A number of reagent-free infrared spectroscopic diagnostic and analytical methods have been established previously, making use of dry biofluid films. For example, this approach has successfully measured high concentration analytes of serum and urine. However, a number of low concentration diagnostically relevant analytes presently elude detection by infrared spectroscopy. This is due in part to their relatively low concentration and in part to spectral interference by other strongly absorbing constituents. The applicability of the technique would be broadened substantially if it were possible to separate and concentrate, lower concentration analytes, e.g. serum creatinine and urine proteins, from the obscuring presence of relatively high concentration compounds. The invention disclosed achieves this is through microfluidic sample preconditioning based upon laminar fluid diffusion interfaces. Preferential separation of certain low concentration serum and urine analytes of clinical interest that presently lie just below the threshold of detection by infrared spectroscopy is provided.