Sampling University of Zambia Advanced Chemistry.ppt

Sampling University of Zambia Advanced Chemistry

1. CHE1010
Introductory Chemistry for
medical and health sciences
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2. Sampling of Different Sample Types
LECTURE 1
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4. Sampling
Sampling is the process of obtaining a representative sample.
(We can not analyze the whole thing!)
A sample is the representative of the whole bulk system. It’s
composition should closely reflect the composition of the bulk.
Sampling is done for monitoring purposes, as well as for
research.
Critical step in analysis as the significance and accuracy
depends on sampling.
Sample can be solid, liquid, gas and heterogeneous or
homogenous.
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5. Sampling
Homogenous Sample: A grab sample is often OK. For
instance, in clinical lab, gross sample (blood, urine) can be
analyzed directly as it is homogenous.
Heterogeneous Sample: Several individual samples are
taken. E.g., analyzing average protein content of shipment of
grains, one has to collect little grain from each bag during
loading/unloading using a sampling spear (sack sampler) and
combine to obtain a gross sample.
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6. Sampling
Gross Sample consists of
several portion of the material
to be tested.
Laboratory Sample consists
of a small portion of gross
sample made homogenous.
Analysis Sample is that
which is actually analyzed.
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7. Sampling
Steps involved in a
measurement process
Steps involved in
Sampling process
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8. Steps involved in sampling bulk material
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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.
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9. Sampling of solids
Solid samples are often the most difficult to sample
(Inhomogeneous, variation of particle size, variation within
particles).
Solids are usually heterogeneous and samples must be
collected carefully.
Solids come in variety of forms, each of which is sampled
differently.
Examples
oLarge particulates such as ores
oSmaller particulates
oSheet material
oTissue samples
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10. Grab Sample
A grab sample is a sample
taken at random and
assumed to representative.
1/50th of the total bulk
Easiest but less reliable
Satisfactory only
for homogenous
samples
Best and easy
time for sampling:
during transportation
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11. Collection of sediment
Bottom grab sampler is used.
It has jaws that close when they are in
contact with sediments, scooping up
sediments in the process.
Advantage
oEase of use and ability to collect large
samples.
Disadvantages
oinclude tendency to lose fine grained
sediments as water flows out of
samples.
oLoss of spatial information both laterally
and with depth due to mixing of
samples.
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12. Collection of surface soil
Soil samples collected at depths of up to 30 cm are easily
collected with scoops or shovels.
Even small containers of solid materials may be sampled using
spatula or scoops.
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13. Collection of soil at greater depth
Soil samples collected at greater depth are obtained by digging
a trench and collecting samples.
Drill a hole till the required depth. Remove soil from the drill
and analyze.
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14. Sampling from bags
Simple bag sampling
spears (sack samplers)
are most commonly used
for taking samples from
bags because they are
relatively cheap and sample quickly.
To obtain a good cross section of sample, the spear
should be 40 to 45 cm in length.
Tapered type of sampling spears penetrates bags
easily.
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15. Sampling of powder
Small particulate materials
such as powder are best
collected with a sample
thief.
Sample thief allow
material to be collected
simultaneously from
several locations.
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16. Powder Thief
The Powder Thief is suitable for sampling free flowing powders
and granules.
Operation:
1.Insert the sampler into the product, ensure that the tip is
inside the sampler body.
2.At the required depth pull up the body to expose the tip.
Powder will flow in around the tip.
3.Push down body of the sampler to trap the sample.
4.Withdraw sampler.
5.Pull up body to release the sample.
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17. Sampling of metals and alloys
Samples of these materials are obtained by sawing , milling or
drilling.
When sampling a metal, it is usually necessary to obtain
materials from both the surface and the interior.
Wire……cutting off pieces of appropriate length.
Drilling can also be done.
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18. Sampling of biological tissues
It is done by removing the entire organ which is then
homogenized before smaller portions are taken for the
analysis.
Alternately small portions may be combined to form a
composite sample.
The composite sample is then homogenized and analyzed.
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19. REDUCTION OF SAMPLE SIZE
Reducing particle size
Sub-sampling of gross sample
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20. Coning and quartering
Gross sample is piled as
cone
Flattened
Divided into quarters
Separation of quarters
Discarding opposite
quarters
Process is repeated
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21. Riffling
Riffle tank is used
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22. Sampling of Liquids
Examples
Those drawn from containers of commercial solvents
Beverages
Natural waters
Biological fluids
Suspensions in the form of oral medication
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23. Sampling of liquids
Tend to be homogeneous and much easier to sample.
A Grab sample is often sufficient.
If natural diffusion of water is slow (pond), shake/stir it before
sampling
Large bodies of water are bests sampled after a transfer, or if
in a pipe, after passing through a pump.
Large stationary liquids can be sampled using thief sampler.
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24. Biological Fluids
Timing for biological samples is important.
E.g., blood composition changes before and
after meal. Sampling after 12h fasting is often
recommended. 24h urine collection is better
than a single specimen.
In case of blood, a grab sample is OK. Syringes are used to
collect blood samples.
Glass or plastic with stainless steel needle are available.
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25. Biological Fluids
Blood can be analyzed as whole blood or separated into serum
and palsma
Serum: Fluid separated from clotted blood. Serum does not
clot
Plasma: Fluid separated from unclotted blood. It contains
fibrinogens
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26. Storage of Biological Fluids
Preservatives (such as NaF for glucose) or anticoagulant
(Heparin) are often added to blood samples.
Vacutainers (evacuated tubes with rubber cap) are Used to
collect blood samples.
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27. Thief sampler
It is used for the
sampling of large
stationary liquids.
The separate portions of
liquids can be analyzed
individually and results
combined.
Portions can be
combined into one gross
sample and replicate
analysis is performed.
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28. Pipets
Used for sampling of
homogeneous samples.
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29. Grab sampler
Liquid samples are often
obtained with a device
known as grab sampler.
It consists of metal
container into which a
small bottle is placed.
The top of bottle is
closed with a stopper
connected to a link held
by the person doing the
sampling.
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30. Grab sampler
It is lowered to the desired depth in the liquid ,the wire attached
to it is jerked to open the sample bottle .
It is filled.
It is automatically sealed.
Cable connected to it is used to lift the sampler.
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32. Sampling of gases
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Assessment of air quality.
Workers during their work at many workplaces are exposed to
gases and vapours (inorganic gases and organic (solvent)-
vapours).
These exposures need be categorized and measured
analytically,
Sometimes, a grab sample is OK
Usual method is displacement of liquids (not reacting with or
solubilizing the samples)

