Page 177 - Analytical Electrochemistry 2d Ed - Jospeh Wang

162 POTENTIOMETRY

FIGURE 5-17 Flow injection potentiometric determination of potassium in serum. (Repro-
duced with permission from reference 47.)

routinely employed in most hospitals for the high-speed determination of physio-
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logically important cationic electrolytes (e.g., K ,Na ,Ca 2‡ ,Mg 2‡ , and H )or
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anions (e.g. Cl ) in body ¯uids. The corresponding ISEs are usually placed in
series, along the ¯ow channel. Additional advantages accrue from the coupling of
arrays of potentiometric detectors with chemometric (statistical) procedures (see
Section 6-4). The transient nature of ¯ow-injection potentiometric measurements
(e.g., Figure 5-17) nicely addresses the potential-drift problem common to analogous
batch measurements. It can also be exploited for enhancing the selectivity by
operating under kinetic (rather than equilibrium) control. Improved selectivity can
also be achieved by deliberately adding the interfering ion to the ¯ow-injection
carrier solution. Several designs of low-volume potentiometric ¯ow detectors have
been reported (47,48). The simplest design consists of an ISE ®tted tightly with a
plastic cap, with an inlet and outlet for the ¯owing stream (Figure 5-18). The
reference electrode is usually placed downstream from the ISE. It can also be
immersed in a parallel (potassium chloride) ¯owing stream. Other common detector
designs include the ¯ow-through tubular ISE (used in Figure 5-17), and tangential or
wall-jet ISEs. Multi-ion detectors, based on ion-sensitive ®eld-effect transistors
(discussed in Section 6-3) have been combined with miniaturized micromachined
¯ow-injection systems (50). Such coupling offers improved response times and
reduced consumption of samples and reagents.
In addition to automated analysis, ISEs can be used to detect ionic species in
chromatographic ef¯uents. Particularly powerful is the coupling of modern ion