MOLECULAR EMISSION SPECTROSCOPY: RAMAN SPECTROSCOPY

RAMAN SPECTROSCOPY

• more of light-scattering than emission, but it is discussed here to be differentiated from Fluorometry.

PRINCIPLE

The Raman effect is analogous to fluorescence except:

• It is NOT wavelength-dependent (as we use wavenumber = same as IR spectrum) 

• Does NOT require the molecule to have a chromophore 

• Energy shift in cm^-1is measured instead of wavelength

PRINCIPLE

• The shifts measured correspond to the wavenumbers of the bands present in the middle-IR spectrum of the molecule.

INSTRUMENTATION

• lasers are used to provide high-intensity radiation in the visible region, generally somewhere between 450 and 800 nm (NIR lasers) (can use the near IR laser) 

  • it does not excite fluorescence in molecules 

  • good penetration properties

COMPARISON OF FT-RAMAN AND FT-IR SPECTRA

• bands that absorbed weakly in middle -IR region will absorb strongly in the Raman region and vice versa 

• they provide complementary information

Applications

• similar to IR: Has potential for identifying complex samples, e.g. drugs in formulations and in pack (in packaging)

• Samples such as peptide pharmaceuticals (proteins as drugs) can be analysed for changes in their three-dimensional structure 

• Provides additional fingerprint identity information complementary to middle-IR spectroscopy

Strengths

• Complementary to middle-IR spectroscopy but requires very little sample preparation since near-IR (NIR) radiation with its good penetration properties can be used for the analysis 

• Increasingly a readily available option on middle-IR FT-IR instruments.

Limitations

• Not yet fully established as a quantitative technique; only for identification

• The solvent may interfere if samples are run in solution.