Chapter 1 - Spectroscopy Methods

CHM260

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CHAPTER 1

The study of the interaction betweenELECTROMAGNETIC (EM) RADIATION

and MATTER

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covers

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ATOMICSPECTROSCOPY

(atomic absorption)

MOLECULARSPECTROSCOPY

(molecular absorption)

What is Electromagnetic Radiation? is a form of energy that has both Wave and

Particle Properties. It is produced by oscillating electric and magnetic

disturbance, or by the movement of electricallycharged particles traveling through a vacuum ormatter.

For example: Ultraviolet, visible, infrared,microwave, radio wave.

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EM radiation is conveniently modeled as wavesconsisting of perpendicularly oscillating electric andmagnetic fields, as shown below.

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Direction ofpropagation

o At 90° to the direction of propagation is an oscillation inthe ELECTRIC FIELD.

o At 90° to the direction of propagation and 90° from theelectric field oscillation (orthagonal) is the MAGNETICFIELD oscillation.

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Period (p)the time required for one cycle to pass a fixed point inspace.

Frequency (V @ f )the number of cycles which pass a fixed point in space persecond. Unit in Hz or s-1

Amplitude (A)The maximum length of the electric vector in the wave(Maximum height of a wave).

Wavelength (λ)The distance between two identical adjacent points in awave (usually maxima or minima).

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Wavenumber (ν)The number of waves per cm in units of cm-1.

Radiant Power ( P )The amount of energy reaching a given area per second.

Unit in watts (W) Intensity ( I )The radiant power per unit solid angle.

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c = 3.00 x 108 m/s = 3.00 x 1010 cm/sc = 3.00 x 108 m/s = 3.00 x 1010 cm/s

Speed of light = Wavelength x Frequency

c = VWhere as is the wavelength of the waves V is the frequency of the waves c is the speed of light

Speed of light = Wavelength x Frequency

c = VWhere as is the wavelength of the waves V is the frequency of the waves c is the speed of light

Wavelength is inversely proportional to frequency ∝ 1/V

The Higher the Frequency the Shorter the Wavelength andvv.

Wavelength is inversely proportional to frequency ∝ 1/V

The Higher the Frequency the Shorter the Wavelength andvv.

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800 nm

Infrared radiationV = 3.75 x 1014 s-1

Ultraviolet radiationV = 7.50 x 1014 s-1

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EMR is viewed as a stream of discrete particles ofenergy called photons.We can relate the energy, E of photon to itswavelength, frequency and wavenumber by

E = hV = = hch = Planck’s constanth = 6.63 x 10-34 J.s

hc

E = hV = hc=

Therefore wavenumber,

= 1/ = V/c

Unit of wavenumber is cm-1

hc

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What is the energy of a 500 nm photon?

V = c/= (3 x 108 m s-1)/(5.0 x 10-7 m)

V = 6 x 1014 s-1 @ Hz

E = hV= (6.626 x 10-34 J•s)(6 x 1014 s-1)= 4 x 10-19 J

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Visible

Ultraviolet

Radio

wave

Gammaray

Hzcmcm-1Kcal/mol eV

TypeQuantum Transition

Typespectroscopy

TypeRadiation

Frequencyυ

Wavelengthλ

WaveNumber VEnergy

9.4 x 107 4.9 x 106 3.3 x 1010 3 x 10-11 1021

9.4 x 103 4.9 x 102 3.3 x 106 3 x 10-7 1017

9.4 x 101 4.9 x 100 3.3 x 104 3 x 10-5 1015

9.4 x 10-1 4.9 x 10-2 3.3 x 102 3 x 10-3 1013

9.4 x 10-3 4.9 x 10-4 3.3 x 100 3 x 10-1 1011

9.4 x 10-7 4.9 x 10-8 3.3 x 10-4 3 x 103 107

X-ray

Infrared

Micro-wave

Gamma rayemission

X-rayabsorption,emission

UV absorption

IR absorption

Microwaveabsorption

Nuclearmagneticresonance

Nuclear

Electronic(inner shell)

