Near-IR excited FT Raman spectroscopy

Near-IR excited FT Raman spectroscopy was developed only recently (1986). Within a short time, however, it has become a very useful technique and is well suited to studies of research and industry samples.

The reason why FT Raman is so successful compared to conventional Raman spectroscopy (visible laser excitation) is that:

(1) for most samples, spectra are free of fluorescence, so that it’s applicable to many samples that could not be examined by conventional Raman spectroscopy,
(2) spectra can be acquired rapidly, and
(3) spectral subtraction is accurate.

In the near-ill excited FT Raman, sample fluorescence is suppressed (or eliminated) due to sample excitation at 1064 nm where most materials do not absorb. Spectra are obtained rapidly due to the well known signal-to-noise advantages associated with FT instruments.

FT RAMAN SPECTROSCOPY

Both conventional and FT Raman spectroscopies are based on the same principle. FT Raman differs from conventional Raman in two important ways;

(1) the laser wavelength used to excite samples lies in the near-IR and
(2) instead of using dispersive gratings a Michelson interferometer is used to analyze scattered light.

An FT Raman instrument consists of the following components;
(1) a laser (most often a 1064 nm Nd:YAG laser) for sample excitation,
(2) one or more filters to effectively block the Rayleigh scattering,
(3) an efficient interferometer,
(4) a highly sensitive detector, and
(5) a capability to do a fast Fourier-transform on an acquired interferogram.

The laser chosen is usually a near-IR laser to avoid any sample fluorescence that might arise. An FT instrument is built around an interferometer. Such an instrument has several advantages. Two of the most important advantages associated with the FT approach are Jacquinot and Felgett (also known as “multiplex”) advantages. The latter advantage allows simultaneous detection of all the wavelengths of light and is the primary reason why an FT instrument records a spectrum in a shorter time than a grating instrument. The high throughput advantage of the inteferometer is called the Jacquinot advantage. The twin advantages more than offset the loss in scattering efficiency as a result of longer wavelength excitation (compared to visible). An additional advantage of FT Raman spectroscopy is the accuracy of the wavenumber values in a spectrum. This is important when spectra are to be subtracted.

to be continued....