Experiments and numerical simulations are combined in an attempt to elucidate the mechanism by which colloidal particles suspended in physiologically relevant (i.e., high ionic strength) media can be electrokinetically sampled on a surface. Results are reported for the trapping of 210 nm fluorescently labeled polystyrene spheres under the influence of a high frequency non-uniform electric field created by planar quadrupolar microelectrodes deposited on an oxidized silicon chip. 3D computer modeling is subsequently used to yield the spatial profiles of electric field intensity, temperature, fluid velocity and force balance on the particles. The results suggest that the experimentally observed rapid particle trapping is achieved by the synergistic action of dielectrophoresis and electrothermal fluid flow. Specifically, electrothermal fluid flow was found responsible for the transport of the particles from the bulk of a suspension to the surface, where dielectrophoretic forces, which become significant only at very small length scales from the surface, cause their stable capture. The calculations show that the time scales associated with the transport and capture of particles when a non-uniform AC field is used are smaller by at least one order of magnitude when compared with those corresponding to a diffusion-limited system. This study can provide valuable insights in the design and operation of biosensors for rapid and in-situ detection of pathogens from small sample volumes, or the development of point-of-care diagnostic devices and micro-total analysis systems (micro-TAS) that operate on AC electrokinetic principles.