Instrumental Techniques for
Environmental Analysis
Chapter 1 : Introduction
Rezaul Karim
Environmental Science and Technology
Jessore Science and Technology University
1
Chapter outline
Analytical chemistry; its scope
and application;
instrumental analysis;
instruments for analysis,
representative sample; sample
storage, its pre-treatment and
preparation,
sample pre-treatment,
calibration of instrumental
methods;
selecting an analytical methods
2
Reference
Skoog, Holler & Crouch 2007,
Instrumental Analysis, Brooks Cole
Cengage Learning, USA.
Gray, Cakvin & Bhatia, 2009,
Instrumental Methods of Analysis, 1st
edition, CBS, New Deli, India
3
Analytical chemistry
Analytical Chemistry deals with methods
for determining the chemical composition
of samples of matter.
A qualitative method yields information
about the identity of atomic or molecular
species or the functional groups in the
sample.
A quantitative method provides numerical
information as to the relative amount of
one or more of these components.
4
Classification of analytical
methods
Classical
methods
◦ sometimes
called wet-
chemical
method
◦ preceded by a
century or more
◦ Gravimetric or
by volumetric
measurements
Instrumental
methods
◦ physical properties
as conductivity,
electrode potential,
light absorption or
emission, mass-to-
charge ratio
◦ paralleled the
development of the
electronics and
computer
industries.
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Types of instrumental methods
Characteristic
Properties
INSTRUMENTAL
METHODS
Radiation
e.g. Emission,
adsorption,
scattering, refracting,
rotating,
Diffraction, refraction
Spectrophotometer and
photometry
Electrical charge coulometry
Electrical current Amperomtry; palaeography
Mass Gravimetric
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Instruments for analysis
An instrument for chemical analysis converts
information about the physical or chemical
characteristics of the analyte to information
that can be manipulated and interpreted by a
human.
An analytical instrument can be viewed as a
communication device between the system
under study and the investigator.
To retrieve the desired information from the
analyte, it is necessary to provide a stimulus,
which is usually in the form of
electromagnetic, electrical, mechanical, or
nuclear energy.
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responseStimulus
System
under study
Data domain
The measurement process is aided by a wide
variety of devices that convert information from
one form to another.
It is important to understand how information can
be encoded (represented) by physical and
chemical characteristics and particularly by
electrical signals, such as current. voltage, and
charge.
The various modes of encoding information are
called data domains.
types of domain
nonelectrical domains
electrical domains8
Nonelectrical domains The measurement process begins and ends
in nonelectrical domains.
The physical and chemical information / characteristics are length, density, chemical composition, intensity of light. pressure, and etc.
The information representing the mass of the object in standard units is encoded directlyby the experimenter who provides information processing by summing the masses to arrive at a number.
Home task: TABLE 1-2 Some Examples of Instrument Components
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Data domain Map
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Electrical domains The modes of encoding information as electrical
quantities are :
analog domains
time domains,
the digital domain
For example,
◦ the measurement of the molecular fluorescence
intensity of a sample of tonic water containing a
trace of quinine
◦ The intensity of the fluorescence is significant
in this context because it is proportional to the
concentration to the quinine in the tonic water,
which is ultimately the information that we desire.
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A block diagram of fluorometer
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(a) a general diagram of the instrument,
(b) a diagrammatic representation of the flow of information through
various data domains in the instrument
(c) the rules governing the data-domain transformations during the
measurement process.
The intensity of the fluorescence
emission, which is nonelectrical
information, is encoded into an
electrical signal by a special type of
device, called an input transducer.
◦ In this example, the input transducer converts
the fluorescence from the tonic water to an
electrical current I, proportional to the
intensity of the radiation.
◦ The current is then passed through a resistor
R, which according to Ohm's law produces a
voltage V that is proportional to I, which is in
turn proportional to the intensity of the
fluorescence.13
◦ The mathematical relationship between the electrical output and the input radiant power impinging on its surface is called the transfer function of the transducer
Finally, V is measured by the digital voltmeter to provide a readout proportional to the concentration of the quinine in the sample.◦ Devices that serve to convert data from
electrical to non-clectrical domains are called output transducers e.g. Voltmeters, alphanumeric displays, electric motors, computer screens
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Analog-Domain Signals
is encoded as the
magnitude of one of the
electrical quantities -
voltage, Current, charge,
or power.
