13.19 : Raman Spectroscopy Instrumentation: Overview

A conventional Raman spectrophotometer includes a laser source, a sample holding system, a wavelength selector, and a detector.

The monochromatic laser source, typically using visible or near-infrared radiation, generates a highly focused beam of light. This light interacts with the molecules of the sample, scattering some of the light. Liquid and gaseous samples are usually tested in ordinary glass capillaries, while solids can be analyzed as powders packed in capillaries or as potassium bromide pellets. The scattered light is collected using a separate lens and focused onto the entrance of a monochromator, which disperses the light into its constituent frequencies.

To ensure accurate results, the output is filtered extensively to remove stray laser radiation and Rayleigh scattering, which can interfere with the Raman signal. The optical signal is then converted into an electrical signal within the detector, often a charge-coupled device or photomultiplier tube, allowing it to be processed and visualized as a Raman spectrum.

In some cases, high-quality bandpass and notch filters are used in fiber-optic Raman spectrometers to minimize Rayleigh-scattered radiation. Another variation, the Fourier Transform Raman instrument, replaces the monochromator with a Michelson interferometer and uses a continuous-wave laser. After passing through the filters, the radiation is focused onto a cooled germanium detector for analysis.