How do I What’s
obtain
in a
mysample
sample?
Qualitative
Analysis
for testing?
Sampling
The sampling process
• Sampling is the process of selecting a portion of
material to represent or provide information about a
larger body of material.
• Consideration needs to be given to whether the sample
is part of something larger, how it should be obtained,
and how it should be stored and treated before
measurement.
Total amount of
shampoo produced
at the same time is
the batch.
Batch of shampoo
An amount of material
taken at random from
the batch, is a bulk
sample.
• The procedure by which the sample is obtained is
called sampling.
• The sample will contain both the analyte and the matrix.
Bulk sample
(bottles taken at random)
• The analyte is the chemical being identified (qualitative
analysis) or measured (quantitative analysis).
Lot 1
• The matrix is all other components of the sample,
other than the analyte.
Lot 2
Lot 3
The bulk sample is then
divided into lots.
Sample lots
(taken at random)
One of the lots if then sent
to the laboratory for testing this is the laboratory sample.
Sampling is important because…...
• It is not cost effective for a laboratory to test all of
the material in a batch (e.g. every bottle of shampoo).
Laboratory sample
• The test portion that is measured in the laboratory
must be representative of the batch.
Test sample
• Poor sampling can lead to inaccurate results and so
waste time and money.
Example: The analyst wants
to confirm that the colours
listed on the shampoo label
are present in the product.
Looking at this shampoo
label the colours are the
analyte (highlighted in
yellow boxes) and all other
ingredients are the matrix.
Subject to Crown licence, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical,
including photocopying, recording or any retrieval system, without the written permission of the copyright holder. © LGC Limited, 2005. All rights reserved.
Once the laboratory sample
has been divided into smaller
portions these are called the
test samples.
The test sample may be further
divided into test portions.
This is the portion that the
analyst tests.
Test portion
The content of each test portion
will be measured.
Measurement
1304/CB/1005
What’s in my sample?
Analysis
What’s Qualitative
in my sample?
Qualitative analysis
Which metals are present in these salts?
• Qualitative analysis is the process of
determining if any of a particular
substance or class of substances is
present in a sample.
Calcium salt
Platinum wire
• The elements and compounds in the
sample are identified.
• Some examples of types of qualitative
analysis are:
- Simple chemical tests (e.g. flame tests)
- Mass spectrometry
- Infrared spectroscopy.
• Qualitative analysis can tell you what
chemicals are present in a sample but
not how much is present.
Quantitative analysis
• Quantitative analysis usually provides
more information than qualitative
analysis of a sample.
• It determines the absolute or relative
amounts of elements and compounds
in a sample (e.g. their concentration).
• Some examples of types of quantitative
analysis are:
- Volumetric analysis:
• Acid - base titrations
• Redox titrations
• Complexometric titrations
- Colorimetry
- High performance liquid chromatography (HPLC)
- Gas chromatography (GC).
Flame colour
Cation
Intense yellow
Brick-red
Apple green
Blueish green
Sodium
Calcium
Barium
Copper
How much calcium is present in a sample
of water?
In volumetric analysis, the
concentration of a solution
(e.g. the amount of calcium in
a sample of water), is determined
by measuring the volume of a
standard solution (e.g. EDTA) that
will react with a known volume of
a solution (e.g. water sample).
The procedure of adding one
solution to another, in a measured
way, until the reaction is complete
is called a titration.
0
10
20
30
40
50
If the titration is performed
correctly the final results can be
very accurate.
Volumetric analysis is commonly
used to obtain accurate results in
industrial and research
laboratories.
Substances arrive at an analytical laboratory that need
to be identified (qualitative analysis) and on some
occasions the amount of a substance needs to be
determined (quantitative analysis).
Subject to Crown licence, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical,
including photocopying, recording or any retrieval system, without the written permission of the copyright holder. © LGC Limited, 2005. All rights reserved.
1304/CB/1005
How reliable
is myisexperiment?
How reliable
my experiment?
Precision,
Bias and
Accuracy
Precision,
Bias and
Accuracy
• Precision and bias are two terms which
are commonly used to describe the
performance of an analytical method.
• Precision is a measure of the spread
of results. “If I make repeated
measurements on the same sample,
by how much can I expect my results
to change?”
• Bias is a measure of the difference
between the average of repeated
• An archer is practising shooting some
arrows, the aim is to get all the arrows
as close together and as near the
centre of the target as possible.
• The results that
the archer wants to
achieve are precise
with insignificant
bias as shown
on this target.
• The results of some of the archer's
attempts are shown below on
targets A-D.
measurements and the true value.
“How close are my results to the
right answer?”
• Results that are precise and unbiased
are described as accurate.
Poor precision with bias
Poor precision with no bias
Subject to Crown licence, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical,
including photocopying, recording or any retrieval system, without the written permission of the copyright holder. © LGC Limited, 2005. All rights reserved.
Good precision with bias
Precise and unbiased
‘Accurate’
1304/CB/1005