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224 Modern Analytical Chemistry

the original sample (if the extraction is 100% efficient). The preconcentration of
metal ions is accomplished by a liquid–liquid extraction with a metal chelator. For
example, before their analysis by atomic absorption spectrophotometry, metal ions
in aqueous samples can be concentrated by extraction into methyl isobutyl ketone
(MIBK) using ammonium pyrrolidine dithiocarbamate (APDC) as a chelating
agent. Typically, a 100-mL sample is treated with 1 mL of APDC, and extracted with
ten mL of MIBK. The result is a ten-fold increase in the concentration of the metal
ions. This procedure can be adjusted to increase the concentrations of the metal
ions by as much as a factor of 40.

7 I KEY TERMS

breakthrough volume (p. 196) judgmental sampling (p. 184) recovery (p. 202)
composite sample (p. 186) laboratory sample (p. 199) sampling plan (p. 182)
coning and quartering (p. 199) masking (p. 207) separation factor (p. 203)
convenience sampling (p. 185) masking agent (p. 208) size-exclusion chromatography (p. 206)
dialysis (p. 206) Nyquist theorem (p. 184) stratified sampling (p. 185)
distribution ratio (p. 216) partition coefficient (p. 211) systematic–judgmental
extraction (p. 212) preconcentration (p. 223) sampling (p. 184)
grab sample (p. 185) purge and trap (p. 214) systematic sampling (p. 184)
gross sample (p. 193) random sample (p. 183) supercritical fluid (p. 215)
in situ sampling (p. 186)

7 J SUMMARY

An analysis requires a sample, and how we acquire the sample is When the analytical method’s selectivity is insufficient,
critical. To be useful, the samples we collect must accurately repre- it may be necessary to separate the analyte from potential
sent their target population. Just as important, our sampling plan interferents. Such separations can take advantage of physical
must provide a sufficient number of samples of appropriate size so properties, such as size, mass or density, or chemical proper-
that the variance due to sampling does not limit the precision of ties. Important examples of chemical separations include mask-
our analysis. ing, distillation, and extractions.
A complete sampling plan requires several considerations, In a solid-phase extraction the analytes are first extracted
including the type of sampling (random, judgmental, systematic, from their solution matrix into a solid adsorbent. After wash-
systematic–judgmental, stratified, or convenience); whether to col- ing to remove impurities, the analytes are removed from the
lect grab samples, composite samples, or in situ samples; whether adsorbent with a suitable solvent. Alternatively, the extraction
the population is homogeneous or heterogeneous; the appropriate can be carried out using a Soxhlet extractor.
size for each sample; and, the number of samples to collect. In a liquid–liquid extraction, the analyte (or interferent) is ex-
Removing a sample from its population may induce a change tracted from one liquid phase into a second, immiscible liquid
in its composition due to a chemical or physical process. For this phase. When the analyte is involved in secondary equilibrium re-
reason, samples are collected in inert containers and are often pre- actions, it is often possible to improve selectivity by carefully ad-
served at the time of collection. justing the composition of one or both phases.