Ramen Spectroscopy

Question 1

What is Raman Spectroscopy

Answer 1

Raman spectroscopy is a spectroscopic technique typically used to determine vibrational modes of molecules

Question 2

How does Raman scattering arise ?

Answer 2

• Raman scattering arises as a result of exchange of energy between light and matter.
  - Light from an incident laser source is inelastically scattered and shows up
  as a shift in frequency.
  - The energy associated with this shift in frequency is that of a molecular
  vibration.
  - A spectrum of intensity against wavelength difference shows peaks for
  each mode of vibration (similar to an infrared spectrum).

Question 3

Describe the Raman experiment steps (3)

Answer 3

• A sample is irradiated with monochromatic light (much like a laser pointer)
• The inelastically scattered light is collected (the small proportion which has changed wavelength, or energy)
• The intensity collected as a function of frequency difference gives a vibrational spectrum.

Question 4

The Raman experiment explained

Answer 4

• Most of the radiation has the same frequency as the incident radiation & is called Rayleigh radiation.
• A small fraction of the light sets molecules into vibration. The energy needed to do this comes from the incident radiation, thus its frequency (energy) changes.
• Bands will be observed at the different energies associated with the different modes of vibration of molecules in the sample.

Question 5

What does Raman spectra show

Answer 5

• Raman spectra can be very complex – some features such as C=C, CH3 or CH2 stretching may be identified, but frequently the spectrum is used as a fingerprint.

Question 6

How are mixtures observed in Raman Spectra

Answer 6

• Mixtures are observed as the sum of the spectra of contributing (non-interacting) components, in proportion with their relative concentrations.

Question 7

Mixture vs pure spectra

Answer 7

• Changes as a result of component interactions will also be distinguishable when compared to spectra of the pure components.

Question 8

Advantages of Raman spectroscopy

Answer 8

• Minimal sample preparation
• Very general
• Rich in information
• Aqueous samples

Question 9

Disadvantages of Raman Spectroscopy

Answer 9

• Effect is weak
• Interference from fluorescence
• Expensive

Question 10

Applications of Raman Spectra

Answer 10

• Compound identification (drugs, excipients, mixtures)
• Reaction monitoring (organic reactions, enzyme inhibitor activity)
• Identification of crystalline polymorphs
• Polymer analysis (e.g. backbone conformation)
• Molecular (and polymer) interactions