The trapping of charged microparticles under confinement in a converging diverging microchannel, under a symmetric AC field of tunable frequency, is studied. We show that at low frequencies, the trapping characteristics stem from the competing effects of positive dielectrophoresis and the linear electrokinetic phenomena of electroosmosis and electrophoresis: It is found, somewhat :unexpectedly; that electroosmosis and electrophoresis significantly affect the concentration profile of the trapped analyte, even for a symmetric AC field. However; at intermediate frequencies, the microparticle trapping mechanism is predominantly a consequence of positive dielectrophoresis. We substantiate our :experimental results for the micro-partieje concentration distribution,, along the converging diverging micro channel, with a detailed theoretical analysis that takes into account all of the relevant frequency-dependent electrokinetic phenomena. This study; should be useful in understanding the response of biological components such as cells to applied AC fields. Moreover, it will have potential,applications in the design of efficient point-of-care diagnostic devices-for-detecting biomarkers and also possibly in some recent strategies in cancer therapy using AC fields.