topic 1intro spectrophotometry
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Spectrochemical
Methods ofAnalysis
CHM580
Fakulty of Applied SciencesUiTM
FTIR
AAS
UV-VISICP-OES
NMR
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Why do we need an instrumentalanalysis course?
Scientists involve in chemical analysissamples of interest (analytes) usually askthese QUESTIONS
• What is this sample composed of?
• How much of each component is
present?
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Why do we need an instrumental
analysis course?
Chemical analysis relies on
• accurate measurements
• careful interpretation of results
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CHM 580
In this course, you will appreciate
• The methods and instruments used tomake measurements
• The principles behind thesemeasurements
SPECTROSCOPY
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Classification of InstrumentalMethods
Characteristic Properties Instrumental MethodsEmission of radiation Emission (ICP-OES) and
Fluorescence spectroscopy
Absorption of radiation AA, UV-vis, IR, NMR spectroscopy
Scattering of radiation Raman spectroscopy
Mass-to-charge ratio MS
Chemical and Physical Properties Used in Instrumental Methods
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Definitions
Spectrometer /Spectrophotometer
• An instrument
Spectroscopy• The use of the spectrometer
Spectrometry
• The measurement of a spectrumSpectrum
• Output of the instrument (further definition)
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Spectrometric methods
A group of techniques that relies onthe interaction of electromagnetic
radiation and matter
There are many type of methods that are
based on either molecular or atomic interactions
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What is light?
Visible light
• the form of light which we can see
•a form of energy made up of waves knownas electromagnetic radiation
What we perceive as light is actually onlya very small part of the electromagneticspectrum.
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Electromagnetic spectrum andthe uses in everyday life
How do we make use of the variedproperties of light?There are thousands of applications.
Here are a few examples:Radio waves - Television, radio, cellphones,satellitesMicrowaves - Satellites and microwaveovensInfra-Red - Toaster ovens, broilers, Night-vision, F.L.I.R. (forward looking infra-red) Ultra-violet - Sunbeds, night club lightingX-rays - Medical imaging, material science
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Electromagnetic radiation
• Represented as electric and magneticfields that undergo in-phase, sinusoidaloscillations at right angles to each other
and to the direction of propagation
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Properties of electromagnetic
radiation
Dual properties
• Wave
• Particle
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Wave characteristics
• Amplitude
• Wavelength
• Frequency
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Wave characteristics
• Amplitude, A is the length of the electricvector at a maximum in the wave
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Wave characteristics
Wavelength (λ) is the distance between two
equivalent points on successive waves, and it canbe measured with a base unit
of meters (m) (such as km, cm, m, angstroms (Å))
Frequency () is the number of cycles of a wave to
pass some point in a second.The basic unit of frequency is cycles per second
(s-1), or Hertz (Hz)
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Velocity of radiation
Wavelength and frequency arerelated by the velocity of
radiation (c), a fundamentalconstant
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Velocity of radiation
In vacuum, c
• Is independent of wavelength
•
Is at its maximum• 2.99792 x 108 m s-1
c
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Velocity of radiation
In medium containing matter
• Propagation of radiation is slowed by theinteraction of EMR with bound electrons inmatter
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Frequency
andwavelength
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Wavenumber,
• Reciprocal of wavelength of radiation,
• Unit of cm-1
• The wavelength must be measured in cm
1
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Prefixesatto a 10-18
femto f 10-15 pico p 10-12 nano n 10-9 micro 10-6 milli m 10-3 centi c 10-2 deci d 10-1
kilo k 103
mega M 106 giga G 109 tera T 1012
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1 Å = 10-10 m = 10-8 cm
1 nm = 10-9 m = 10-7 cm
1m = 10-6
m = 10-4
cm
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Electromagnetic radiation
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Electromagnetic radiation
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The electromagnetic spectrum
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Quantum-mechanical properties of radiation
Planck's theory was based on the idea that
black bodies emit light (and other electromagnetic radiation) as a stream of
discrete particles called
• photons or quanta
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Energy states of chemical species
Postulates of quantum theory• atoms, ions and molecules can exist only
in certain discrete states
– change their state
– they absorb or emit energy exactlyequal to the energy difference betweenthe states
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Energy states of chemical species
• when species absorb or emit radiation,the or of the radiation is related tothe energy difference
hch E E E
01
whereh is Planck's constant λ is the wavelength is the frequency
c is the speed of light.
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Energy of a given EMR
hcc
hhE
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Atom
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Atomic orbitals
Orbital shapes representing boundary surfaces enclosing regions
of space where electrons are most likely to be found in the first
three shells.