33. Sampling of gases
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34. Breath sample: The person blows into an evacuated bag.
Auto-exhaust is collected in a large evacuated plastic bag.
Breath sample: The person blows into an evacuated bag.
Auto-exhaust is collected in a large evacuated plastic bag.
Sampling with enrichment: Substance (gas) is adsorbed onto a
solid collection phase or absorbed into a solution.
In active sampling procedures, workplace air is drawn onto a
collection medium using especially designed sampling pumps.
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35. T and P is important in determining volume and hence
concentration.
For direct sampling, gas storage vessels or gas sample bags
(TedlerTMbags) are used.
Bag is connected to pump and air is drawn at 1 to 3 L/min
V= r x t V= volume, r= flow rate (L/min), t= timing of sampling
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36. Measurement Principles
To estimate the exposure of hazardous substances, in principal
the workplace air is drawn through an adequate collection
medium using a pump. The complete measurement
procedure comprises the following steps:
1.Sampling
2.Transport and storage
3.Analytical determination
4.Assessment
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37. Measurement Principles
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38. Sampling
Workplace air can also be analyzed directly in its original state
or enriched first.
The gases and vapours that occur in workplace air can be
separated and enriched in the collection medium
Substance (gas) is adsorbed onto a solid collection phase or
absorbed into a solution. During enrichment by adsorption or
absorption the analyte is collected in its original state.
Chemical Reactions: Respective reactions of the analysed
substances can be used also for derivatization and enrichment.
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39. Sampling with enrichment
For sampling with enrichment, the substances to be
determined are either adsorbed onto a solid collection phase,
or absorbed in a solution. The samples afterwards need to be
prepared so that they can be subsequently analysed.
Sampling pumps are often used
V= r x t V= volume (L), r= flow rate (L/min), t = timing (min) of sampling
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40. Adsorption Tubes
For sampling with enrichment using adsorption tubes, the
substances to be determined are adsorbed onto a solid
collection phase (adsorbent) placed inside a tube The
adsorption tubes are heated in a compatible thermal desorber,
whereupon the collected substances are transferred into a cold
trap using a carrier gas. When desorption is complete, the cold
trap is heated abruptly and thereby transferring the sample
onto the GC column for subsequent separation.
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41. Reaction samplers
A sampling that utilizes chemical reactions
The sampling occurs using either a bottle, in which the reagent
is dissolved in an appropriate liquid, or with sample media
(filters or adsorbents) that have been pretreated with a reagent.
The reaction between the investigated substance and the
reagent takes place in situ during sampling
Examples include the collection
i.of aldehydes using enrichment systems impregnated with
dinitrophenylhydrazine,
ii.of diisocyanatescollected on filters impregnated with 2-
methoxyphenyl-piperazine
iii.hydrogen peroxide in solutions containing titanium.
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42. Sampling without enrichment
The method can either deliver the result instantly on location
(e. g., via direct reading), or by analysis in the laboratory after
sample collection in a suitable vessel.
For air sampling without enrichment, gas storage vessels (e. g.,
gas mouse) or gas sample bags (e. g. Tedlaräbags) can be
used.
This type of sampling is also used for the determination of
organic gases that occur permanently in the air of workplaces
(e. g. propane, propene)
For the analysis of gases and vapours
collected without enrichment, a gas-tight
syringe is normally used for sampling from
the collection device
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43. Sampling of gases
Different types of sampling devices are used for the collection
of gaseous samples.
GAS SAMPLING BAG
STAINLESS STEEL CONTAINER
FILTERATION UNIT
GAS ADSORPTION TUBES
IMPINGERS
CENTRIFUGAL COLLECTOR
ELECTROSTATIC SAMPLERS
THERMAL PRECIPITATOR
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44. Impingers
Impingers are special glass tubes designed to
collect airborne contaminants by bubbling the
sampled air at a high flow rate through a
method specific asbsorbing liquid inside. The
liquid used can then be analyzed to determine
airborne contaminate levels.
Air drawn into the impinger is forced through a
nozzle, which is covered by a liquid such as
high purity water. The pollutant dissolves in
the liquid and is subsequently analysed,
usually by colorimetric techniques.
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45. Centrifugal collectors
The particles are removed by the
application of a centrifugal force.
The polluted gas stream is forced
into a vortex. The motion of the
gas exerts a centrifugal forces on
the particles, and they get
deposited on the inner surface of
the cyclone.
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