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Shake it up!
Different sizes,
shapes, and densities can be the source of segregation in many
materials. If we could sample individual particles (or
molecules) completely at random, this would not matter. But we
don't! We sample particles (and molecules) in groups and not
completely at random. So we have a better chance of obtaining a
more representative sample if we mix the material beforehand.
The E-Newsletter
"Are Your Samples Degenerate?" presented alternate shoveling as
a useful technique for splitting an entire lot of loose, solid
material, such as grains, pellets, and soil, into two fractions.
Alternate scoops go into two different piles, and one of the
piles is chosen at random to be the sample.
The problem, of
course, is that if the material is not well-mixed, then the two
fractions might be very different, neither being very
representative of the original lot. Mixing the material
beforehand can substantially reduce Gy's Grouping and
Segregation Error.** An extension of alternate shoveling is
fractional shoveling. Here, we move the entire lot to several
smaller piles and then select one at random to be the sample.
The idea of
fractional shoveling can be applied to liquids with fractional
pouring. If the container of liquid is small enough to
manipulate, we can shake it up by hand or mechanically to mix
it. We then pour small amounts successively into other
containers, repeating until all the original material has been
distributed. We select one container at random to be the sample.
If the
material consists of two liquids with very different densities,
just pouring some off the top will be disastrous for getting a
representative sample. This would also be a mistake for liquids
containing solid particles that are floating, or for liquids
with residue or sediment at the bottom. Fractional pouring can
help reduce the Grouping and Segregation Error.
Caution: Shaking is not always successful for mixing. We
must understand the type of material we have. For example, with
mixtures of gases and fine solid particles, the contrast between
the densities can make homogenization nearly impossible.** For
large lots, mixing may not even be realistic. In these cases, it
becomes even more important to reduce the effects of the other
sampling errors.
Moral: Mixing can often reduce the
Grouping and Segregation Error and increase the chances
of obtaining more representative samples.
Share your
experiences! If you have examples you can share, let me know and
I'll pass them along to the other readers. And, I welcome your
comments on this and other E-Newsletters.
More information about sampling
equipment and procedures for solids, liquids, and gases is
available in the short introductory book
A Primer for Sampling Solids, Liquids, and Gases,
by Dr. Patricia L. Smith.
** Gy, Pierre (1998). Sampling for
Analytical Purposes, New York: John Wiley and Sons, 153
pages.
Patricia L. Smith, Ph.D., Owner
Phone: 806.778.0224 NEW!!
Fax: 806.798.0224 NEW!!
E-mail:
alpha_alphastat.com
Web: www.alphastat.com
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