How is Raman Scattering Measured

The laser is directed onto the sample via a focusing lens, which in modern day systems is usually a microscope objective, often that of a commercially manufactured instrument (e.g., Olympus, Leica, Nikon). Such an instrument allows a selection of objectives from the turret with varying magnification and numerical aperture and, for example, long working distances. With an objective 

A comparison of spectral contributions from different illumination/col-lection optics. The spectra were excited from a clean chromium surface at 830 nm excitation (32mW) and with an integration time of 30 s. 

Spectrometer lengths are typically 300 mm for medium resolution or 800 mm for high resolution systems. The dispersion per pixel is thus typically 1 cm for a 300 mm length spectrometer with an 1800 mm grating operating at 633 nm and can be as low as 0.25 cm for a high resolution system. The resolution of 

A further feature of modern Raman spectroscopic microscopes is that the laser is polarized, allowing determination of, for example, depolarization ratios, and molecular orientation in crystals or liquid crystals. Care must be taken to account for the polarization response of the vertically ruled diffraction grating however. To date, there have been few or no polarization dependent studies of biological materials although recent studies have demonstrated that polarization dependent Raman can detect structural changes in the extracellular matrix associated with basal cell carcinoma.  

The above describes the commonly utilized instrumentation required for dispersive Raman spectroscopic microscopy. In the past, Fourier transform Raman spectroscopy provided an alternative for coloured and fluorescent samples but the use of near-IR lasers at 1.064 pm together with In Ga As detectors reduced the sensitivity. Recent developments in laser rejection Alters and CCD technologies have rendered dispersive techniques the preferred option. 

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