seminar

Question 1

Transmission

FT-IR technique

Answer 1

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

Question 2

Reflection FT-IR technique

Answer 2

• 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

Question 3

FT-IR reflection techniques

Answer 3

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)

Question 4

ATR

Answer 4

• 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

Question 5

Diffuse reflectance

Answer 5

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

Question 6

Specular reflectance

Answer 6

• Sample must be reflective or on a reflective surface

- Information provided is from the thin layers

Question 7

Attenuated total reflectance

Answer 7

• 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

Question 8

SUMMARY OF ATR

Answer 8

• 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

Question 9

Specular vs diffuse reflection

Answer 9

• 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

Question 10

Specular reflection = external reflection spectroscopy

Answer 10

• 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

Question 11

Specular reflection summary

Answer 11

• 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

Question 12

Diffuse reflection- diffuse reflectance infrared fourier transform spectroscopy

Answer 12

• 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

Question 13

DRIFTs sample preparation

Answer 13

• 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

Question 14

Diffuse reflection suited for

Answer 14

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

Question 15

FT-IR reflection techniques

Summary

Answer 15

• 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