Page 59 - Modern Analytical Chemistry

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42 Modern Analytical Chemistry

Since equimolar concentrations of analyte and interferent were used
(C A = C I ), we have

S I 6
K AI , = = =015
.
S A 40
(b) To achieve an accuracy of better than ±0.50% the term K A,I ´C I in
equation 3.8 must be less than 0.50% of C A; thus

0.0050 ´C A ³K A,I ´C I
Solving this inequality for the ratio C I /C A and substituting the value for
K A,I determined in part (a) gives
.
.
C I 0 0050 0 0050
£ = =0 033
.
.
C A K A I, 015
Therefore, the concentration of 6-methoxycodeine cannot exceed 3.3% of
codeine’s concentration.

Not surprisingly, methods whose signals depend on chemical reactivity are often
less selective and, therefore, more susceptible to interferences. Problems with selec-
tivity become even greater when the analyte is present at a very low concentration. 6

3 5 Robustness and Ruggedness
D.
For a method to be useful it must provide reliable results. Unfortunately, methods
are subject to a variety of chemical and physical interferences that contribute uncer-
tainty to the analysis. When a method is relatively free from chemical interferences,
it can be applied to the determination of analytes in a wide variety of sample matri-
ces. Such methods are considered robust.
robust Random variations in experimental conditions also introduce uncertainty. If a
A method that can be applied to analytes method’s sensitivity is highly dependent on experimental conditions, such as tem-
in a wide variety of matrices is perature, acidity, or reaction time, then slight changes in those conditions may lead
considered robust.
to significantly different results. A rugged method is relatively insensitive to changes
in experimental conditions.
rugged
A method that is insensitive to changes
in experimental conditions is considered 3 D.6 Scale of Operation
rugged.
Another way to narrow the choice of methods is to consider the scale on which the
analysis must be conducted. Three limitations of particular importance are the
amount of sample available for the analysis, the concentration of analyte in the
sample, and the absolute amount of analyte needed to obtain a measurable signal.
The first and second limitations define the scale of operations shown in Figure 3.6;
the last limitation positions a method within the scale of operations. 7
The scale of operations in Figure 3.6 shows the analyte’s concentration in
weight percent on the y-axis and the sample’s size on the x-axis. For convenience,
we divide analytes into major (>1% w/w), minor (0.01% w/w – 1% w/w), trace
–7
–7
(10 % w/w – 0.01% w/w) and ultratrace (<10 % w/w) components, and we
divide samples into macro (>0.1 g), meso (10 mg – 100 mg), micro (0.1 mg –
10 mg) and ultramicro (<0.1 mg) sample sizes. Note that both the x-axis and the
y-axis use a logarithmic scale. The analyte’s concentration and the amount of