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Molecules and compounds
A molecule is a group of two or more atoms in a definite arrangement held together by
chemical bonds. Examples; H2, H2O
A compound is a molecule that contains atleast two different elements
Examples: H2O, NaCl
All compounds are molecules but not allmolecules are compounds
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Molecules
3D 3D2D
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Atomic and molecular orbitals
Atomic orbitals
Molecular orbitals
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Interaction of radiation and matter
Spectroscopists use the interaction of radiation with matter
• To obtain information about MATTER
Sample is stimulated by applying ENERGY
which can be in the form of
• Heat
• Electrical energy
• RADIATION
•
Chemical reaction
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Overall process of aninstrumental measurement
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Interaction of radiation with matter
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Interaction of radiation and matter
Initially, the matter (molecules, atoms or ions) is in its ground state
• Lowest energy stateSome analyte species undergoes a
TRANSITION to an excited state
• Higher energy level
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Interaction of radiation and matter
We obtain information about the sample bymeasuring EMR
• Emitted
• Absorbed
• Scattered
as a result of excitation
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Method of interactions
•Absorption – Radiation is absorbed by an atom, molecule
or ion taking it to a higher energy state
• Emission
– The release of photon by an atom, moleculeor ion, taking it to a lower energy state
• Scattering
– an excitation to a virtual state lower in energythan a real electronic transition
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Three General Types of Spectra
Continuous spectrum
Emission line spectrum
Absorption line spectrum
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The absorption process
Electronic transition
• Changes in the distribution of outer electrons about atoms or molecules
Molecular
→ *
Atomic
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Atomic absorption
With atoms, the simplestcase, it is still a relativelycomplex absorptionprocess.
Even for hydrogen atom,the line spectrum iscomplex due to major electronic transitions andthe sublevels – s, p, d, f
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Quantitative absorption methods
Measurements of two beams, Po and P
• Passes through the medium that containsthe analyte, Po
• Part of the radiation has been absorbed bythe matter, P
Two terms related to the ratio of P and Po
• Transmittance
• Absorbance
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Beer’s Law • A beam of monochromatic radiation of radiant power P0
is directed at a solution• The solution contains a sample
• Absorption takes place• The beam of radiation leaving the solution has radiant
power P
b
PoP
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Transmittance, T
T =
% Transmittance, %T = 100 T = x 100%
Absorbance, A
A = - log T = - log
0P
P
0P
P
0P
P
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Absorbance vs. %Transmittance
A10T
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Beer’s Law
Absorbance is linearly related to theconcentration of the absorbing species c and the pathlength b of the absorbing
medium A = abc
c has the units of g L-1
b has the unit of length, cma, absorptivity, has the units of L g-1 cm-1
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Beer’s Law
When c is in mole/L or M, b in cm,
• the proportionality constant is
•called molar absorptivity
• has the units of L mol-1 cm-1
A = bc
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Q & A
• A compound of formulaweight 280 adsorbed85% of the radiation in a2.5 cm cell at a
concentration of 15 ugmL-1. Calculate its molar absorptivity at thewavelength
A= 0.839
ε = A/bc
= 0.839/2.5 x Molar?
=0.3356 x 5.36 x 10-5
M=1.8 x 10-5 (unit?)
The SI units for ε are
m2/mol, but in practice,
they are usually taken asM−1 cm−1 or L mol−1 cm−1.
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Emission of EMR
• Atoms, molecules and ions can be excitedvia a number of processes
• When they relax, they release excess
energy• In some cases, the relaxation causes the
emission of EMR
• The type of EMR emission is often thecharacteristic of the species
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Emission of EMR
Energy
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Emission of EMR
Continuous spectra
• Produced by many solids that are
heated until they glow• Radiation is emitted over a wide
energy range
• Maximum λ is a function of thetemperature of the materials
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Spectra in the visible region
Continuous
Emission
or
Bright line
Absorption
or
Dark line
Visible region
Emission of EMR
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Emission of EMRType of spectra
Atomic species – line spectrarelatively narrow lines but still complex
• Several major electronic transition and
sublevels
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Emission
The intensity of the radiation is directlyproportional to the concentration of species being measured
I = k cwhere
I is the intensity of lightk is the proportionality constant
c is the concentration
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Instrumental methods
Characteristic properties SpectrometersEmission of radiation
Absorption of radiation
Scattering of radiation
Mass-to-charge ratio
ICP-OESFluorescence
UV-visFTIR
NMR
AAS (flame and GF)
Raman
Mass
Common spectroscopic methods
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Common spectroscopic methodsbased on EMR
Types of spectroscopy Wavelength
range
Type of quantum
transition
Gamma-ray emission 0.005 – 1.4 Å Nuclear
X-ray (A, E, F, D) 0.1 – 100 Å Inner electron
Vacuum UV absorption 10 – 180 nm Bonding electronsUV-vis (A, E, F) 180 – 780 nm Bonding electrons
IR absorption and Raman
scattering
0.78 – 300 m Rotation/vibration of
molecules
Microwave absorption 0.75 – 375 mm Rotation of molecules
Electron spin resonance 3 cm Spin of electrons in amagnetic field
Nuclear magnetic resonance 0.6 – 10 m Spin of nuclei in a
magnetic field