Biomedical Engineering Reference
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Neutral carrier-based ion-selective electrode
Sample
Polymeric membrane
Inner filling solution
R
IL
R
IL
I
A
I
A
IL
R
L
L
Charged carrier-based ion-selective electrode
IL
R
IL
I
A
I
A
IL
R
L
L
Ion exchanger-based ion-selective electrode
R
I
R
I
I
A
I
A
I
R
FIGURE 4.6
Schematic view of the equilibria between sample, ion-selective membrane, and inner fi ll-
ing solution for three important classes of solvent polymeric ion-selective membranes. Top: electrically neu-
tral carrier (L) and lipophilic cation exchanger (R
); center: charged carrier (L
) and anion exchanger (R
+
);
and bottom: cation exchanger (R
).
4.2.2 Response characteristics: selectivity and detection limits
Sensor selectivity improvement is often desired for specifi c applications. While the
progress in the design of new highly selective ionophores for ISEs is discussed below,
here we are focused on the selectivity as a sensor characteristic, methods of its deter-
mination, and the general ways of its enhancement. Unfortunately, there are large dis-
crepancies in selectivity data, published over decades, mainly due to inappropriate
methods used for selectivity determination. IUPAC recommended two procedures for
selectivity determination as the separate solution method (SSM) and fi xed interference
method (FIM) [28]. In the former the potential measurements are performed in two
separate solutions, each containing the salt of the primary or interfering ion only. The
Nikolski selectivity coeffi cient is then calculated according to the following equation:
⎛
⎞
zFE
(
E
RT
)
a
⎟
⎜
pot
I
J
I
I
(7)
log
K
log
IJ
2 303
.
zz
/
a
⎝
⎠
I
J
J
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