Flow injection potentiometry for enzymatic assay of cholesterol with a tungsten electrode sensor

Flow injection potentiometry (FIP) for the enzymatic determination of cholesterol is reported. The assay utilises a combination of three enzymes: cholesterol esterase (CE), cholesterol oxidase (COD) and peroxidase (POD). The method is developed by the use of a tungsten wire electrode as a sensor vs. Ag/AgCl in conjunction with a redox mediator ferrocyanide. CE converts esterified cholesterol to free cholesterol, which is then oxidised by COD with hydrogen peroxide as product. Ferrocyanide is converted to ferricyanide by hydrogen peroxide, catalysed by POD, and the tungsten electrode responds to the ratio of ferricyanide to ferrocyanide. Flow injection potentiometry gave well-defined peaks for cholesterol samples with a fast response (30 s). Linear calibration was obtained from 0.05 to 3.0 mM cholesterol, with a slope of 60.2 mV/decade change in cholesterol concentration, and detection limit 0.01 mM cholesterol (S/N=3). Repeatability was 3% (CV). Interferences from commonly found species were shown to be negligible. The sensor cell is simple to construct, and it was free from surface contamination problems over long periods of use. The application of the sensor for the determination of serum cholesterol was demonstrated.