TOPIC 1intro Spectrophotometry

7/28/2019 TOPIC 1intro Spectrophotometry

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Spectrochemical

Methods ofAnalysis

CHM580

Fakulty of Applied SciencesUiTM

FTIR

AAS

UV-VISICP-OES

NMR



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?



Why do we need an instrumental

analysis course?

Chemical analysis relies on

• accurate measurements

• careful interpretation of results



CHM 580

In this course, you will appreciate

• The methods and instruments used tomake measurements

• The principles behind thesemeasurements

SPECTROSCOPY



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



Definitions

Spectrometer /Spectrophotometer

• An instrument

Spectroscopy• The use of the spectrometer

Spectrometry

• The measurement of a spectrumSpectrum

• Output of the instrument (further definition)



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



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.



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



Electromagnetic radiation

• Represented as electric and magneticfields that undergo in-phase, sinusoidaloscillations at right angles to each other

and to the direction of propagation



Properties of electromagnetic

radiation

Dual properties

• Wave

• Particle



Wave characteristics

• Amplitude

• Wavelength

• Frequency




• Amplitude, A is the length of the electricvector at a maximum in the wave




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)

http://imagine.gsfc.nasa.gov/docs/dict_jp.html

http://imagine.gsfc.nasa.gov/docs/dict_ad.html

http://imagine.gsfc.nasa.gov/docs/dict_ei.html

http://imagine.gsfc.nasa.gov/docs/dict_ei.html

http://imagine.gsfc.nasa.gov/docs/dict_ad.html

http://imagine.gsfc.nasa.gov/docs/dict_jp.html



Velocity of radiation

Wavelength and frequency arerelated by the velocity of

radiation (c), a fundamentalconstant




In vacuum, c

• Is independent of wavelength

•

Is at its maximum• 2.99792 x 108 m s-1

c




In medium containing matter

• Propagation of radiation is slowed by theinteraction of EMR with bound electrons inmatter



Frequency

andwavelength



Wavenumber,

• Reciprocal of wavelength of radiation,

• Unit of cm-1

• The wavelength must be measured in cm

1



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



1 Å = 10-10 m = 10-8 cm

1 nm = 10-9 m = 10-7 cm

1m = 10-6

m = 10-4

cm



The electromagnetic spectrum



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



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



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.

http://en.wikipedia.org/wiki/Planck's_constant

http://en.wikipedia.org/wiki/Speed_of_light

http://en.wikipedia.org/wiki/Speed_of_light

http://en.wikipedia.org/wiki/Planck's_constant



Energy of a given EMR

hcc

hhE



Atom



Atomic orbitals

Orbital shapes representing boundary surfaces enclosing regions

of space where electrons are most likely to be found in the first

three shells.



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

http://en.wikipedia.org/wiki/Atom

http://en.wikipedia.org/wiki/Chemical_bond

http://education.jlab.org/qa/element.html

http://education.jlab.org/qa/element.html

http://en.wikipedia.org/wiki/Chemical_bond

http://en.wikipedia.org/wiki/Atom



Molecules

3D 3D2D

http://en.wikipedia.org/wiki/Image:Atisane3.png

http://en.wikipedia.org/wiki/Image:Atisane3.png



Atomic and molecular orbitals

Atomic orbitals

Molecular orbitals

http://upload.wikimedia.org/wikipedia/commons/1/11/Electron_orbitals.svg






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



Overall process of aninstrumental measurement



Interaction of radiation with 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




We obtain information about the sample bymeasuring EMR

• Emitted

• Absorbed

• Scattered

as a result of excitation



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



Three General Types of Spectra

Continuous spectrum

Emission line spectrum

Absorption line spectrum



The absorption process

Electronic transition

• Changes in the distribution of outer electrons about atoms or molecules

Molecular

→ *

Atomic



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



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



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

http://en.wikipedia.org/wiki/Image:Beer_lambert.png



http://localhost/var/www/apps/conversion/tmp/scratch_10//upload.wikimedia.org/wikipedia/commons/3/32/Beer%E2%80%93Lambert_law_in_solution.JPG




Transmittance, T

T =

% Transmittance, %T = 100 T = x 100%

Absorbance, A

A = - log T = - log

0P

P

0P

P

0P

P



Absorbance vs. %Transmittance

A10T



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



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



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.

http://en.wikipedia.org/wiki/SI

http://en.wikipedia.org/wiki/Molar_concentration

http://en.wikipedia.org/wiki/Molar_concentration

http://en.wikipedia.org/wiki/SI



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



Emission of EMR

Energy

http://en.wikipedia.org/wiki/File:AtomicLineSpEm.png



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



Spectra in the visible region

Continuous

Emission

or

Bright line

Absorption

or

Dark line

Visible region

Emission of EMR



Emission of EMRType of spectra

Atomic species – line spectrarelatively narrow lines but still complex

• Several major electronic transition and

sublevels



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



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



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

TOPIC 1intro Spectrophotometry

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