2. SAMPLING AND EVALUATION OF
EXPERIMENTAL DATA
Population - The group of things, items or
units under investigation
Sample - Obtained by collecting
information only about some members
of a "population“
Sampling – Act of collecting sample to
produce meaningful information.
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3. SAMPLING
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Sampling is the process to get a representative and
homogeneous sample.
Representative means that content of analytical
sample reflects content of bulk sample.
Homogeneous means that the analytical sample
has the same content throughout.
4. SAMPLING
Deciding how to obtain a sample for analysis depend
on:
A. The size of the bulk to be sampled.
B. The physical state of the fraction to be analyzed (solid,
liquid, gas)
C. The chemistry of the material to be assayed.
(Nothing can be done that would destroy or alter the
identity or quantity of the analyte)
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5. In a chemical analysis :
A chemical analysis is usually
performed on only small portion of
the material collected to be
characterized.
If the amount of material is very small
and it is not needed for further use,
then the entire samples may be
used for analysis.
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9. 9
Obtaining a representative sample is the first step of an analysis.
The gross sample is several small portions of the sample.
This is reduced to provide a laboratory sample.
An aliquot of this sample is taken for the analysis sample.
10. 10
Identify the population from
which the sample is to be
obtained.
Collect a gross sample that
is truly representative of the
population being sampled.
Reduce the gross sample to
a laboratory sample that is
suitable for analysis.
Steps involved in sampling bulk material
11. SAMPLING GASES
Tend to be homogeneous.
Large volume of samples is required because of their
low density.
Liquid displacement method: The sample must has
little solubility in the liquid and does not react with
the liquid
Breath sample: The subject could blow into evacuated
bag.
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12. SAMPLING SOLID
Inhomogeneity of the material, make sampling of solids
more difficult.
The easiest way to sample a material is grab sample –
the sample taken at random and assumed to be
representative.
The gross sample must be reduced in size to obtain a
laboratory sample.
Solid samples may need drying
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13. CONING AND QUARTERING
This process is continued until the gross sample is
small enough to be transported to the laboratory.
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Sampling Solids
15. 3. There is no specific technique that can be used for
taking the samples. Using an example, explain how
to sample either a solid, liquid, or gas sample. (5
marks)
i) Sampling solid
Using the method cone and quarter.
Divide a pile of material into quarter.
Take a sample from each quarter of the pile and
crush these sample and form into a smaller conical
pile.
Flatten the conical pile and cut into equal quarters.
Two opposite quarters are chosen at random.
Crush the quarter further.
The whole steps are repeated until a laboratory
samples obtain.
16. SAMPLING LIQUIDS
Liquid samples are homogeneous and are much easier
to sample.
The gross sample can be relatively small.
If liquid samples are not homogeneous, and have only
small quantity, they can be shaken and sampled
immediately.
Sampling depends on the types of liquids:
i) large volume of liquids (impossible to mix)
ii) large stationary liquids (lakes, rivers)
iii) biological fluids
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17. F R E E P O W E R P O I N T T E M P L A T E :
W W W . B R A I N Y B E T T Y . C O M 17
• If water sample is taken from the river,
then the water samples is collected at
the SURFACE, MIDDLE and at the
BOTTOM of the river bed.
18. SAMPLE STORAGE AND
PRESERVATION
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Samples storage purpose:
There is a time gap between when the sample
is taken and the actual analysis is being carried
out.
For liquids samples, make sure that it is kept in
bottles with stoppers.
Acidic liquid samples can be stored in glass container
whereas basic liquid samples in plastic container.
Solid samples is easier to keep and have less chance to be
adulterated by foreign matters.
Sometimes it can also get absorbed or adsorbed to the wall
of the container.
19. What are the problems encounter during storage of
samples?
The sample can be contaminated by foreign matter .
There is a lost of analyte during storage.
Decomposition of sample.
The sample should not react with the wall of the
container or get adulterated.
During storage of samples, for example liquid samples,
sometimes there is a lost of analyte if it is volatile .[so
the container should be closed tightly]
F R E E P O W E R P O I N T T E M P L A T E :
W W W . B R A I N Y B E T T Y . C O M 19
20. SAMPLE STORAGE AND
PRESERVATION
Preparing a laboratory sample
Converting the sample to a useful form:
Solids are usually ground to a suitable particulate size to get a
homogeneous sample.
Dry the samples to get rid of absorption water. 20
An important aspect of the sampling process
Samples are preserved to prevent from:
Decomposition
Precipitation of metals from water samples
Loss of water from hygroscopic material
Loss of volatile analytes from water samples
21. DEFINING REPLICATE SAMPLES
Replicate samples are always performed unless the
quantity of the analyte, expense or other factors
prohibit.
Replicate samples are portion of a material of
approximately the same size that is carried through
an analytical procedure at the same time and the
same way.
Obtaining replicate data on samples improves the
quality of the results and provides a measure of their
reliability.
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22. PREPARING SOLUTIONS OF THE
SAMPLE
A solvent is chosen that dissolves the whole sample
without decomposing the analyte.
Sources of error :
i) Incomplete dissolution of the analyte.
ii) Losses of analyte by the volatilization.
iii) Introduction of analyte as a solvent
contamination.
iv) Contamination from the reaction of the solvent
with vessel walls.
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23. THANK YOU!
F R E E P O W E R P O I N T T E M P L A T E :
W W W . B R A I N Y B E T T Y . C O M 27
