uvvis-spectroscopy p bambang

8/6/2019 UVVIS-Spectroscopy p Bambang

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SPECTROSCOPY

Light interacting with matter as an

analytical tool


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X-ray:

core electronexcitation

UV:

valanceelectronic

excitation

IR:

molecularvibrations

Radio waves:

Nuclear spin states(in a magnetic field)

Electronic Excitation by UV/Vis Spectroscopy :


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Spectroscopic Techniques and

Chemistry they Probe

UV-vis UV-vis region bonding electrons

Atomic Absorption UV-vis region atomic transitions (val. e-)

FT-IR IR/Microwave vibrations, rotations

Raman IR/UV vibrations

FT-NMR Radio waves nuclear spin states

X-Ray Spectroscopy X-rays inner electrons, elementalX-ray Crystallography X-rays 3-D structure


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Spectroscopic Techniques and Common Uses

UV-vis UV-vis region Quantitativeanalysis/Beers Law

Atomic Absorption UV-vis region Quantitative analysisBeers Law

FT-IR IR/Microwave Functional Group Analysis

Raman IR/UV Functional GroupAnalysis/quant

FT-NMR Radio waves Structure determination

X-Ray Spectroscopy X-rays Elemental Analysis

X-ray Crystallography X-rays 3-D structure Anaylysis


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Different Spectroscopies

UV-vis electronic states of valence e/d-orbital transitions for solvated transitionmetals

Fluorescence emission of UV/vis by certainmolecules

FT-IR vibrational transitions of molecules

FT-NMR nuclear spin transitions X-Ray Spectroscopy electronic transitions

of core electrons


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Quantitative Spectroscopy

Beers Law

Al1 = el1bc

e is molar absorptivity (unique for a

given compound at l1)

b is path lengthc concentration


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Beers Law

A = -logT = log(P0/P) = ebc

T = Psolution /Psolvent = P/P0

Works for monochromatic light

Compound x has a unique e at different wavelengths

cuvette

source slit

detector


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Characteristics of

Beers Law Plots One wavelength

Good plots have a range of

absorbances from 0.010 to 1.000

Absorbances over 1.000 are not that

valid and should be avoided

2 orders of magnitude


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Standard Practice

Prepare standards of knownconcentration

Measure absorbance at max Plot A vs. concentration

Obtain slope

Use slope (and intercept) to determinethe concentration of the analyte in theunknown


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Typical Beers Law Plot

y = 0.02x

0

0.2

0.4

0.6

0.8

1

1.2

0.0 20.0 40.0 60.0

concentration (uM)

A


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UV-Vis Spectroscopy

UV- organic molecules

Outer electron bonding transitions

conjugation

Visible metal/ligands in solution

d-orbital transitions

Instrumentation


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Characteristics of UV-Vis spectra of

Organic Molecules Absorb mostly in UV unless highly

conjugated

Spectra are broad, usually to broad forqualitative identification purposes

Excellent for quantitative Beers Law-

type analyses The most common detector for anHPLC


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Molecules have quantized energy levels:

ex. electronic energy levels.

energy

hv

energy

} = hvQ: Where do these quantized energy levels come from?

A: The electronic configurations associated with bonding.

Each electronic energy level(configuration) has

associated with it the many

vibrational energy levels we

examined with IR.


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Broad spectra

Overlapping vibrational and rotational

peaks Solvent effects


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Molecular Orbital Theory

Fig 18-10


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2s 2s

2p 2pn


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C C

hv

C C

H

HH H

HH

max

= 135 nm (a high energy transition)

Absorptions having max < 200 nm are difficult to observe because

everything (including quartz glass and air) absorbs in this spectral

region.

Ethane


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C C

hv

Example: ethylene absorbs at longer wavelengths: max = 165 nm = 10,000

=hv

=hc/


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hv

n

n

C O

n

The n to pi* transition is at even lower wavelengths but is notas strong as pi to pi* transitions. It is said to be forbidden.

Example:

Acetone: n max = 188 nm ; = 1860

n max = 279 nm ; = 15


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C C

C C

C O

C O

H

> 135 nm

> 165 nm

n > 183 nm weak

> 150 nm

n > 188 nmn > 279 nm weak

A

180 nm

279 nm

C O


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C C

HOMO

LUMO

Conjugated systems:

Preferred transition is between Highest Occupied Molecular Orbital(HOMO) and Lowest Unoccupied Molecular Orbital (LUMO).

Note: Additional conjugation (double bonds) lowers the HOMO-

LUMO energy gap:

Example:

1,3 butadiene: max = 217 nm ; = 21,000

1,3,5-hexatriene max = 258 nm ; = 35,000


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O

O

O

Similar structures have similar UV spectra:

max = 238, 305 nm max = 240, 311 nm max = 173, 192 nm


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Lycopene:

max = 114 + 5(8) + 11*(48.0-1.7*11) = 476 nm

max (Actual) = 474.


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Metal ion transitions

Degenerate

D-orbitals

of naked Co

D-orbitalsof hydrated Co2+

Octahedral Configuration

E


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Co2+

H2O

H2O

H2O

H2O

H2O

H2

O

Octahedral Geometry


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Instrumentation

Fixed wavelength instruments

Scanning instruments

Diode Array Instruments


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Fixed Wavelength Instrument

LED serve as source

Pseudo-monochromatic light source

No monochrometer necessary/ wavelength selection occurs

by turning on the appropriate LED 4 LEDs to choose from

photodyode

sample

beam of light

LEDs


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Scanning Instrument

cuvette

Tungsten

Filament (vis)

slit

Photomultiplier

tube

monochromator

Deuterium lamp

Filament (UV)

slit

Scanning Instrument


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sources

Tungten lamp (350-2500 nm)

Deuterium (200-400 nm)

Xenon Arc lamps (200-1000 nm)


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Monochromator

Braggs law, nl = d(sin i + sin r)

Angular dispersion, dr/d = n / d(cos r)

Resolution, R = / = nN,

resolution is extended by concave

mirrors to refocus the divergent beam at

the exit slit


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Sample holder

Visible; can be plastic or glass

UV; you must use quartz


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Single beam vs. double beam

Source flicker


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Diode array Instrument

cuvette

Tungsten

Filament (vis)

slit

Diode array detector

328 individual detectors

monochromator

Deuterium lampFilament (UV)

slit

mirror


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Advantages/disadvantages

Scanning instrument

High spectral resolution (63000), /

Long data acquisition time (several minutes)

Low throughput

Diode array

Fast acquisition time (a couple of seconds), compatible withon-line separations

High throughput (no slits)

Low resolution (2 nm)


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HPLC-UV

Mobile

phase

HPLC

Pump

syringe

6-port

valveSample

loop

HPLC

column

UV

detector

Solvent

waste

uvvis-spectroscopy p bambang

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