Technology

Spectrometer technology

NIR FT Raman

Raman microscopy

Avoiding fluorescence

 

 

 

Instrumental Setup

Typically, a Raman spectrometer contains at least 4 units:

A high sensitivity is obtained by using the intense light of a focused laser beam, a good collection efficiency for the Raman scattered light, high throughput optics and a sensitive detector unit. A computer controls the complete system. Moreover, it displays the Raman spectra or directly the result of a spectral analysis.

Lasers and detectors

Raman spectra can be obtained by illuminating a sample with lasers emitting at almost any wavelength. The wavelength is chosen due to the availability of lasers and the required sensitivity for the analysis (the Raman scattering intensity is inversely proportional to 4th power of the excitation wavelength). Moreover, using selected wavelengths the resonance Raman effect can be utilized or fluorescence can be avoided.

Lasers cover the complete wavelength range from the ultraviolet (UV) and visible (VIS) to the near infrared (NIR). For a high sensitivity of the Raman spectrometer appropriate detectors have to be used. For the visible range, these are typically Silicon based CCD detectors. In the NIR, Raman spectra are detected by Indium Gallium Arsenide (InGaAs) or Germanium (Ge) single point detectors.

FT Raman or dispersive Raman spectroscopy?

While using NIR lasers, the Raman scattering intensity is reduced at least by a factor of 1⁄4. Additionally, InGaAs or Ge detectors are less sensitive than CCD detectors for the detection of the inherently low intensity of the Raman scattering. Interferometer based spectrometers can partly compensate this loss by a higher throughput compared to dispersive instruments that are based on gratings or filters. An interferometer provides an interferogram that can be converted by a Fourier transformation (FT) into a Raman spectrum.

Usually, the dispersive CCD/diode laser systems are preferred for dilute components of mixtures and solutions, weak scatterers and thermolabile samples. However, the FT Raman systems offer a better wavelength accuracy, larger spectral range, higher resolution and generally they have an ultimately lower fluorescence interference, especially when excited at 1064 nm.

Thus, the choice of the spectrometer type depends on the sample.