Taibah
University
Faculty of
Science
Chemistry
Department
SELECTED TOPICS
IN ANALYTICAL
CHEMISTRY
(CHEM 437)
1
CHEM 437
Instructor: Bader Altayeb
E-mail: BTAYEB@taibahu.edu.sa
Required textbook: “Sample
Preparation Techniques in
Analytical Chemistry”
2
Fundamentals of Sample Preparation
Schedule of Assessment Tasks for Students During the Semester
Assessment task Week Due Proportion of
Total
Assessment
1 Participation Through the term 5
2 Activity/homework Through the term 5
3 Written Exam (1) To be notified
20
4 Written Exam (2) To be notified
20
5 Oral presentation/or research
paper
To be notified 10
6 Final Exam (theoretical) To be notified
40
Total 100
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1. Introduction
4
Content:
1.1.What is meant by sample preparation?
1.2.Purpose Of Sample Preparation
1.3.Overview On Sample Preparation
1.4.Types of Sample
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Fundamentals of Sample Preparation
1.1.What is meant by sample
preparation?
v In analytical chemistry, sample preparation refers to the ways in
which a sample is treated prior to its analyses.
q It is the processes in which a representative piece of material is extracted
from a larger amount and readied for analysis.
q Sampling and sample preparation have a unique meaning and special
importance when applied to the field of analytical chemistry
q Most analysis is still done by taking a part (or portion) of the object under
study (referred to as the sample) and analyzing it in the laboratory (or at
the site).
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Fundamentals of Sample Preparation
• The purpose of an analytical study is to
obtain information about some object
or substance. The substance could be a
solid, a liquid, a gas, or a biological
material.
• The information to be obtained can be
varied. It could be the chemical or
physical composition, structural or
surface properties, or a sequence of
proteins in genetic material.
• It is not possible to find every bit of
information of even a very small number
of samples. For the most part, the state
of current instrumentation has not
evolved to the point where we can take
an instrument to an object and get all
the necessary information.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Some common steps involved in the process are shown in Figure
1.1.
The first step is sampling, where the sample is obtained from the
object to be analyzed. This is collected such that it represents the
original object.
Sampling is done with variability within the object in mind. For
example, while collecting samples for determination of Ca+2 in a
lake, it should be kept in mind that its concentrations can vary
depending on the location, the depth, and the time of year.
The next step is sample preservation. This is an important step,
because there is usually a delay between sample collection and
analysis. Sample preservation ensures that the sample retains its
physical and chemical characteristics so that the analysis truly
represents the object under study.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
sample preparation is the next step, Most samples are not
ready for direct introduction into instruments.
For example, in the analysis of pesticides in fish liver, it is
not possible to analyze the liver directly. The pesticides
have to be extracted into a solution, which can be analyzed
by an instrument. There might be several processes within
sample preparation itself.
Some steps commonly encountered are shown in Figure
1.2. However, they depend on the sample, the matrix, and
the con-centration level at which the analysis needs to be
carried out.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Once the sample preparation is complete, the analysis is carried
out by an instrument of choice. A variety of instruments are used
for different types of analysis, depending on the information to be
acquired: For example, chromatography for organic analysis,
atomic spectroscopy for metal analysis, capillary electrophoresis
for DNA sequencing, and electron microscopy for small
structures.
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SAMPLE PREPARATION: AN ANALYTICAL
PERSPECTIVE
The sample preparation depends on the analytical
techniques to be employed and their capabilities. For
instance, only a few microliters can be injected into GC.
Sampling, sample preservation, and sample preparation
are all aimed at producing those few microliters that
represent what is in the fish. It is obvious that an error
in the first three steps cannot be rectified by even the
most sophisticated analytical instrument. So the
importance of the prior steps, in particular the sample
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Common analytical instrumentation and the sample preparation associated with
them are listed in Table 1.1.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Methods of Quantitation
Almost all measurement processes, including sample preparation
and analysis, require calibration against chemical standards.
