What is confocal Raman microscopy?

Raman microscopy couples a Raman spectrometer to a standard optical microscope, allowing high magnification visualisation of a sample and Raman analysis with a microscopic laser spot.  Raman microscopy is easy: simply place the sample under the microscope, focus, and make a measurement.

Just adding a microscope to a Raman spectrometer does not give a controlled sampling volume - for this a spatial filter is required.  Confocal Raman microscopy refers to the ability to spatially filter the analysis volume of the sample, in the XY (lateral) and Z (depth) axes.

There are several methods in use today (for example, true confocal aperture, or pseudo confocal slit-binning techniques) and some are better than others.  However, it is well established that by using a true confocal Raman microscope, it is possible to analyse individual particles or layers with dimensions down to 1 µm and below.

For a true confocal design, the limits of spatial resolution are defined principally by the laser wavelength and quality of the laser beam that is used and the type of microscope objective selected and so on. For the highest spatial resolution, a correctly matched high magnification objective and visible laser excitation will often produce the optimum results.  Typical spatial resolution is in the order of 0.5-1 µm.

A confocal Raman spectral image of 0.9 µm polystyrene beads.
A confocal Raman spectral image of 0.9 µm polystyrene beads.
Cross section of a layered polymer structure, showing the capability of confocal Raman microscopy for analysis of µm thick layers. Total scan depth (Z) is 10 µm.
Cross section of a layered polymer structure, showing the capability of confocal Raman microscopy for analysis of µm thick layers. Total scan depth (Z) is 10 µm.
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