Molecularvibration

Electronic(outer shell)

Molecularrotation

Magneticallyinduced spinstates

Spectral Properties, Application and Interactions ofElectromagnetic Radiation

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Region Wavelength Range

UV 180 – 380 nm

Visible 380 – 780 nm

Near-IR 780 – 2500 nm

Mid-IR 2500 – 50000 nm

Region Unit Definition (m)

X-ray Angstrom unit, Å 10-10 m

Ultraviolet/visible Nanometer, nm 10-9 m

Infrared Micrometer, μm 10-6 m

Wave Number (cycles/cm)

X-Ray UV Visible IR Microwave

200nm 400nm 800nm

Wavelength (nm)

Spectral Distribution of Radiant Energy

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Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

Longest wavelength EM waves Uses:

◦ TV broadcasting◦ AM and FM broadcast radio◦ Avalanche beacons◦ Heart rate monitors◦ Cell phone communication

Wavelengths from 1 mm- 1 m Uses:

◦ Microwave ovens◦ Bluetooth headsets◦ Broadband Wireless Internet◦ Radar◦ GPS

Wavelengths in between microwaves andvisible light

Uses:◦ Night vision goggles◦ Remote controls◦ Heat-seeking missiles

Only type of EM wave able to be detected bythe human eye

Violet is the highest frequency light Red light is the lowest frequency light

Shorter wavelengths than visible light Uses:

◦ Black lights◦ Sterilizing medical equipment◦ Water disinfection◦ Security images on money

Tiny wavelength, highenergy waves

Uses:◦ Medical imaging◦ Airport security◦ Inspecting industrial welds

Smallest wavelengths, highest energy EMwaves

Uses◦ Food irradiation◦ Cancer treatment◦ Treating wood flooring

Interactions of radiation with matter is to obtain theinformation about a sample.

The sample is stimulated by applying energy in theform of heat, electrical energy, light, particles, or achemical reaction.

The analyte is predominately in its lowest-energy orground state. The stimulus then causes someanalyte species to undergo a transition to a higher-energy or excited state.

In emission the analyte is stimulated by theapplication of heat, electrical energy, or a chemicalreaction.

The energy required for the transition for analytefrom a lower energy state to a higher energy state isdirectly related to the frequency of electromagneticradiation that causes the transition.

Information about the analyte can be obtained by:-◦ measuring the electromagnetic radiation emitted as

it returns to the ground state◦ or by measuring the amount of electromagnetic

radiation absorbed as a result of excitation.

Process involved in emission and chemiluminescencespectroscopy

Absorption methods

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Photoluminescence method(Fluorescence and phosphorescence)

Absorption: A transition from a lower level to a higher level with transfer ofenergy from the radiation field to an absorber, atom, molecule, or solid.

Emission: A transition from a higher level to a lower level with transfer ofenergy from the emitter to the radiation field. If no radiation is emitted, thetransition from higher to lower energy levels is called nonradiative decay.

1.3.1 Radiation Absorption

• This process transfers energy to the molecule and results in adecrease in the intensity of the incident electromagneticradiation.• Absorption of the radiation thus attenuates the beam inaccordance with the absorption law.

Transmittance

Transmittance (T) is defined as the amount of light passing through thesample solution (P) divided by the amount of incident radiation (Po).

Absorbance, A of solution is related to the transmittance inlogarithmic manner.

As the absorbance increases, transmittance decreases.

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Absorbance

where I is the light intensity after it passes through the sampleand I o is the initial light intensity. The relation between A and T is:A = -log T = - log (I / I o ).

The Beer-Lambert law (or Beer's law) is thelinear relationship between absorbance andconcentration of an absorbing species.

The amount of radiation absorbed may be measured in a numberof ways:

Transmittance, T = P / P0% Transmittance, %T = 100 TAbsorbance,

A = log10 P0 / PA = log10 1 / TA = log10 100 / %TA = 2 - log10 %T

The last equation, A = 2 - log10 %T , is worthremembering because it allows you to easily calculateabsorbance from percentage transmittance data.

