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186 Modern Analytical Chemistry

Direction of
groundwater flow

= Sample site

Major axis
of plume

Landfill
Figure 7.4
Systematic–judgmental sampling scheme for
monitoring the leaching of pollutants from a
landfill. Sites where samples are collected
are represented by the solid dots.

composite sample A composite sample consists of a set of grab samples that are combined to
Several grab samples combined to form a form a single sample. After thoroughly mixing, the composite sample is analyzed.
single sample.
Because information is lost when individual samples are combined, it is normally
desirable to analyze each grab sample separately. In some situations, however, there
are advantages to working with composite samples. One such situation is in deter-
mining a target population’s average composition over time or space. For example,
wastewater treatment plants are required to monitor and report the average compo-
sition of treated water released to the environment. One approach is to analyze a se-
ries of individual grab samples, collected using a systematic sampling plan, and av-
erage the results. Alternatively, the individual grab samples can be combined to
form a single composite sample. Analyzing a single composite sample instead of
many individual grab samples, provides an appreciable savings in time and cost.
Composite sampling is also useful when a single sample cannot supply sufficient
material for an analysis. For example, methods for determining PCBs in fish often
require as much as 50 g of tissue, an amount that may be difficult to obtain from a
single fish. Tissue samples from several fish can be combined and homogenized,
and a 50-g portion of the composite sample taken for analysis.
A significant disadvantage of grab samples and composite samples is the need to
remove a portion of the target population for analysis. As a result, neither type of sam-
ple can be used to continuously monitor a time-dependent change in the target popu-
in situ sampling lation. In situ sampling, in which an analytical sensor is placed directly in the target
Sampling done within the population population, allows continuous monitoring without removing individual grab samples.
without physically removing the sample.
For example, the pH of a solution moving through an industrial production line can be
continually monitored by immersing a pH electrode within the solution’s flow.