The present invention an apparatus and method of injecting a liquid borne sample into a field flow fractionator and a method of forming a top plate and spacer. In an embodiment, the field flow fractionation unit includes a top plate including a sample injection inlet port, a sample injection outlet port, and a spacer including a separation channel cavity defining at least a portion of the separation channel, a sample injection inlet cavity configured to be in fluid contact with the separation channel and located substantially beneath the sample injection inlet port, a sample injection outlet cavity configured to be in fluid contact with the separation channel and located substantially beneath the sample injection outlet port, such that the injection inlet and outlet paths are configured to define an injection channel that is essentially perpendicular to the length of the separation channel spanning the width of the separation channel cavity.