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Chapter 0

CHEM 2AA3 Chapter Notes - Chapter 0: Macadamia, Graph Paper, Analyte


Department
Chemistry
Course Code
CHEM 2AA3
Professor
Philip Britz- Mc Kibbin
Chapter
0

Page:
of 3
!
80 The Analytical Process
The diagram below shows steps in transforming a
complex substance into individual samples that can
be analyzed. A lot is the total material (for example, a
railroad car full of grain or a carton of macadamia
chocolates) from which samples are taken. A bulk
sample (also called a gross sample) is taken from the
lot for analysis and archiving (storing for future refer-
ence). The bulk sample must be representative of the
lot or the analysis will be meaningless. From the repre-
sentative bulk sample, a smaller, homogeneous labora-
tory sample is formed that must have the same compo-
sition as the bulk sample. For example, you might
obtain a laboratory sample by grinding an entire solid
bulk sample to a fine powder, mixing thoroughly, and
keeping one bottle of powder for testing. Small test
portions (called aliquots) of the laboratory sample are
used for individual analyses.
the field into 20 000 small patches that are 10 centime-
ters on a side. A random sample is collected by taking
portions from the desired number of regions chosen at
random. Assign a number to each small patch and use
a computer to generate 100 numbers at random from
1 to 20 000. Then harvest and combine the grass from
each of the selected 100 patches to construct a repre-
sentative bulk sample for analysis.
In a segregated heterogeneous material, large
regions have obviously different compositions. To
obtain a representative specimen of such a material,
you need to construct a composite sample. For exam-
ple, the field in panel b has three different types of
grass in regions A, B, and C. You could draw a map of
the field on graph paper and measure the area in each
region. In this case, 66% of the area lies in region A,
14% lies in region B, and 20% lies in region C. To
construct a representative bulk sample from this segre-
gated material, take 66 small patches from region A, 14
from region B, and 20 from region C. You could do so
by drawing random numbers from 1 to 20 000 to select
patches until you have the desired number from each
region.
Box 0-1 Constructing a Representative Sample
Representative
bulk sample
Homogeneous
lab sample
Lot
Aliquot
Sampling
Sample
preparation
Aliquot Aliquot
Sampling is the process of selecting a representative bulk
sample from the lot. Sample preparation converts a bulk
sample into a homogeneous laboratory sample. Sample
preparation also refers to steps that eliminate interfering
species or that concentrate the analyte.
In a random heterogeneous material, differences
in composition are random and on a fine scale. When
you collect a portion of the material for analysis, you
obtain some of each of the different compositions. To
construct a representative sample from a heterogeneous
material, you can first visually divide the material into
many small regions. For example, if you want to
measure the magnesium content of the grass in the
10-meter 3 20-meter field in panel a, you could divide
10 cm !
10 cm
patches
chosen
at
random
10 meters
20 meters
Random heterogeneous material
(a)
20 meters
A 66%
C 20%
B 14%
Segregated heterogeneous material
10 meters
(b)
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