Intro Lab Methods
Chapter 6
Sampling
Scientific testing absolutely depends on a logical link between the laboratory results
for a sample and the bulk of the material from which it came. If there is no valid link,
there is no point in the test because it will tell you nothing about the bulk supply.
The best equipment, technical expertise and hard work cannot compensate for a
poor sample.
Sampling terminology
Sample:
a small portion of a large mass of material and must be
representative of that mass
Representative sample: must be identical in its chemical and physical
characteristics to the whole
Specimen:
a portion or single part of the whole
Random sampling:
the sampler should gather material in a widely distributed
pattern but the pattern should not bias the removal to only
one particular type of material
Bulk or gross sample:
the end result of the collection of material in a sampling
program
Sub-sampling:
the process used to reduce the size of a sample in a
representative manner so as to obtain a more convenient
quantity for laboratory work
Laboratory sample:
the portion of the bulk sample provided to the laboratory
for its testing purpose
Analytical sample:
that portion of the laboratory sample which is actually
tested.
The following techniques are procedures used to obtain a representative sample
1. Repeated coning to homogenise a bulk solid sample
2. Riffling used to sub-sample a gross sample
3. Coning and quartering used to sub-sample a gross sample
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Practical work 6.1
Validation of sampling
This practical task is designed to demonstrate whether the sampling procedure used
has produced a sample that truly reflects the composition of the bulk material. You
will create a bulk supply of material of known composition (10% salt in sand) and you
will sample it for laboratory testing. You will measure its true salt content and
compare your answers to the expected values.
Practical work 6.2
Sampling liquids
You will practice using dip tubes with different profiles to sample heterogeneous
liquids of known composition and check your technique is able to get the expected
answers.
You will need to decide which tube diameters suit which liquid samples and what
measuring cylinders give the best reproducibility.
You will then determine the percentage by volume of each liquid present in masked
samples.
Practical work 6.3
Sampling equipment
Record the sample identity which you have been allocated by the tide zone from
which it was obtained.
You will need to reduce (two or three) bulk samples of approximately 5kg (1.8L) to
approximately 100g, using the riffles provided.
Accurately record the mass of each of your laboratory samples.
Select a nest of sieves and clean them thoroughly as demonstrated by the teacher.
Record the aperture sizes and assemble them so that the aperture decreases from
biggest at the top to the smallest next to the catch pan.
Transfer a laboratory sample to the top of a nest of sieves.
Shake the sieves (with the lid on) for 5 minutes
Using the A3 paper method demonstrated by the teacher, carefully capture and
record the mass of each fraction.
Repeat the procedure with the other samples.
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Student Name:
Practical:
Validation of sampling
Practical Number: 6.1
Date Performed:
Text book References
Date Submitted:
Procedure:
You will be put into teams to perform these operations. The intention is to have each
team member carry out every step at least once. Because of the large amount of
handling needed, team work will be necessary and different team members will
rotate through different tasks each time the cycle is performed.
1. Mix sand (250g) and sodium chloride (25g) together by coning. Each team
member needs to satisfy the teacher that proper technique is being used. Other
team members will start working on step 5 and onwards.
This will be your team's bulk supply.
2. Use coning and quartering to obtain an approximately 25g sample. Recombine all
leftovers into one cone. The next member of the team will cone and quarter the
remainder to get the next 25g sample and so on.
Each 25g will be the laboratory sample.
3. Weigh accurately about 5 g of this sample. Each team member to test their own
lab sample.
This will be the analytical sample.
4. Other team members are to repeat steps 2 – 3 to obtain two more laboratory and
analytical samples.
5. Label and weigh 3 filter papers
6. Label and weigh 3 evaporating basins and get them to constant weigh in the
110°C oven.
Treat each of your 5g analytical samples as follows:
7. Place each sample in a beaker and add approximately 25mL of distilled water to
dissolve the salt.
