chapter 1 - spectroscopy methods
DESCRIPTION
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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