine the lowest value detectable.