Page 216 - Modern Analytical Chemistry

1400-CH07 9/8/99 4:03 PM Page 199

Chapter 7 Obtaining and Preparing Samples for Analysis 199

(a) (b)

Figure 7.9
Illustration showing the method of coning
and quartering as a means of reducing a
gross sample for subsampling. (a) The gross
sample is first piled into a cone and
(b) flattened. Looking down from above,
(c) the cone is divided into four quarters,
(c) (d) (d) which are then separated.

Reducing Particle Size A reduction in particle size is accomplished by a combina-
tion of crushing and grinding the gross sample. The resulting particulates are then
thoroughly mixed and divided into samples of smaller mass containing the appro-
priate number of particles. The process seldom occurs in a single step. Instead, sam-
ples are cycled through the process several times until a laboratory sample of de- laboratory sample
sired mass is obtained. Sample taken into the lab for analysis
Crushing and grinding uses mechanical force to break larger particles into after processing the gross sample.
smaller ones. A variety of tools are used depending on the particle’s size and hard-
ness. Large particles are crushed using jaw crushers capable of reducing particles to
diameters of a few millimeters. Ball mills, disk mills, and mortars and pestles are
used to further reduce particle size.
Significant changes in composition may occur during crushing and grinding.
Decreasing particle size increases available surface area. With more surface area
there is a greater risk of losing volatile components, a problem made worse by the
frictional heat accompanying the crushing and grinding. An increase in surface area
also means that portions of the sample are freshly exposed to the atmosphere where
oxidation may alter the sample’s composition. Other problems include contamina-
tion from the mechanical abrasion of the materials used to crush and grind the
sample, and differences in the ease with which particles are reduced in size. Softer
particles are reduced in size more easily and may be lost as dust before the rest of
the sample has been processed. This is a problem since the analyte’s distribution
may not be uniform between particles of different size.
To ensure that all particles are reduced to a uniform size, the sample is intermit-
tently passed through a sieve. Processing of those particles not passing through the
sieve continues until the entire sample is of uniform size. The sample is then mixed
thoroughly to ensure homogeneity, and a secondary sample obtained with a riffle or coning and quartering
by coning and quartering. The latter approach is outlined in Figure 7.9. The gross A process for reducing the size of a gross
sample is piled into a cone, flattened, divided into four quarters, and two diagonally sample.