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Application of IR Raman Spectroscopy

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Raman Spectroscopy 3 IR regions Structure and Functional Group Absorption IR Reflection IR Photoacoustic IR IR Emission Micro Mid-IR Mid-IR absorption Samples Placed ... – PowerPoint PPT presentation

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Title: Application of IR Raman Spectroscopy


1
Application of IRRaman Spectroscopy
  • 3 IR regions
  • Structure and Functional Group
  • Absorption IR
  • Reflection IR
  • Photoacoustic IR
  • IR Emission
  • Micro

2
Mid-IR
  • Mid-IR absorption
  • Samples
  • Placed in cell (salt)
  • Combined with oil
  • Need cell that does not absorb IR
  • KBr, NaCl
  • Tends to absorb water
  • Gases
  • Solutions
  • Solvent issues
  • Dissolution of cell

3
Analysis
  • Can calculate group frequencies
  • C-H, CO, CC, O-H
  • Variations of frequencies for group
  • Fingerprint region
  • Compare to standards
  • Absorption of inorganics
  • Sulphate, phosphate, nitrate, carbonate
  • Search spectra against library

4
Mid-IR
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Interpretation
  • Alcohols and amines display strong broad O-H and
    N-H stretching bands in the region 3400-3100 cm-1
  • bands are broadened due to hydrogen bonding and a
    sharp 'non-bonded' peak can around 3400 cm-1 .
  • Alkene and alkyne C-H bonds display sharp
    stretching absorptions in the region 3100-3000
    cm-1
  • bands are of medium intensity often obscured
    (i.e., OH).
  • Triple bond stretching absorptions occur in the
    region 2400-2200 cm-1
  • Nitriles are generally of medium intensity and
    are clearly defined
  • Alkynes absorb weakly unless they are highly
    asymmetric
  • symmetrical alkynes do not show absorption bands
  • Carbonyl stretching bands occur in the region
    1800-1700 cm-1
  • bands are generally very strong and broad
  • Carbonyl compounds (acyl halides, esters) are
    generally at higher wave number than simple
    ketones and aldehydes
  • amides are the lowest, absorbing in the region
    1700-1650 cm-1
  • Carbon-carbon double bond stretching occurs in
    the region around 1650-1600 cm-1
  • bands are generally sharp and of medium intensity
  • Aromatic compounds will display a series of sharp
    bands
  • Carbon-oxygen single bonds display stretching
    bands in the region 1200-1100 cm-1
  • bands are generally strong and broad

9
Quantitative IR
  • Difficult to obtain reliable quantitative data
    based on IR
  • Deviations from Beers law
  • Narrow Bands and wide slit widths required
  • Require calibration sources
  • Complex spectra
  • Weak beam
  • Lack of reference cell
  • Need to normalize refraction
  • Take reference and sample with same cell

10
Other methods
  • Reflectance IR
  • Measurement of absorbance from reflected IR
  • Surface measurement
  • Photoacoustic IR
  • can use tunable laser
  • Near IR
  • 700 nm to 2500 nm
  • Quantitative analysis of samples
  • CH, NH, and OH
  • Low absorption
  • Emission IR

11
Raman Spectroscopy
  • Scattering of light
  • Fraction of scattered light in the visible
    differs from incident beam
  • Difference based on molecular structure
  • Based on quantized vibrational changes
  • Difference between incident and scattered light
    is in mid-IR region
  • No water interference
  • Can examine aqueous samples
  • Quartz or glass cells can be used
  • Competition with fluorescence

12
Raman Spectroscopy
  • Theory
  • Instrumentation
  • Application
  • Method
  • Excitation with UV or NIR
  • Measurement of scatter at 90
  • Measurement 1E-5 of incident beam

13
Theory
  • 3 types of scattered radiation
  • Stokes
  • Lower energy than Anti-Stokes
  • Named from fluorescence behavior
  • More intense
  • Used for Raman measurements
  • Anti-Stokes
  • No fluorescence interference
  • Rayleigh
  • Most intense
  • Same as incident radiation
  • Shift patterns independent of incident radiation
    wavelength

14
Theory
  • Excitation
  • From ground or 1st vibrationally excited state
  • Population of excited state from Boltzmanns
    equation
  • Molecule populates virtual states with energy
    from photon
  • Can be effected by temperature
  • Elastic scattering is Rayleigh
  • Energy scatteredenergy incident
  • Energy difference due to ? ground and 1st excited
    state
  • hn-DE is Stokes scattering
  • HnDE is anti-Stokes scattering

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Theory
  • Variation in polarizability of bond with length
  • Electric field (E) due to excitation frequency
    with E0
  • Dipole moment (m) based on polarizability of bond
    (a)
  • For Raman activity a must vary with distance
    along bond
  • a0 is polarizability at req

17
Theory
  • Equation has Rayleigh, Stokes, and Anti-Stokes
    component
  • Complementary to IR absorbance
  • Overlap not complete

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Instrumentation
  • Laser source
  • Ar (488 nm, 514.5 nm)
  • Kr (530.9 nm, 647.1 nm)
  • He/Ne (623 nm)
  • Diode (782 nm or 830 nm)
  • Nd/YAG (1064 nm)
  • Tunable lasers
  • Intensity proportional to n4
  • Consider energy and chemical effect of absorbing
    energy

20
Instrumentation
  • Sample holder
  • Glass
  • Laser focusing allows small sample size
  • Liquid and solid samples can be examined
  • Use of fiber optics

21
Applications
  • Laser microprobes
  • Use of laser permits small sampling area
  • Resonance Raman
  • Use electronic absorption peak
  • Low concentrations can be examined
  • Lifetimes on 10 fs
  • Surface enhanced Raman
  • Increase of sensitivity by 1000 to 1E6

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