Page 178 - Analytical Electrochemistry 2d Ed - Jospeh Wang

5-3 ON-LINE AND IN-VIVO POTENTIOMETRIC MEASUREMENTS 163

FIGURE 5-18 Flow-through potentiometric cell-cap design. A, reference electrode; B,
iodide electrode; C, ¯ow-through cap; D, inlet; E, outlet. (Reproduced with permission from
reference 49.)

chromatography with potentiometric detection (51). Similarly, liquid-membrane
microelectrodes have been used as small-dead-volume detectors in open tubular
column liquid chromatography (52). Miniaturization has also permitted the recent
adaptation of ISEs as on-column detectors for capillary zone electrophoresis in
connection with femtoliter detection volumes (53,54). The small dimensions in
capillary electrophoresis require proper attention to the positioning of the ISE
detector. Both micropipette and coated-wire ISEs have been useful for this task, with
the latter offering a simpli®ed electrode alignment (54). Micropipette ISEs have also
been used as tips in scanning electrochemical microscopy (55; see Section 2-3). The
suitability to miniaturization is attributed to the fact that the potentiometric signal
does not depend on the sensing area; but further miniaturization of ISEs to the
nanometer domain is limited by the electrical resistance of the bulk liquid
membrane.
Potentiometric microelectrodes are very suitable for in-vivo real-time clinical
monitoring of blood electrolytes, intracellular studies, in-situ environmental surveil-
lance or industrial process control. For example, Simon's group described the utility
of a system for on-line measurements of blood potassium ion concentration during
an open-heart surgery (56); Buck and co-workers (57) reported on the use of ¯exible
planar electrode arrays for the simultaneous in-vivo monitoring of the pH and
potassium ion in the porcine beating heart during acute ischemia (Figure 5-19).