Detection sensitivity in CE-ICP-MS has been improved by introducing a preconcentration step by large volume stacking with polarity switching. This implies the modification of the interface to be able to deliver either the solution buffer when performing preconcentration or the make-up solution when separation occurs. This procedure has been evaluated on a mixture of proteins in terms of efficiency and sensitivity and has shown an improvement in detection limits. It was shown that injection volumes can be increased 10 times without any substantial loss in resolution, leading to a sensitivity enhancement by a factor of 6. This technique was applied successfully in studies on the interactions of cadmium with a mixture of test-proteins. The experiments performed have demonstrated the ability of CE-ICP-MS in the fast screening of weak metal/protein interactions. Interactions of Cd2+ with several proteins can be revealed in less than 35 minutes.