Raman spectroscopy is a technique of probing matter that requires little to no sample preparation. In a Raman spectrometer, a monochromatic light (near UV, visible, or near IR) illuminates a sample or analyte. Although the majority of light scatters from the molecules elastically (Rayleigh scattering), a small fraction scatters inelastically, causing the scattered light to have either a longer wavelength compared to the incident light (Stokes shift), or shorter (Anti-Stokes shift). Both Stokes and Anti-Stokes shifts provide information on the vibrational states of the molecule responsible for the scattering of the incident light. Because these states are unique to a molecule, Raman spectrum provides information on the chemical composition of the sample.
Raman spectroscopy has a number of significant advantages over other techniques. Because the technique requires no, or minimal, sample preparation, the Raman spectrometer can be implemented as a portable, handheld, device allowing for a “point and shoot” spectrum acquisition. The device can use visible light to probe infrared molecular transitions, eliminating the need for a more expensive infrared light source and infrared image sensor. The absence of Raman scattering by water is another crucial advantage, especially useful in obtaining spectra of biological samples. These advantages make Raman spectrometers widely used in applications such as detection of controlled substances in forensics, authentication of artwork, and “through package” quality control of pharmaceuticals.
Raman signal is very weak because only 1 in about 10 million photons scatters inelastically. However, the signal can be increased if the energy of the excitation light is close to the energy of the electronic transitions in the molecule (resonant Raman scattering) or if the analyte is near a rough surface of a noble metal, such as silver or gold [surface-enhanced Raman scattering (SERS)]. Surface-enhancement of the Raman signal is large enough for detection and identification even of a single molecule. SERS has been used, for example, in the detection of cancer cells in vivo, bacterial and viral infections, and offending molecules (food allergens) in foods.
A laser light source with narrow spectral width and high wavelength stability is used as a light source for Raman spectroscopy.
The spectrometer disperses Raman scattered light. A polychromator with a diffraction grating is typically used.
Since Raman scattered light is extremely faint, a detector with high sensitivity is required. It is also beneficial to simultaneously detect the multiple wavelengths that are dispersed, and for this reason, a linear image sensor is used. The appropriate sensor needs to be selected to match the measurement wavelength (wavenumber) range by considering the wavelength of the laser light source.
Raman spectrum of polystyrene plate (using spectroscopic module C13560 without SERS substrate)
Hamamatsu offers a lineup of Raman spectrometers for “normal” and surface-enhanced applications. Every unit features an image sensor (CCD or CMOS), dispersive grating, and necessary optics. The spectrometer for the surface-enhanced applications comes with the built-in excitation laser. All of the spectrometers are fully equipped to operate with a computer. In addition, Hamamatsu offers individual components, such as image sensors and light sources, to original equipment manufacturers.
The mini-spectrometers are provided in a compact, thin case that houses optical elements, an image sensor, and a driver circuit. Spectrum data can be acquired by guiding measurement light into a mini-spectrometer through an optical fiber and transferring the measured results to a PC via the USB connection. Several types are available for Raman spectroscopy.
This is an InGaAs linear image sensor designed for Raman spectroscopy measurement using a 1064 nm laser. Designed specifically for measuring the Raman spectral range, the cutoff wavelength has been reduced from that of the previous product (G11508-512SA) to achieve low dark current.
These CCD linear image sensors can be used in Raman spectrometers that use 532 nm or 785 nm lasers.
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