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202 Modern Analytical Chemistry

where k A is the analyte’s sensitivity.* In the presence of an interferent, equation 7.9
becomes
7.10
S samp = k A C A + k I C I
where k I and C I are the interferent’s sensitivity and concentration, respectively. A
method’s selectivity is determined by the relative difference in its sensitivity toward
the analyte and interferent. If k A is greater than k I , then the method is more selec-
tive for the analyte. The method is more selective for the interferent if k I is greater
than k A .
Even if a method is more selective for an interferent, it can be used to deter-
mine an analyte’s concentration if the interferent’s contribution to S samp is insignifi-
cant. The selectivity coefficient, K A,I , was introduced in Chapter 3 as a means of
characterizing a method’s selectivity.
k I
K A,I =
k A 7.11
Solving equation 7.11 for k I and substituting into equation 7.10 gives, after simplifying
S samp = k A(C A + K A,I ´C I) 7.12
An interferent, therefore, will not pose a problem as long as the product of its con-
centration and the selectivity coefficient is significantly smaller than the analyte’s
concentration.

K A,I ´C I << C A
When an interferent cannot be ignored, an accurate analysis must begin by separat-
ing the analyte and interferent.

7 E General Theory of Separation Efficiency

The goal of an analytical separation is to remove either the analyte or the interferent
from the sample matrix. To achieve a separation there must be at least one signifi-
cant difference between the chemical or physical properties of the analyte and inter-
ferent. Relying on chemical or physical properties, however, presents a fundamental
problem—a separation also requires selectivity. A separation that completely re-
moves an interferent may result in the partial loss of analyte. Altering the separation
to minimize the loss of analyte, however, may leave behind some of the interferent.
A separation’s efficiency is influenced both by the failure to recover all the ana-
recovery lyte and the failure to remove all the interferent. We define the analyte’s recovery,
The fraction of analyte or interferent R A , as
remaining after a separation (R).
C A
R A =
( C Ao )
where C A is the concentration of analyte remaining after the separation, and (C A ) o
is the analyte’s initial concentration. A recovery of 1.00 means that none of the ana-
lyte is lost during the separation. The recovery of the interferent, R I , is defined in
the same manner
C I
R I = 7.13
()
C Io
*In equation 7.9, and the equations that follow, the concentration of analyte, C A , can be replaced by the moles of
analyte, n A , when considering a total analysis technique.