NURUL AUNI ZAINAL ABIDIN
FACULTY OF APPLIED SCIENCE
UITM NEGERI SEMBILAN
Sampling
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.
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The analysis may be classed as:
 Method
i) Meso
ii) Semimicro
iii) Micro
Sample Weight (mg) Sample Volume (L)
>100
>100
10 – 100
50 - 100
1 – 10
< 50
iv) Ultramicro
<1
 Classification of the constituents in a sample
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
i)
Major
>1%
ii)
Minor
0.1 – 1 %
iii) Trace
< 0.1 %
iv) Ultratrace
in the range of few parts per million
or less.
Sampling
 Deciding how to obtain a sample for analysis
depend on:
 i.
The size of the bulk to be sampled.
 ii.
The physical state of the fraction to be analyzed.

(solid, liquid, gas)
 iii.
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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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.
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Steps involved in sampling bulk material
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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Sampling Solids
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2008
 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.
 For reliable results, it is best to take 1/50 to 1/100 of
the total bulk. The larger the particle size, the larger the
gross sample should be.
 The gross sample must be reduced in size to obtain a
laboratory sample.
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Method of Sampling Solids
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2006

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Coning and Quartering
This process is continued until the gross sample is
small enough to be transported to the laboratory.
Cone and Quarter
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2006
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.
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Sampling Liquids
 Sampling techniques will depend on the types of
liquid.
i) Large volume of liquids (impossible to mix)
ii) Large stationary liquids (lakes, rivers)
iii) Biological fluids
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2006
Sampling Gases
 Tend to be homogeneous.
 Large volume of samples is required because of their low
density.
 Air analysis: Use a `Hi-Vol’ sampler that is containing filters
to collect particulates.
 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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Sample Storage and Preservation
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.
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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.
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Modern balances are electronic. They still compare one mass against another since
they are calibrated with a known mass. Common balances are sensitive to 0.1 mg.
Fig. 4.1. Electronic
analytical balance.
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Weighing bottles are used for drying samples. Hygroscopic samples are
weighed by difference, keeping the bottle capped except when removing
the sample.
Fig. 4.2. Weighing bottles.
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A weighing dish or boat is used for direct
weighing of samples.
Fig. 4.3. Weighing dish.
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Volumetric flasks are
calibrated to contain an
accurate volume. See the
inside back cover of the text
for tolerances of Class A
volumetric glassware.
Fig. 4.4. Volumetric flask.
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Volumetric pipets accurately deliver a fixed volume.
A small volume remains in the tip.
Fig. 4.5. Transfer of volumetric pipets.
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Measuring pipets are straight-bore pipets marked at different volumes.
They are less accurate than volumetric pipets.
Fig. 4.6. Measuring pipets.
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Syringe pipets precisely deliver microliter volumes.
They are commonly used to introduce samples into a gas chromatograph.
Fig. 4.7. Hamilton microliter syringe.
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These syringe pipets can reproducibly deliver a selected volume.
They come in fixed and variable volumes. The plastic tips are disposable.
Fig. 4.8. Single-channel and
multichannel digital
displacement pipets and
microwell plates.
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A 50-mL buret is marked in 0.1 mL increments.
You interpolate to 0.01 mL, good to about ±0.02 mL.
Two readings are taken for every volume measurement.
Fig. 4.9. Typical buret.
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Position the black field just below the meniscus.
Avoid parallax error by reading at eye level.
Fig. 4.10. Meniscus
illuminator.
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Place the flask on a white background.
Place the buret tip in the neck of the flask while your swirl.
Fig. 4.11. Proper technique
for titration.
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Use these for quantitative transfer of precipitates and solutions,
and for washing precipitates.
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Fig. 2.20. Wash bottles: (a) polyethylene, squeeze type;
(b) glass, blow type.
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.
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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.
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2007
iii) Introduction of analyte as a solvent
contamination.
iv) Contamination from the reaction of the solvent with
vessel walls.
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SAMPLE PREPARATION AND DISSOLUTION
 Sample dissolution is the digestion or mineralization of a sample
to render it soluble and to destroy organic matter that may interfere
with the recovery of the analyte.
 Sample dissolution procedures can be divided into 3:
i)
Dry ashing – Performed at a high temperature (400 –
700 oC) in a muffle furnace. Atmospheric O2 serves as the
oxidant, that is organic matter is burned off, leaving inorganic
residue.
ii) Oxidative ashing
iii) Wet digestion – A method for the decomposition of an
organic material, such as resins or fibers, into an ash by
treatment with nitric or sulfuric acids.
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DRY ASHING
 Simple dry ashing
- no chemical aids.
- Pb, Zn, Co, Cr, Mo, Sr, Fe traces can be recovered with little
loss by retention and volatilization.
- Usually a porcelain crucible can be used.
- Example: Lead is volatilized at T more than 500 oC, especially if
chlorine is present (blood and urine samples). Pt crucible are
preferred for lead for minimal retention losses. If an oxidizing
material (Mg(NO3)2) is added to sample, the ashing efficiency is
enhanced.
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DRY ASHING
 If the sample are liquids and wet tissues:
- The sample are dried on a stream bath or by gentle heat before
they are placed in a muffle furnace.
- The heat from the furnace should be applied gradually up to full
T to prevent rapid combustion and foaming.
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WET DIGESTION
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 Usually use combination of acids to achieve a complete dissolution.
 A small amount (5 mL) of H2SO4 is used with larger volumes of