The relationship between a detector signal and the amount of
analyte is obtained by recording the response from known
quantities. Similarly, if an extraction step is involved, it is
important to add a known amount of analyte to the matrix and
measure its recovery.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Such processes require standards, which may be
prepared in the laboratory or obtained from a
commercial source. An important consideration in
the choice of standards is the matrix.
For some analytical instruments, such as x-ray
fluorescence, the matrix is very important, but it
may not be as critical for others.
Sample preparation is usually matrix dependent. It
may be easy to extract a polycyclic aromatic
hydrocarbon from sand by supercritical extraction
but not so from an aged soil with a high organic
content
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Calibration Curves
The most common calibration method is to prepare standards of
known
concentrations, covering the concentration range expected in the
sample.
The matrix of the standard should be as close to the samples as
possible.
For instance, if the sample is to be extracted into a certain organic
solvent, the standards should be prepared in the same solvent.
The calibration curve is a plot of detector response as a function of
concentration.
It is used to determine the amount of analyte in the unknown
samples. The calibration can be done in two ways, best illustrated by
an example. Let us say that the amount of lead in soil is being
measured. The analytical method includes sample preparation by
acid extraction followed by analysis using atomic absorption (AA).
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SAMPLE PREPARATION: AN ANALYTICAL
PERSPECTIVE
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Preservation Of Samples
The sample must be representative of the object under investigation. Physical,
chemical, and biological processes may be involved in changing the composition
of a sample after it is collected.
v Physical processes that may degrade a sample are volatilization, diffusion,
and adsorption on surfaces.
v chemical changes include photochemical reactions, oxidation, and
precipitation.
v Biological processes include biodegradation and enzymatic reactions.
- sample degradation becomes more of an issue at low analyte concentrations
and in trace analysis.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
• The sample collected is exposed to conditions different from the original
source.
For example, analytes in a groundwater sample that have never been
exposed to light can undergo significant photochemical reactions when
exposed to sunlight.
• It is not possible to preserve the integrity of any sample indefinitely.
Techniques should aim at preserving the sample at least until the analysis
is completed.
• A practical approach is to run tests to see how long a sample can be held
without degradation and then to complete the analysis within that time.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Ø Common steps in sample preservation are the use of:
proper containers, temperature control, addition of preservatives, and the
observance of recommended sample holding time.
Ø The holding time depends on the analyte of interest and the sample
matrix. stable for months, whereas Cr(VI) is stable for only 24 hours.
Ø Holding time can be determined experimentally by making up a spiked
sample (or storing an actual sample) and analyzing it at fixed intervals to
determine when it begins to degrade.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
The changes that take place in a sample are either chemical
or biological.
ØIn the chemical changes , certain changes occur in the
chemical structure of the constituents that are a function
of physical conditions.
• Metal cations may precipitate as hydroxides or form
complexes with other constituents; cations or anions may
change valence states under certain reducing or oxidizing
conditions; other constituents may dissolve or volatilize
with the passage of time.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
• Metal cations may also adsorb onto surfaces (glass, plastic, quartz, etc.),
such as, iron and lead.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
1.4.4. Chemical Changes
q A wide range of chemical changes are possible.
q For inorganic samples, controlling the pH can be useful in preventing chemical
reactions. For example, metal ions may oxidize to form insoluble oxides or hydroxides.
q The sample is often acidified with HNO3 to a pH below 2, as most nitrates are soluble,
and excess nitrate prevents precipitation.
q Other ions, such as sulfides and cyanides, are also preserved by pH control. Samples
collected for NH3 analysis are acidified with sulfuric acid to stabilize the NH3 as NH4SO4.
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• Biological changes taking place in a sample may change the valence of an element
or a radical to a different valence. Soluble constituents may be converted to
organically bound materials in cell structures, or cell lysis may result in release of
cellular material into solution. as a general rule, it is best to analyze the samples
as soon as possible after collection.
• Samples may also contain microorganisms, which may degrade the sample
biologically. Extreme pH (high or low) and low temperature can minimize microbial
degradation. Adding biocides such as mercuric chloride or pentachlorophenol can
also kill the microbes.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Absorption of Gases from the Atmosphere
-Gases from the atmosphere can be absorbed by the sample during handling, for
example, when liquids are being poured into containers.
