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198 Modern Analytical Chemistry

an auger may be used to drill a hole to the desired depth and the sample col-
lected with a soil punch.
The sampling of particulate material is often determined by the size of the par-
ticles. Large particulate solids, such as coals and ores, can be sampled by randomly
collecting samples with a shovel or by riffling. A riffle (Figure 7.7) is a trough con-
taining an even number of compartments with adjacent compartments emptying
on opposite sides of the riffle. Particulate material dumped into a riffle is divided
into two parts. By repeatedly passing half of the separated material back through the
riffle, a sample of any desired size may be collected. Smaller particulate materials,
Figure 7.7
such as powders, are best collected with a sample thief, which allows material to be
Example of a four-unit riffle. A sample
added through the top is divided into four collected simultaneously from several locations (Figure 7.8). A typical sample thief
piles, two on each of the riffle’s sides. consists of two tubes that are nestled together. Each tube has an identical set of slots
aligned down their length. Before the sample thief is inserted into the material being
sampled, the inner tube is rotated so that slots are closed. When the sample thief is
in place, the inner tube is rotated to open the slots, allowing the powder to enter the
sample thief through each slot. The inner tube is then rotated to the closed position
and the sample thief withdrawn.
When sampling a metal, it usually is necessary to obtain material from both the
surface and the interior. When the metal is in the form of a sheet, random samples
can be collected with a metal punch. Samples can be obtained from a metal wire by
randomly cutting off pieces of an appropriate length. Larger pieces of metal, such as
bars or bricks, are best sampled by sawing through the metal at randomly selected
points and collecting the “sawdust” or by drilling through the metal and collecting
the shavings. A surface coating can be sampled in situ or by dissolving the coating
with an appropriate solvent.
Sampling of biological tissue is done by removing the entire organ, which is
then homogenized before smaller portions are taken for analysis. Alternatively, sev-
eral small portions of tissue may be combined to form a composite sample. The
composite sample is then homogenized and analyzed.

Sample Preservation Without preservation, many solid samples are subject to
changes in chemical composition due to the loss of volatile material, biodegrada-
tion, and chemical reactivity (particularly redox reactions). Samples stored at re-
duced temperatures are less prone to biodegradation and the loss of volatile mate-
rial, but fracturing and phase separations may present problems. The loss of volatile
material is minimized by ensuring that the sample completely fills its container
without leaving a headspace where gases can collect. Samples collected from mate-
rials that have not been exposed to O 2 are particularly susceptible to oxidation reac-
tions. For example, the contact of air with anaerobic sediments must be prevented.

Sample Preparation Unlike gases and liquids, which generally require little sample
preparation, solid samples usually need some processing before analysis. There are two
reasons for this. First, as discussed in Section 7B.3, sampling variance is a function of
Figure 7.8 the number of particles sampled, not their combined mass. For extremely heteroge-
Schematic diagram of a sample thief. neous populations consisting of large particulates, the gross sample may be too large to
Rotating the inner cylinder opens and closes analyze. For example, a boxcar containing a load of a Ni-bearing ore with an average
the openings along the outer cylinder’s
shaft. particle size of 5 mm may require a sample weighing one ton to obtain a reasonable
sampling variance. Reducing the sample’s average particle size allows the same number
of particles to be sampled with a smaller, more manageable combined mass.
Second, the majority of analytical techniques, particularly those used for a
quantitative analysis, require that the analyte be in solution. Solid samples, or at
least the analytes in a solid sample, must be brought into solution.