Counter-flow dialysis for microvolume desalting

Some analytical techniques are not compatible with physiological salt concentrations. An array of desalting approaches exists, but the conventional implementation requires large sample volumes, which is not compatible with fingerprick blood samples for molecular diagnostics. With dialysis being identified as the most suitable method for on-chip microvolume desalting, the aim of this work was to develop a microvolume dialyser that can desalt biological samples to any required salt concentration. Phase-separation ultrafiltration membranes were first characterised to determine their porosity, an important parameter for the dialysis efficiency. Desalting of a microchamber-confined sample over these membranes was then systematically investigated, first by static dialysis against a bulk water phase and next in a dialyser where also the water phase was enclosed in a laser micromachined chamber, with periodic water refreshment. Although the water content of the channel was approximately 400-fold smaller than the initial bulk water volume, similar salt removal rates could be achieved with high refreshment rates. A dialyser that enables a continuous flow of the sample and of the water phase in opposite directions was then constructed. It was established that by selecting the appropriate combination of flow rates the extent of desalting can be exactly controlled. This extensive experimental data set was in good agreement with predictions from mass transfer theory, which also indicated that a further increase in dialysis performance would require a different membrane structure. The developed counter-flow microvolume dialyser could be operated for a prolonged period of time, also for samples with a high protein content and also for blood serum. Finally, protein samples were dialysed under different sample and water flow conditions and the effect of a range of residual salt concentrations on the quality of mass spectrometer analysis was explored