A COMPARISON OF 3 ELECTRODES FOR THE MEASUREMENT OF PH IN SMALL VOLUMES

An ion-sensitive field effect transistor (ISFET, Sentron(R), Sentron, Inc.) electrode was compared with a glass combination micro-electrode (MI-410, Microelectrodes, Inc.) and a solid-state metal wire oxide pH sensor (Beetrode(R), World Precision Instruments, Inc.) with a liquid junction reference electrode (MERE1, World Precision Instruments, Inc. ). The electrodes were assessed for linearity, reproducibility, accura cy, drift from the initial calibration between pH 4 and pH 7 and the t ime taken to record a stable reading. The ISFET was used to determine the pH in dental plaque samples (1 mg suspended in 20 mu l). The pH va lues correlated with the hydrogen ion concentration for all the electr odes (r = 0.98). The MI-410 fractured before this evaluation was compl eted. Coefficients of variation were 0.65% (pH 4) and 0.08% (pH 7) for the ISFET and 4.69% (pH 4) and 3.46% (pH 7) for the Beetrode. Both el ectrodes gave readings that differed significantly from the initial ca libration, but the drift was greater for the Beetrode (F = 7.93; p = 0 .0005) than the ISFET (F = 1.89; p = 0.1519). However, this drift was smaller than the change in pH as measured in dental plaque samples. Th e Beetrode gave a stable reading after 3.39+/-0.83 s and the ISFET aft er 2.2+/-0.76 s, while the MI-410 required at least 20 s. The ISFET ty pe electrode is suitable for use in small volumes such as plaque suspe nsions, is easier to operate and yields results closer to the initial calibration than the Beetrode and is more robust than the MI-410 and t he Beetrode.