The relationship between absorbance and transmittanceis illustrated in the following diagram:

So, if all the light passesthrough a solution withoutany absorption, thenabsorbance is zero, andpercent transmittance is100%. If all the light isabsorbed, then percenttransmittance is zero, andabsorption is infinite.

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A = ebc tells us that absorbance depends on the total quantity of the absorbingcompound in the light path through the cuvette. If we plot absorbance againstconcentration, we get a straight line passing through the origin (0,0)

Note that the Law is not obeyed at highconcentrations. This deviation from the Law is notdealt with here.

A = -logT = log(P0/P) = ebc T = Psolution/Psolvent = P/P0

Works for monochromatic light Compound x has a unique e at different

wavelengths

cuvettesource

slit

detector

A = bc◦ A is absorbance (no units, since A =

log10 P0 / P )◦ is the molar absorbtivity with units of L

mol-1 cm-1

◦ b is the path length of the sample - thatis, the path length of the cuvette in whichthe sample is contained. In cm

◦ c is the concentration of the compound insolution, expressed in mol L-1

Beer’s law also applies to solutions containingmore than one kind of absorbing substance.Provided that there are no interactions amongthe various species.

The total absorbance for a multicomponentsystem is the sum of the individualabsorbances. In other words,

Atotal = A1 + A2 + … An= 1bc1 + 2bc2 + … + nbcn

where the subscripts refer to absorbingcomponets 1, 2, …, n.

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• deviations in absorptivity coefficients at high concentrations (>0.01M)due to electrostatic interactions between molecules in closeproximity.

• scattering of light due to particulates in the sample.• fluoresecence or phosphorescence of the sample.• changes in refractive index at high analyte concentration.• shifts in chemical equilibria as a function of concentration.• non-monochromatic radiation, deviations can be minimized by using

a relatively flat part of the absorption spectrum such as the maximumof an absorption band.

• stray light = radiation from the instrument that is outside the nominalwavelength band chosen for the determination.

The linearity of the Beer-Lambert law is limited by chemicaland instrumental factors. Causes of nonlinearity include:

Limits to Beer’s LawThere are few exception to the linear relationshipbetween absorbance and path length at a fixedconcentration. We frequently observe deviationsfrom the direct proportionality betweenabsorbance and concentration where b is aconstant. Some of these deviations, called realdeviations, are fundamental and represent reallimitations to the law. Others occur as aconsequence of the manner in which theabsorbance measurements are made or as aresult of chemical changes associated withconcentration changes. These deviations arecalled instrumental deviations and chemicaldeviation respectively.

Limitations (deviations) of Beer’sLaw

High concentration (close proximity ofmolecules affects absorption)

Analyte dissociation to product with differentabsorption characteristics (e.g., pH-dependent indicators)

Polychromatic radiation (i.e., light of morethan one )

Where P’ and P” are powers for ’ and ”,respectively◦ Negative deviation = lowerabsorbance than predicted becausehigher transmittance

◦ Higher T because molecules don’tabsorb one as well as other

Ap P

P Pmeas

log' "

' "

0 0

Stray radiation

Ps = power from stray radiationExtra light hits detector higher T; lower A

Ap P

P Ps

s' log

'

'

0

Absorption spectra

An absorption spectrum is a plot of absorbanceversus wavelength.

Absorbance could also be plotted against wavenumber or frequency.

Occasionally, plots with log A as the ordinateare used. A plot of molar absorptivity as afunction of wavelength is independent ofconcentration.

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Typical absorption spectra of potassium permanganateat five different concentrations

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A 7.25 x 10-5 solution of potassiumpermanganate has a transmittance of 44.1%when measured in a 2.10 cm cell atwavelength of 525nm. Calculate

a) The absorbance of this solutionb) The molar absorptivity of KMnO4

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a) A = -log T = -log 0.441 = 0.355b) = A/bc

= 0.3554/(2.10 x 7.25 x 10-5mol L-1)= 2.33 x 103 L mol-1cm-1

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Electrons bound toatoms have discreteenergies (i.e. not allenergies are allowed).

