Abstract: A valving arrangement and a method are provided by which a biological sample is mixed with a reactant, such as a solvent or a precipitating agent, as it is injected through a multi-position injection valve. A multi-position switching valve is connected to the injection valve, and one or more filters attached to the switching valve remove precipitates of predetermined size from the reacted sample. The sample the flows to conventional analytical columns for liquid chromatography analysis of analytes of interest. Proteins and/or peptides are thus quickly and efficiently removed from the sample as precipitates collected in the filters. The injection valve and the switching valve are then placed in respective cooperating positions to flow a carrier fluid through the injection valve and the switching valve directly to the analysis columns, while a back-flow of a suitable surfactant is pumped through the filters to purge them of collected precipitates.