The flexible dialyser membrane – a source of error in plasma creatinine determination

Introduction Dialysis is frequently used in continuous flow analysis to separate low molecular weight analytes from protein containing samples. The dialyser assembly consists of two clear plastic blocks with matching donor and recipient grooves, between which is sandwiched a semipermeable membrane. The airsegmented donor (diluted sample) stream and recipient streams flow at constant rates along the grooves. This allows partial diffusion of the analyte across the membrane. The analyte is transferred into the recipient stream. On leaving the dialyser, reagents are added to this stream in order to produce a response to the analyte and to provide a quantitative measurement. Detergent is often added to the sample stream to facilitate the analysis. At the detergent concentrations commonly used in continuous flow assays the pressure within the manifold drastically increases when a protein containing sample is analysed [1 ]. The cellophane membranes used are thin and flexible, therefore changes in the pressure across a dialyser membrane affects the conformation of the membrane. The relative volumes and flow rates of the donor and recipient streams within the dialyser are also changed. This phenomenon was investigated particularly for a plasma creatinine assay commonly used in clinical chemistry [2]. Figure shows a schematic flow diagram of the system used. In the assay, the high baseline absorbance of the picric acid reagent is sensitive to momentary changes in flow rate of the recipient stream from the dialyser.