SPECTROSCOPY Light interacting with matter as an analytical tool
Dec 19, 2015
SPECTROSCOPY
Light interacting with matter as an analytical tool
X-ray: core electron excitation
UV: valance electronic excitation
IR: molecular vibrations
Radio waves:Nuclear spin states(in a magnetic field)
Electronic Excitation by UV/Vis Spectroscopy :
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, elemental
X-ray Crystallography X-rays 3-D structure
Spectroscopic Techniques and Common Uses
UV-vis UV-vis regionQuantitative
analysis/Beer’s Law
Atomic Absorption UV-vis regionQuantitative analysis
Beer’s Law
FT-IR IR/Microwave Functional Group Analysis
Raman IR/UVFunctional Group
Analysis/quant
FT-NMR Radio waves Structure determination
X-Ray Spectroscopy X-rays Elemental Analysis
X-ray Crystallography X-rays 3-D structure Anaylysis
Different Spectroscopies
• UV-vis – electronic states of valence e/d-orbital transitions for solvated transition metals
• Fluorescence – emission of UV/vis by certain molecules
• FT-IR – vibrational transitions of molecules• FT-NMR – nuclear spin transitions• X-Ray Spectroscopy – electronic transitions
of core electrons
Quantitative Spectroscopy
• Beer’s Law
Al1 = el1bc
e is molar absorptivity (unique for a given compound at l1)
b is path length
c concentration
Beer’s 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
sourceslit
detector
Characteristics of Beer’s 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
Standard Practice
• Prepare standards of known concentration
• Measure absorbance at max• Plot A vs. concentration• Obtain slope • Use slope (and intercept) to determine
the concentration of the analyte in the unknown
Typical Beer’s Law Plot
y = 0.02x
0
0.20.4
0.6
0.81
1.2
0.0 20.0 40.0 60.0
concentration (uM)
A
UV-Vis Spectroscopy
• UV- organic molecules– Outer electron bonding transitions– conjugation
• Visible – metal/ligands in solution– d-orbital transitions
• Instrumentation
Characteristics of UV-Vis spectra of Organic Molecules
• Absorb mostly in UV unless highly conjugated
• Spectra are broad, usually to broad for qualitative identification purposes
• Excellent for quantitative Beer’s Law-type analyses
• The most common detector for an HPLC
Molecules have quantized energy levels:
ex. electronic energy levels. en
ergy
hv
ener
gy
}= hv
Q: 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.
Broad spectra
• Overlapping vibrational and rotational peaks
• Solvent effects
Molecular Orbital Theory
• Fig 18-10
2s 2s
2p 2pn
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
C C
hv
Example: ethylene absorbs at longer wavelengths:max = 165 nm = 10,000
= hv =hc/
hv
n
n
C O
n
The n to pi* transition is at even lower wavelengths but is not as strong as pi to pi* transitions. It is said to be “forbidden.”Example:
Acetone: nmax = 188 nm ; = 1860nmax = 279 nm ; = 15
C C
C C
C O
C OH
135 nm
165 nm
n183 nm weak
150 nmn188 nmn279 nm weak
A
180 nm
279 nm
C O
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 ; = 21,0001,3,5-hexatriene max = 258 nm ; = 35,000
O
O
O
Similar structures have similar UV spectra:
max = 238, 305 nm max = 240, 311 nm max = 173, 192 nm
Lycopene:
max = 114 + 5(8) + 11*(48.0-1.7*11) = 476 nm
max(Actual) = 474.
Metal ion transitions
Degenerate D-orbitalsof naked Co
D-orbitalsof hydrated Co2+
Octahedral Configuration
E
Co2+
H2O
H2O
H2OH2O
H2O
H2O
Octahedral Geometry
Instrumentation
• Fixed wavelength instruments
• Scanning instruments
• Diode Array Instruments
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
Scanning Instrument
cuvette
Tungsten Filament (vis)
slit
Photomultiplier tube
monochromator
Deuterium lampFilament (UV)
slit
Scanning Instrument
sources
• Tungten lamp (350-2500 nm)
• Deuterium (200-400 nm)
• Xenon Arc lamps (200-1000 nm)
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
Sample holder
• Visible; can be plastic or glass
• UV; you must use quartz
Single beam vs. double beam
• Source flicker
Diode array Instrument
cuvette
Tungsten Filament (vis)
slit
Diode array detector328 individual detectors
monochromator
Deuterium lampFilament (UV)
slit
mirror
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 with on-line separations– High throughput (no slits)– Low resolution (2 nm)
HPLC-UV
Mobile phase
HPLC Pump
syringe
6-port valveSample
loop
HPLC column
UV detector
Solvent waste