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HNO3 (20 to 30 mL).
Usually performed in a Kjeldahl flask.
HNO3 destroys the bulk of organic matter, but it does not get hot
enough to destroy the last traces.
It is boiled off during the digestion process until only H2SO4
remains and dense, white SO3 fumes are evolved and begin to
reflux in the flask.
At this point, the solution gets very hot, H2SO4 acts on the
remaining organic material.
WET DIGESTION
 If the organic matter persists, more HNO3 may added.
 Digestion is continued until the solution clears.
 All digestion procedures must be performed in a fume hood.
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Eliminating Interferences
 Interferences are substances that prevent direct
measurement of the analyte and must be
removed.
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STANDARD SOLUTIONS
 Definition: Standard solutions are solution whose
concentrations are known to a high degree of accuracy.
 Characteristics:
i) Maintain its concentration over a long period of time
(months or years) after preparation. This eliminates the need
for restandardization.
ii) Must be able to undergo rapidly, stoichiometric, and
complete reaction with the analyte.
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 A standard solution can be prepared in either of two
ways:
1. A primary standard is carefully weighed, dissolved, and
diluted accurately to a known volume. Its concentration can be
calculated from this data.
2. A solution is made to an approximate concentration and then
standardized by titrating an accurately weighed quantity of a
primary standard.
 Types of standard solutions:
i)
ii)
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Primary standard
Seconday standard
PRIMARY STANDARD
 Definition: A compound of highest purity and it is used to determine,
directly or indirectly, the concentration of the standard solution for a
titration.
 Ideal primary standards for volumetric titration should have the following
characteristics:
i) Highest purity, up to 99.99% (0.01 to 0.02% impurity).
ii) Stability, the substance should stable at room conditions or during
heating and does not react with constituents of the atmosphere.
iii) Free from hydrated water and should be nonhygroscopic.
iv) Soluble in titration medium.
v) High formula weight to minimize weighing errors.
vi) Easily available at reasonable cost.
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PRIMARY STANDARD
 The number of primary standards available is very limited e. g.
oxalic acid (H2C2O4.2H2O), sodium carbonate (Na2CO3), calcium
carbonate (CaCO3), sodium chloride and arsenic trioxide.
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Why not use HCl or NaOH as the
primary standard?
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2008
 A primary standard should essentially available in pure form ,
stable towards light and heat and react in a stoichiometric
proportion.
 HCl is a gas which is dissolved in water to form the solution
the concentration expressed is very approximate so its not
a primary standard.
 NaOH cannot be weighed in open air because it is highly
hygroscopic.
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SECONDARY STANDARD
 A less pure substance whose composition is reliably known.
 The purity or the concentration of a secondary standard must
be established by careful stoichiometric analysis, usually
against a primary standard.
 Examples: HCl, HNO3, NaOH, KMnO4, and AgNO3.
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