What is spectral resolution, and when is it needed?

Spectral resolution is the ability to resolve spectral features and bands into their separate components.  The spectral resolution required by the analyst or researcher depends upon the application involved.  For example, routine analysis for basic sample identification typically requires low/medium resolution.  In contrast, characterisation of polymorphs and crystallinity often requires high resolution, since these phenomena exhibit only very subtle changes in the Raman spectrum, which would not be visible in a low resolution experiment.

Three spectra recorded at low, medium and high spectral resolution, illustrating how the high resolution mode yields sharper peaks, and separates close lying peaks, which are merged together at low resolution
Three spectra recorded at low, medium and high spectral resolution, illustrating how the high resolution mode yields sharper peaks, and separates close lying peaks, which are merged together at low resolution

Spectral resolution is an important experimental parameter.  If the resolution is too low, spectral information will be lost, preventing correct identification and characterisation of the sample.  If the resolution is too high, total measurement time can be longer than necessary.  What makes resolution “too low” or “too high” depends upon the particular application, and what information is desired from the experiment.

Typically, low/medium resolution is suitable for basic chemical identification, and distinguishing different materials.  Higher resolution becomes necessary to characterise more subtle spectral features – for example, minor changes in the shape or position of a peak.  There are a number of chemical phenomena which cause such subtle spectral changes:

  • Crystallinity – in general Raman peaks become sharper and more intense as a material moves from an amorphous to crystalline structure.  High spectral resolution allows even very small changes in crystallinity to be characterised.
  • Polymorphism – polymorphs are materials which have the same chemical formula but differing solid state forms.  Because the chemical formula is identical, their overall Raman spectral profiles are similar, but the influence of the solid form on individual bond vibrations causes subtle changes in the spectrum.  Thus, it is not unexpected to observe minor changes in peak shapes and positions throughout the spectrum.
  • Intrinsic stress/strain – some materials (most notably semiconductors) display changes in their Raman/photoluminescence spectra when subjected to stress and strain.  In the case of semiconductors, stress/strain is carefully induced within the material to produce semiconducting and/or luminescent properties necessary for use of that material in working devices.  Raman is used as a vital tool to characterise the stress/strain, but often the effect on the spectrum is subtle – thus, high resolution is necessary.
  • Hydrogen bonding – the weak inter- and intramolecular interactions caused by hydrogen bonding can cause small changes in a Raman spectrum, and with high resolution capability the spectrum can be used to investigate such interactions.
  • Protein folding – a protein’s primary structure will of course have a significant impact on the resulting Raman spectra, but the secondary and tertiary structures comprising localised and more widespread folding cause sufficient perturbation to the vibrational modes to affect the spectrum.  Once again, the effect will be subtle, and only high resolution analysis will allow its characterisation.
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