Ø-Gases such as O2, CO2, and volatile organics may dissolve in the samples.
v Oxygen may oxidize species, such as sulfite or sulfide to sulfate.
v Absorption of CO2 may change conductance or pH. This is why pH measurements
are always made at the site.
v CO2 can also bring about precipitation of some metals.
v Dissolution of organics may lead to false positives for compounds that were
actually absent.
-Blanks are used to check for contamination during sampling, transport, and
laboratory handling.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
q ORGANIC SPECIES CAN ALSO UNDERGO CHANGES DUE TO CHEMICAL REACTIONS:
• Organics can also react with dissolved gases; for example, organics can react with trace
chlorine to form halogenated compounds in treated drinking water samples. In this
case, the addition of sodium thiosulfate can remove the chlorine.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Preservation of samples
v Preservation methods are limited to pH control, chemical addition, amber or opaque
bottles, filtration, refrigeration, and freezing.
v To minimize the potential for volatilization or biodegradation between sampling and
analysis, keep the sample as cool as possible without freezing.
v Complete and unequivocal preservation of samples, either domestic sewage,
industrial wastes, or natural waters, is a practical impossibility.
v Regardless of the nature of the sample, complete stability for every constituent can
never be achieved.
v At best, preservation techniques can only retard the chemical and biological
changes that inevitably continue after the sample is removed from the parent
source.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Preservation of samples
o Methods of preservation are relatively limited and are intended generally to
(1) retard biological action,
(2) retard hydrolysis of chemical compounds and complexes,
(3) reduce volatility of constituents,
(4) reduce absorption effects.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Preservation of samples
vSample preservation should be performed immediately upon sample collection
vFor composite samples each aliquot should be preserved at the time of collection.
vWhen use of an automated sampler makes it impossible to preserve each aliquot,
then samples may be preserved by maintaining at 4 deg. C until compositing and
sample splitting is completed.
vChoice of Proper Containers
vContainer may :Plastic (P) or Glass (G).
vFor metals, polyethylene with a polypropylene cap (no liner) is preferred.
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Choice of Proper Containers
q The surface of the sample container may interact with the analyte.
§ The surfaces can provide catalysts (e.g., metals) for reactions or just sites for
irreversible adsorption.
§ For example, metals can adsorb irreversibly on glass surfaces, so plastic containers are
chosen for holding water samples to be analyzed for their metal content. These
samples are also acidified with HNO3 to help keep the metal ions in solution.
• Organic molecules may also interact with polymeric container materials (plastic
container). Plasticizers such as phthalate esters can diffuse from the plastic into the
sample, and the plastic can serve as a sorbent (or a membrane) for the organic
molecules
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
• Consequently, glass containers are suitable for organic analytes.
• Storing the sample in amber bottles prevents photooxidation of organics (e.g.,
polynuclear aromatic hydrocarbons).
• Bottle caps should have Teflon liners to preclude contamination from the plastic
caps.
• Oily materials may adsorb strongly on plastic surfaces, and such samples are usually
collected in glass bottles.
• A sonic probe can be used to emulsify oily samples to form a uniform suspension
before removal for analysis.
• Oil that remains on the bottle walls should be removed by rinsing with a solvent and
be returned to the sample
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SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE
Preservation of samples
v Maximum holding time is 24 hours when sulfide is present.
v Optionally, all samples may be tested with lead acetate paper before the pH
adjustment in order to determine if sulfide is present.
vIf sulfide is present, it can be removed by the addition of cadmium nitrate powder
until a negative spot test is obtained.
vThe sample is filtered and then NaOH is added to pH 12.
vSamples should be filtered immediately on-site before adding preservative for
dissolved metals.
v For samples from non-chlorinated drinking water supplies conc. H2SO4 should
be added to lower sample pH to less than 2. The sample should be analyzed before
14 days.
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Table 1.3 lists some typical preservation methods.
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