Page 210 - Modern Analytical Chemistry

1400-CH07 9/8/99 4:03 PM Page 193

Chapter 7 Obtaining and Preparing Samples for Analysis 193

and that for the second sampling strategy is
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As expected, since the relative method variance is better than the relative
sampling variance, a sampling strategy that favors the collection of more
samples and few replicate analyses gives the better relative error.

7 C Implementing the Sampling Plan

After a sampling plan has been developed, it is put into action. Implementing a
sampling plan normally involves three steps: physically removing the sample from
its target population, preserving the sample, and preparing the sample for analysis.
Except for in situ sampling, the analysis of a sample occurs after removing it from
the target population. Since sampling exposes the target population to potential
contamination, the sampling device must be inert and clean.
Once a sample is withdrawn from a target population, there is a danger that it
may undergo a chemical or physical change. This is a serious problem since the
properties of the sample will no longer be representative of the target population.
For this reason, samples are often preserved before transporting them to the labora-
tory for analysis. Even when samples are analyzed in the field, preservation may still
be necessary.
The initial sample is called the primary, or gross sample and may be a single gross sample
increment drawn from the target population, or a composite of several increments. The initial sample, collected from the
In many cases the gross sample cannot be analyzed without further treatment. Pro- target population without any
processing.
cessing the gross sample may be used to reduce the sample’s particle size, to transfer
the sample into a more readily analyzable form, or to improve its homogeneity.
In the sections that follow, these three steps are considered for the sampling of
liquids (including solutions), gases, and solids.

7C.1 Solutions
Typical examples of liquid samples include those drawn from containers of com-
mercial solvents; beverages, such as milk or fruit juice; natural waters, including
from lakes, streams, seawater, and rain; bodily fluids, such as blood and urine; and,
suspensions, such as those found in many oral medications.

Sample Collection Homogeneous solutions are easily sampled by siphoning, de-
canting, or by using a pipet or syringe. Unfortunately, few solutions are truly homo-
geneous. When the material to be sampled is of manageable size, manual shaking is
often sufficient to ensure homogeneity. Samples may then be collected with a pipet,
a syringe, or a bottle. The majority of solutions, however, cannot be sampled in this
manner. To minimize the effect of heterogeneity, the method for collecting the
gross sample must be adapted to the material being sampled.
The environmental sampling of waters and wastewaters provides a good illus-
tration of many of the methods used to sample solutions. The chemical composi-
tion of surface waters, such as streams, rivers, lakes, estuaries, and oceans, is influ-
enced by flow rate and depth. Rapidly flowing shallow streams and rivers, and
shallow (<5 m) lakes are usually well mixed and show little stratification with