S. Mathison et E. Bakker, EFFECT OF TRANSMEMBRANE ELECTROLYTE DIFFUSION ON THE DETECTION LIMIT OF CARRIER-BASED POTENTIOMETRIC ION SENSORS, Analytical chemistry, 70(2), 1998, pp. 303-309
The detection limit of carrier-based ion-selective electrodes is expla
ined by the presence of a locally elevated concentration of measuring
ions at the sample-membrane phase boundary. Since ion-selective electr
odes are responsive to phase boundary activities, such elevated concen
trations render the potentiometric sensor insensitive to dilute bulk c
oncentration changes. Different mechanisms for the continuous release
of measuring ions from the membrane are conceivable. The extraction of
inner electrolyte into the backside of the ion-selective membrane is
predicted to lead to a concentration gradient of electrolyte across th
e membrane and therefore to a net nux of measuring ions from the inner
filling solution to the sample. This effect is described by an extend
ed model that respects the relevant extraction and diffusion processes
. The extent of coextraction at the backside is predicted on the basis
of potentiometric measurements on the range of anion interference. Th
ese predictions are found to relate well to experimental results with
valino-mycin electrodes. The presence of lipophilic anions in the inne
r electrolyte is found to increase detection limits owing to the incre
ased extraction into the membrane. An upper limit apparently exists be
yond which the detection limit is no longer increased upon increasing
the inner filling solution concentration. Stirring the sample decrease
s the detection limit owing to increased mass transport from the membr
ane surface to the bulk sample.