Page 57 - Modern Analytical Chemistry

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40 Modern Analytical Chemistry
selectivity
A measure of a method’s freedom from
interferences as defined by the method’s
selectivity coefficient.
selectivity coefficient
A measure of a method’s sensitivity for which may be positive or negative depending on whether the interferent’s effect on
an interferent relative to that for the the signal is opposite that of the analyte.* A selectivity coefficient greater than +1 or
analyte (K A,I ).
sensitivity is 0.200, then the method can conceivably detect a difference of as
little as
± 0.0001 g
Dn A = =± 0.0005 g
0.200
in the absolute amount of analyte in two samples. For methods with the same DS A ,
the method with the greatest sensitivity is best able to discriminate among smaller
amounts of analyte.
3 4 Selectivity
D.
An analytical method is selective if its signal is a function of only the amount of an-
alyte present in the sample. In the presence of an interferent, equations 3.1 and 3.2
can be expanded to include a term corresponding to the interferent’s contribution
to the signal, S I,
S samp = S A + S I = k A n A + k I n I (total analysis method) 3.3
S samp = S A + S I = k A C A + k I C I (concentration method) 3.4
where S samp is the total signal due to constituents in the sample; k A and k I are the
sensitivities for the analyte and the interferent, respectively; and n I and C I are the
moles (or grams) and concentration of the interferent in the sample.
The selectivity of the method for the interferent relative to the analyte is de-
fined by a selectivity coefficient, K A,I
k I
K A,I = 3.5
k A
less than –1 indicates that the method is more selective for the interferent than for
the analyte. Solving equation 3.5 for k I
3.6
k I = K A,I ´k A
substituting into equations 3.3 and 3.4, and simplifying gives
S samp = k A (n A + K A,I ´n I ) (total analysis method) 3.7
S samp = k A (C A + K A,I ´C I ) (concentration method) 3.8
The selectivity coefficient is easy to calculate if k A and k I can be independently
determined. It is also possible to calculate K A,I by measuring S samp in the presence
and absence of known amounts of analyte and interferent.
3
EXAMPLE .1
A method for the analysis of Ca 2+ in water suffers from an interference in the
2+
presence of Zn . When the concentration of Ca 2+ is 100 times greater than
2+
that of Zn , an analysis for Ca 2+ gives a relative error of +0.5%. What is the
selectivity coefficient for this method?
*Although k A and k I are usually positive, they also may be negative. For example, some analytical methods work by
measuring the concentration of a species that reacts with the analyte. As the analyte’s concentration increases, the
concentration of the species producing the signal decreases, and the signal becomes smaller. If the signal in the absence
of analyte is assigned a value of zero, then the subsequent signals are negative.