seminar Flashcards

1
Q

Transmission

FT-IR technique

A
  • Excellent for solids, liquids and gases
  • The reference method for quantitative analysis
  • Sample preparation can be difficult
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2
Q

Reflection FT-IR technique

A
  • Collect light reflected from an interface
    Air/sample, solid/sample, liquid/sample
  • Analyse liquids, solids, gels or coatings
  • Minimal sample preparation
  • Convenient for qualitative analysis, frequently used for quantitative analysis
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3
Q

FT-IR reflection techniques

A

Internal reflection spectroscopy- attenuated total reflection ATR

External reflection spectroscopy- specular reflection (smooth surfaces)

Combination of internal and external reflection- diffuse reflection (DRIFTs) (rough surfaces)

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4
Q

ATR

A
  • Infrared beam reflects from a interface via total internal reflectance
  • Sample must be in optical contact with the crystal
  • Collected information is from the surface
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5
Q

Diffuse reflectance

A
  • Solids and powders diluted in a IR transparent matrix if needed
  • Information provided is from the bulk matrix
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6
Q

Specular reflectance

A
  • Sample must be reflective or on a reflective surface

- Information provided is from the thin layers

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7
Q

Attenuated total reflectance

A
  • Introduce in 1960s
  • Light introduced into a suitable prism at an angle exceeding the critical angle for internal reflectance- an evanescent wave at the reflecting surface
  • Sample is in close contact with IRE
  • From the interaction of the evanescent wave (exponential decay) with the sample, a spectrum can be recorded with little or no sample preparation
  • IRE- internal reflection element = ATR crystal
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8
Q

SUMMARY OF ATR

A
  • Versatile and non destructive technique for variety of materials- soft solid materials, liquids, powders, gels, pastes, surface layers, polymer films, samples after evaporation of a solvent
  • Requires minimal or no sample preparation
  • Useful for surface characterization, opaque samples
  • Limitation- sensitivity is typically 3-4 orders of magnitude less than transmission
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9
Q

Specular vs diffuse reflection

A
  • Specular reflection is defined as light reflected from a smooth surface ( such as mirror, any irregularities in the surface are small compared to lamda) at definite angle
  • Diffuse reflection is produced by rough surfaces that tend to reflect light in all directions. There are far more occurances of diffuse reflection then specular reflection in our everyday environment
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10
Q

Specular reflection = external reflection spectroscopy

A
  • Introduced in the 1960s, much wider in 1970s
  • Light is reflected from a smooth (mirror-like) sample at a definite angle to record its spectrum
  • Spectroscopic technique for films deposited on or pressed against reflective surfaces
  • If a surface absorbs a wavelength of light => its relative intensity is decreased
    • Angle of incidence= angle of reflection
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11
Q

Specular reflection summary

A
  • Non destructive measurement of thin layers of monolayers
  • Coating on polished metals
  • Surface characterization
  • Limitation- spectra depend on refractive index
  • Used for testing lubricated surfaces of hard disks, to degradation studies of a protective coating on the surface, analysis of polymers on the surface of food containers and many others
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12
Q

Diffuse reflection- diffuse reflectance infrared fourier transform spectroscopy

A
  • Spectra powders and rough surfaces can be recorded by illuminating these surfces and collecting sufficient scattered radiation with ellipsoids and paraboloids
  • Fast measurement of powdered samples
  • Low repeatability of spectral data
  • Complicated physical description of the effect- shape of particles, compactness of samples. Refractive index of particles. Reflectivity and absorption characteristics of particles
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13
Q

DRIFTs sample preparation

A
  • Powder simply placed into the sample cup and analysed
  • If the sample is to absorbent, it must be diluted in a non-absorbent matrix (KBr, KCl…)
  • No pellet pressing required
  • Particle size smaller than 10 micrometers (i.e nont exceeding the wavelength of the incident radiation) preferred
  • Or paper disc coated on a surface by SiC as an abrasive material-> grinding of sample
  • Ideal for pharmaceutical and forensic application
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14
Q

Diffuse reflection suited for

A
  • Highly diluted samples
  • Small particle samples
  • The scattering coefficient is a function of sample size and packing
  • Sample layer of at lease 1.5mm
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15
Q

FT-IR reflection techniques

Summary

A
  • ATR- structural information from the surface- thick layers
  • Specular reflection (smooth surfaces)- measurement of think layers or monolayers (coatings on metals, surface characterization)
  • DRIFTS (rough surfaces)- structural information is from the bulk matrix
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