These quantities are
continuous in both
amplitude and time.
Magnitudes of analog
quantities can be
measured continuously,
they can be sampled at
specific points in time
dictated by the needs at
a particular experiment
or instrumental.
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Time-Domain Information
Information is stored in the
time domain as the time
relationship of signal
fluctuations.
The time relationships
between transitions of
the signal from HI to LO
or from LO to HI contain
the information of
interest.
For instruments that
produce periodic signals,
the number of cycles of
the signal per unit time
is the frequency and the
time required for each
cycle is its period.16
Digital information
Data are encoded in the
digital domain in a two-
level scheme.
The characteristic that
these devices share is that
each of them must be in
one of only two states.
For example, lights and
switches may be only ON
or OFF and logic-level
signals may be only HI or
LO.
The measurement task is
to count the pulses during
a fixed period of time
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Detector
Detector refers to a mechanical,
electrical or chemical device that
identifies, records. or indicates a
change in one of the variables in its
environment, e.g.
◦ pressure,
◦ temperature,
◦ electrical charge,
◦ electromagnetic radiation,
◦ nuclear radiation.
An example is the UV (ultraviolet)
detector often used to indicate and
record the presence of eluted analytes
in liquid chromatography.
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Transducer
those devices that convert
information in nonelectrical
domains to information in
electrical domains and the
converse.
◦ photodiodes,
◦ photomultipliers, and
◦ other electronic photodetectors
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Sensor
analytical devices that are
capable of monitoring specific
chemical species continuously
and reversibly.
◦ the glass electrode
◦ ion-selective electrodes,
◦ the Clark oxygen electrode, and
◦ liber-optic sensors (optrodes)
Sensors consist of a transducer
coupled with a chemically
selective recognition phase.
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Readout device
A readout device is a transducer that
converts information from an
electrical domain to a form that is
understandable by a human observer.
Usually, the transduced signal takes
the form of
the alphanumeric or graphical output of
a cathode-ray tube,
a series of numbers on a digital display,
the position of a pointer on a meter
scale,
a tracing on a recorder paper.
21
Computer in instruments
Most modern analytical
instruments contain or are
attached to one or more
sophisticated electronic devices
and data-domain converters.
operational amplifiers,
integrated circuits,
analog-to-digital
digital-to-analog converters,
microprocessors, and
Computers.
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A complete analysis
five main steps:
Sampling; selecting a
representative sample of the
material to be analyzed
Dissolution of the sample
Conversion of the analyte into a
form suitable for measurement
Measurement and
Calculation and interpretation of
the measure.
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Representative sample
The aims of analysis are
understood and an appropriate
sampling procedure adopted.
Heterogeneous material have to
be homogenized prior to obtaining
a laboratory sample if an average
or bulk composition is required.
Typical examples
Surface water, rivers, seawaters
Ores, minerals and alloys
Laboratory , industrial or urban
atmosphere
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Sample storage After collecting sample, it may
elapse several days or weeks
before they are received by the
laboratory.
The working load
Sample containers and storage
conditions must be controlled
◦ Temperature,
◦ humidity,
◦ light levels and
◦ exposure to the atmosphere
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Effects for consideration
i. Increases temperature leading to the loss
of volatile analytes
ii.Decrease in temperature that lead to the
formation of deposits or the precipitation
iii.Changes in humidity that affect the
moisture content of hygroscopic solids
and liquids
iv.UV radiation that induces photochemical
reactions, photo-decompostion, or
polymerisation
v. Air induced oxidation
vi.Physical separation of the sample
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Sample pre-treatment
Samples arriving in an analytical
laboratory come in a very wide
assortment of sizes, conditions, and
physical forms
They can contain analytes from major
constituents down to ultra-trace levels.
They have moisture content
Examples of pre-treatment
Dry at 100-120ºC to remove moisture
Weighing before and after drying
Separating into groups e.g. Distillation,
filtration
Concentrating analytes
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Sample preparation A laboratory sample generally needs
to be prepared for anlytical
measurement by treatment with
reagents, that converts the analyte
into an appropriate chemical form for
the selected technique and method.
◦ if the material are readily soluble in
aqueous or organic solvents, a simple
dissolution step may suffice.
◦ Some solution need to be decomposed to
release the analyte and facilitate specific
reactions in solution.
