A microfluidic device comprising at least one inlet port, at least one flow path coupled to the inlet port, and at least one fluid separation element coupled to the flow path, wherein the fluid separation element comprises a packing material and is adapted for separating different components of a fluid, wherein the microfluidic device comprises at least one retaining device for keeping the packing material of the fluid separation element fixed in place and protecting the microfluidic device from debris polluting the analyte.