Thus, only photons ofcertain energy caninteract with theelectrons in a givenatom.

Transitions betweenelectronic levels of theelectrons produce linespectra.

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Consider hydrogen, thesimplest atom.

Hydrogen has a specificline spectrum.

Each atom has itsown specific linespectrum (atomicfingerprint).

For an electromagnetic radiation, at thegive wvlgth of 562 nm

i) calculate the frequency in Hz Ii) Name the EMR at the given wvlgth Iii) Determine the energy in (joules) of

this radiation

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A spectroscopy experiment wasconducted using a 1.0 cm cuvette. Themolar absorptivity of MnO4

- is 2.3 x 103

M-1cm-1.i) Calculate the conc of permanganate

solution which would give anabsorbance 0.8

ii) Calculate the % transmittance ofsolution in (i)

iii) Solution (i) is diluted to half of itsoriginal conc. Calculate thetransmittance of the diluted solution

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Partial energy level diagramfor sodium.

Involve excitation from groundstate to higher state.

Occurs by absorption ofphoton of radiation

Transitions between twodifferent orbitals are termedelectronic transition.

Atomic absorption is measuredat a single wavelength using avery narrow, nearlymonochromatic source.

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The energy of photon that can promote electronsto excite/jump to a higher energy level dependson the energy difference between the electroniclevels.

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Each atom has a specific set of energy levels, andthus a unique set of photon wavelengths with whichit can interact.

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Absorption and emissionfor the sodium atom in thegas phase.

The diagram illustrate thetransitions (excitation andemission) of electronsbetween different energylevels in sodium atom.

ΔEtransition = E1 - E0 = hv = hc/

The energy, E, associated with the molecular bands:Etotal = Eelectronic + Evibrational + Erotational

In general, a molecule may absorb energy in 3 ways:1. By raising an electron (or electrons) to a higher

energy level. (electronic)2. By increasing the vibration of the constituent

nuclei. (vibrational)3. By increasing the rotation of the molecule about

the axis. (rotational)

Eo

hn

Absorption

En

Eo

hn

Emission

En

hn

Rotationalabsorption

Vibrationalabsorption

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Absorption spectrum◦ A plot of the absorbance as a function of

wavelength or frequency.

Emission spectrum◦ A plot of the relative power of the emitted

radiation as a function of wavelength orfrequency.

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The two peaks arise from the promotion ofa 3s electron to the two 3p states

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Absorption Spectrum of Na

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Electronic Transition Vibrational TransitionSuperimposed on theElectronic Transition

Absorption Band –A series of closelyshaped peaks

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In solvents the rotationaland vibrationaltransitions are highlyrestricted resulting inbroad bandabsorption spectra.

Three types ofspectra:◦ Lines◦ Bands◦ Continuum

spectra

72Emission spectrum of a brine sample

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Emission X* X + h

Excitation needs energy!

•Particle bombardment (e-)

•Electrical currents (V)

•Fluorescence

•Heat

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Individual atoms, well separated, in a gas phase

Made up of a series of sharp, well-defined peaks.

Caused by excitation of individualatoms.

Atomic transitions are usually verydiscrete changes of electrons from onequantum state to another energy level(shells, spins, etc).

Only electronic transition is quantized No vibrational or rotational transition.

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Small molecules and radicals

Encountered in spectralsources when gaseousradicals or small moleculesare present. Molecular transitionconsists of 3 processes:

i) Rotational transitionii) Vibrational transitioniii) Electronic transition

∆E = ∆Eelectronic + ∆Evibrational +∆E rotational

Band spectra is produceddue to vibrational androtational transitions.

Continuum spectra: A beam of light that contains abroad, smooth distribution of photon wavelengths.

Produced when solid are heated to incandescence. Blackbody Radiation (Thermal Radiation)

Line spectra

Band spectra

Continuum spectra