◦ Need to be diluted or concentrated
◦ Stabilizations of solutions
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Calibration of instrumental
methods Calibration determines the relationship
between the analytical response and
the analyte concentration.
Usually this is determined by the use
of chemical standards.
Almost all analytical methods require
some type of calibration with chemical
standards.
do not rely on calibration
◦ Gravimetric methods and
◦ some coulometric methods
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Several types of calibration
procedures Comparison with Standards
Direct comparison
Titrations
External standard calibration
The least Squares methods
Errors in external calibrations
Multivariate calibration
Standard addition methods
The internal standard methods
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Direct comparison
Some analytical procedures involve comparing a property of the analyte (or the product of a reaction with the analyte) with standards such that the property being tested matches or nearly matches that of the standard.
For example, colorimeters, the colorproduced as the result of a chemical reaction of the analyte was compared with the color produced by reaction of standards.
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Titrations
In a titration, the analyte reacts
with a standardized reagent (the
titrant) in a reaction.
Usually the amount of titrant is
varied until chemical equivalence is
reached, as indicated by the color
change of a chemical indicator or by
the change in an instrument
response.
The titration is thus a type of
chemical comparison.32
External standard calibration
An external standard is prepared
separately from the sample.
External standards are used to
calibrate instruments and
procedures when there are no
interference effects from matrix
components in the analyte solution.
A series of such external standards
containing the analyte in known
concentrations is prepared.
Ideally, three or more such solutions
are used in the calibration process.
However, in some routine analyses,
two-point calibrations can be
reliable. 33
Calibration is accomplished by obtaining
the response signal (absorbance, peak
height, peak area) as a function of the
known analyte concentration.
A calibration curve is prepared
◦ by plotting the data or
◦ by fitting them to a suitable mathematical
equation e.g. the method of linear least squares.
In the prediction step, the response signal is
◦ obtained for the sample and
◦ used to predict the unknown analyte
concentration, cx
from the calibration curve or
best-fit equation.
The concentration of the analyte in the original
bulk sample is then calculated from cx, by
applying the appropriate dilution factors from
the sample preparation steps.
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The Least-Squares Method:
two assumption i. there is actually a linear relationship
between the measured response and
the standard analyte concentration:
regression model, Y= mx + b; where b
is the y intercept (the value of y when
x is zero), m is the slope of the line.
ii. We also assume that any deviation of
the individual points from the straight
line arises from error in the
measurement.
we assume there is no error in the x
values of the points (concentrations).
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Basic least-squares analysis may not be
appropriate
i. whenever there is significant uncertainty in
the x data, a more complex correlation
analysis may be necessary.
ii. when the uncertainties in the y values vary
significantly with x. In this case, it may be
necessary to apply a weighted least-squares
analysis
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the determination of isooctane
in a hydrocarbon sample.Here, a series of isooctane standards was
injected into a gas chromatograph.
the area of the isooctane peak was obtained
as a function of concentration.
The ordinate is the dependent variable, peak
area, and the abscissa is the independent
variable, mole percent (mol %) of isooctane.
As is typical and usually desirable, the plot
approximates a straight line, because of the
indeterminate errors in the measurement
process to draw the "hest“ straight line
among the data points.
Regression analysis provides the means for
objectively obtaining such a line and also
for specifying the uncertainties associated
with its subsequent Line.
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38
Errors in External-Standard Calibration
When external standards are used, it
is assumed that the same responses
will be obtained when the same
analyte concentration is present in
the sample and in the standard.
Thus, the calibration functional
relationship between the response
and the analyte concentration must
apply to the sample as well.
Usually, in a determination, the raw
response from the instrument is not
used. Instead, the raw analytical
response is corrected by measuring
a blank.
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An ideal blank is identical to the
sample but without the analyte.
In practice, with complex samples, it
is too time-consuming or impossible
to prepare an ideal blank and a
compromise must be made.
Most often a real blank is used in
sample preparation
a solvent blank , containing the same
solvent in which the sample is dissolved,
a reagent blank, containing the solvent
plus all the reagents
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Matrix effects, due to extraneous species in
the sample that are not present in the
standards or blank, can cause the same
analyte concentrations in the sample and
standards to give different responses.
systematic errors can occur during the
calibration process.
i. if the standards are prepared incorrectly,
an error will occur.
ii. errors can occur if the calibration function
is obtained without using enough
standards to obtain good statistical
estimates of the parameters.
