What factors affect spectral resolution in a Raman spectrometer?

Spectral resolution in a dispersive Raman spectrometer is determined by four main factors.  In the discussions below, the effect of each factor is considered under the assumption that all other factors remain unchanged.  In real life all of these factors can exist in many varied permutations, which makes direct comparison of a system’s performance and capabilities difficult.

  • Spectrometer focal length – the longer the focal length (e.g., the distance between the dispersing grating and detector) of the spectrometer the higher the spectral resolution.  Typical Raman spectrometers have focal lengths ranging from 200mm (for low/medium resolution) through to 800mm and higher (for high resolution).  It is sometimes forgotten that a long focal length spectrometer is not limited to high resolution work only – with suitable choice of grating (see below), a high resolution spectrometer can be run in a low resolution mode.  In this way, it is ideally suited for low/medium resolution analysis for routine screening, and yet can also offer high resolution analysis for more specialised applications.
  • Diffraction grating – the higher the groove density of the grating (typically measured as number of grooves per millimetre), the higher the spectral resolution.  Typical gratings used for Raman vary from perhaps 300gr/mm (low resolution) through to 1800gr/mm (high resolution) – more specialised gratings (including 2400gr/mm and 3600gr/mm) are also available, but have certain limitations, and should not be considered general purpose.  The use of higher groove density gratings cannot be applied ad infinitum to increase spectral resolution, since they will have fixed practical and physical limits linked with the spectrometer itself.  Thus gratings provide an initial way to improve resolution, but once their limit is reached it is necessary to move to a longer focal spectrometer.
  • Laser wavelength – the dispersing power of a grating/spectrometer pair can usually be considered constant in terms of wavelength.  However, Raman spectra use an energy related unit (Raman shift, or wavenumber, cm-1) which means that the spectral resolution decreases as the laser excitation is changed from infra-red to visible to ultra-violet wavelengths.  As an example, if a 600gr/mm grating is used with an infra-red laser, a 1200gr/mm or 1800gr/mm will be required with a green laser to achieve a similar resolution.
  • Detector – most systems have a single detector, so practically the user does not have control of this factor.  However, it should be noted that different detectors can be configured with different pixel sizes.  The smaller the pixel the higher the achievable spectral resolution
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