Measurement with a chemical sensor and a dialysis probe supported by a null method

A kind of null method was introduced to continuous monitoring of chemicals with a dialysis probe and a chemical sensor to construct a system which was not affected by the change of recovery of the probe or sensor drift. Reference solutions with high and low concentrations, a bypass allowing the mixture of the reference solutions to reach the sensor directly without perfusing the dialysis probe, and a three-way valve as well as a feedback control were added to the system. The flow of the reference solutions mixed each other before the three-way valve and then perfused the bypass and the dialysis probe alternately before reaching the sensor. The mixing ratio of the reference solutions was controlled so that the difference of the sensor output between the two paths became smaller than a given threshold. Then, the concentration of the mixture calculated from the ratio was regarded as the concentration of the objective solution. This method was tested in measuring the glucose concentration in vitro using a system consisting of a dialysis probe with a bundle of semipermeable cuprammonium rayon hollow fibers and a glucose sensor. The glucose concentration of the objective solution was measured precisely, even when the performance of the probe declined or the glucose sensor showed a large (nearly 40%) drift, with the largest error of 13.5% between 100 and 200 mg/dl (5.16-11.1 mM). The procedure time to obtain the first output value was between 8 and 16 min.