Random errors can also influence the
accuracy of results obtained from calibration
curves.
41
Standard-Addition Methods Standard-addition methods are
particularly useful for analyting complex
samples in which the likelihood of
matrix effects is substantial.
A standard-addition common forms
involves adding one or more increments
of a standard solution to sample aliquots
containing identical volumes. This
process is often called spiking the
sample.
Each solution is then diluted to a fixed
volume before measurement.
Measurements are made on the original
sample and on the sample plus the
standard after each addition.
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The internal standard method An internal standard is a substance that is
added in a constant amount to all samples,
blanks, and calibration standards in an
analysis.
Alternatively, it may be a major constituent of
samples and standards that is present in a
large enough amount that its concentration
can be assumed to be the same in all cases.
Calibration then involves plotting the ratio of
the analyte signal to the internal-standard
signal as a function of the analyte
concentration of the standards.
This ratio for the samples is then used to
obtain their analyte concentrations from a
calibration curve.
An internal standard, if properly chosen and
used, can compensate for several types of
both random and systematic errors.
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selecting an analytical
method Defining the Problem
To select an analytical method intelligently, it
is essential to define clearly the nature of
the analytical problem. Such a definition
requires answers to the following questions:
1. What accuracy is required?
2. How much sample is available?
3. What is the concentration range of the
analyte?
4. What components of the sample might
cause interference?
5. What are the physical and chemical
properties of the sample matrix?
6. How many samples are to be analyzed?
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Performance Characteristics of
Instruments: Numerical Criteria i. Precision: Absolute standard
deviation, relative standard deviation,
coefficient of variation. variance
ii. Bias: Absolute systematic error,
relative systematic error
iii. Sensitivity: Calibration sensitivity.
analytical sensitivity
iv. Detection limit: Blank plus three times
standard deviation of the blank
v. Dynamic range: Concentration limit of
quantitation (LOQ) to concentration
limit of linearity (LOL)
vi. Selectivity: Coefficient of selectivity
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Other Characteristics to Be
Considered in Method Choicei. Speed
ii. Ease and convenience
iii. Skill required of operator
iv. Cost and availability of
equipment
v. Per-sample cost
46
Precision
Precision provides a measure of the
random, or indeterminate, error of an
analysis.
Figures of merit or precision include
◦ Absolute standard deviation (S=
◦ Relative standard deviation, RSD
◦ Standard error of the mean, Sm
◦ Coefficient of variation, CV and
◦ Variance, S2
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Bias
Bias provides a measure of the
systematic, or determinate,
error of an analytical method.
Bias , is defined hy the
equation:
Δ=μґ
Where μ is the population mean
for the concentration of an
analyte in a sample and ґ is
the true value,
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Sensitivity
There is general agreement that the
sensitivity of an instrument or a
method is a measure of its ability to
discriminate between small
differences in analyte concentration.
Two factors limit sensitivity:
◦ the slope of the calibration curve and
◦ the reproducibility or precision of the
measuring device.
A corollary to this statement is that
if two methods have calibration
curves with equal slopes, the one
that exhibits the better precision will
be the more sensitive.
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Detection Limit
Detection limit is that it is the
minimum concentration or mass of
analyte that can be detected at a
known confidence level.
This limit depends on
◦ the ratio = the magnitude of the
analytical signal
◦ the size of the statistical
fluctuations in the blank signal.
The analytical signal is larger than the
blank by some multiple k of the
variation in the blank due to random
errors, it is impossible to detect the
analytical signal with certainty.
50
Dynamic Range the delinition of the
dynamic range of an
analytical method
It extends from the
lowest concentration
at which quantitative
measurements can be
made (limit of
quantitation, or LOQ)
to the concentration at
which the calibration
curve departs from
linearity by a specified
amount (limit of
linearity, / LOL).
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Dynamic Range Usually, a deviation of
5 % from linearity is
considered the upper
limit.
Deviations from
linearity are common
at high concentrations
because of no ideal
detector responses or
chemical effects.
The lower limit of
quantitative
measurements is
generally taken to be
equal to ten times the
standard deviation of
repetitive
measurements on a
blank.
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Selectivity
Selectivity of an analytical method refers to the degree to which the method is free from interference by other species contained in the sample matrix.
Unfortunately, no analytical method is totally free from interference from other species
Frequently steps must be taken to minimize the effects of these interferences.
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