8. Filter each sample and collect the filtrate in the evaporating basin.
9. Wash the remaining sand in each beaker into its filter paper with a further 25mL
of water.
10. Dry each filter paper in a 110°C oven and reweigh.
11. Evaporate each water sample on a steam bath. Oven dry the salt residue in the
basin in a 110°C oven and measure the mass of salt.
12. Clean out each basin with dry paper and check its empty mass. It should be the
same as in step 6
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Results
Bulk
Supply
Mass of sand
Mass of salt
Sand + salt
total
% sand
% salt
Sampling details
Samples
Mass of laboratory
sample (approx 25g)
Mass of analytical
sample used in test
(5g weighed to 4 dec)
1
2
----------------------------
3
----------------------------
Analysis details for analytical samples
Mass of empty
filter paper
Mass of
paper + sand
Mass of empty
evaporating
basin
Mass of basin +
salt residue
Mass check on
cleaned out empty
basin (if necessary)
1
2
3
Analysis calculations
Mass of sand
recovered (a)
Mass of salt
recovered (b)
Recovered
sand + salt
total (a) + (b)
Original mass of
analytical sample
used in test
% sand
% salt
1
2
3
Questions:
1. Discuss any differences between the composition of the original bulk supply and
the test results for your recovered samples.
2. Comment on your recovery check. (the agreement between the mass of each of
your analytical samples and the recovered sand + salt masses after the analysis.
This tells how reliable your results might be.
3. Suggest how you could improve the method to achieve better % composition and
recovery check agreement.
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Student Name:
Practical:
Sampling of liquids
Practical Number: 6.2
Date Performed:
Text book References
Procedure:
Date Submitted:
Results:
You are required to sample two liquid reservoirs, each of which has water as one of
its two components. The other liquid is organic based and one is less dense than
water and the other is more dense.
You will practice on unmasked reservoirs and hence you will have access to the
correct answers for "percentage of each layer".
You will be assessed on masked reservoirs for which you will not know the answers.
This is a team based task and you need to consult the table on the next page and
with your partners plan your actions for getting the best possible answer for the
masked reservoirs.
The problems you need to solve are:
1. Which dip tubes give the best answers?
2. What sample size is best - how many dip tube aliquots should you recover?
3. How do you intend to measure the volume of each layer of your dip tube
samples?
4. How should you clean the dip tubes between samples?
5. How do you intend to measure the volume of each layer of your bulk reservoir?
6. What safety problems need to be solved?
7. What waste management problems need to be solved?
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Result table for liquid sampling
Practice session on the two different unmasked bulk supplies before sampling
Sample code
Sample A
Sample B
bottom layer volume
top layer volume
Total volume
% top layer
% bottom layer
Unmasked supply sample data
Sample code
Sample A
Sample B
Dip tube identifier
Total volume
bottom layer volume
top layer volume
% top layer
% bottom layer
Best dip tube ()
Masked supply sample data. Use the best dip tube suggested by your trials on the unmasked
supplies. Do several measurements and calculate the average.
Sample 1
Sample
code
Dip tube
identifier
Run 1
Run 2
Run 3
avg
Sample 2
spare
spare
Run 1
Run 2
Run 3
avg
spare
Total
volume
bottom
layer
volume
top layer
volume
% top layer
% bottom
layer
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spare
Intro Lab Methods
Student Name:
Practical:
Sampling Equipment
Practical Number: 6.3
Date Performed:
Date Submitted:
Text book References
Procedure:
Results:
location on the beach from which the
Sample number
Sample mass
sample was taken:
1
2
3
Sieve
size
Mass
sand
% sand in
fraction
Mass
sand
% sand in
fraction
Mass
sand
% sand in
fraction
Total
mass
Questions:
1. Did you recover 100% of the initial sample? If not where did you gain or lose sample in
the method?
2. Did you have good agreement between your duplicate samples? You should use the
laboratory computer to graph your three sets of results to see how well they compare.
Submit hardcopy with your log book.
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