Page 60 - Modern Analytical Chemistry

1400-CH03 9/8/99 3:51 PM Page 43

Chapter 3 The Language of Analytical Chemistry 43

10 –10 %
–9
10 %
Ultratrace
–8
10 %
–7
10 % ppb
–6
10 %
–log(% analyte as %w/w) 10 % ppm 1 g sample, 1% analyte
–5
10 %
Trace
–4
–3
10 %
–2
10 %
0.1 g sample, 10% analyte
Minor
0.1%
1% 0.01 g sample, 100% analyte
Major 10% 100 mg 10 mg 100 µg 10 µg

100% 1 mg 1 µg
1 0.1 0.01 g
100 10 1 0.1 0.01 mg
100 10 1 0.1 0.01 µg
Macro 100 10 1 ng
Meso
Micro
Ultramicro Figure 3.6
Scale of operation for analytical methods.
–log(Weight of sample) Adapted from references 7a and 7b.

sample used provide a characteristic description for an analysis. For example,
samples in a macro–major analysis weigh more than 0.1 g and contain more than
1% analyte.
Diagonal lines connecting the two axes show combinations of sample size and
concentration of analyte containing the same absolute amount of analyte. As shown
in Figure 3.6, for example, a 1-g sample containing 1% analyte has the same
amount of analyte (0.010 g) as a 100-mg sample containing 10% analyte or a 10-mg
sample containing 100% analyte.
Since total analysis methods respond to the absolute amount of analyte in a
sample, the diagonal lines provide an easy way to define their limitations. Consider,
for example, a hypothetical total analysis method for which the minimum de-
tectable signal requires 100 mg of analyte. Using Figure 3.6, the diagonal line repre-
senting 100 mg suggests that this method is best suited for macro samples and
major analytes. Applying the method to a minor analyte with a concentration of
0.1% w/w requires a sample of at least 100 g. Working with a sample of this size is
rarely practical, however, due to the complications of carrying such a large amount
of material through the analysis. Alternatively, the minimum amount of required
analyte can be decreased by improving the limitations associated with measuring
the signal. For example, if the signal is a measurement of mass, a decrease in
the minimum amount of analyte can be accomplished by switching from a con-
ventional analytical balance, which weighs samples to ±0.1 mg, to a semimicro
(±0.01 mg) or microbalance (±0.001 mg).