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THE SPECTRO-PAEDIA

click on any item in the list for its wikipedia entry if available.


absorbance
adsorption
Atomization
bandwidth
Beamsplitters
bioluminescence
chemiluminescence
chromatography
electroluminescence
electromagnetic
emission
Emissivity
Fluorescence
luminescence
Michelson
monochromators
photo-multiplier
Phosphorescence
photodiodes
photoelectric
photoluminescence
Rayleigh
Raman
spectrofluorometer
spectrometer
spectrophotometer
Spectrum
Transmittance
ultraviolet
Visible
wavelength
Wavenumber
UV Light
Beer's Law
Diode Array
Deuterium Discharge Lamp
Low Pressure Zinc Discharge Lamp
High Pressure Mercury Discharge Lamp
Low Pressure Cadmium Lamp
Tungsten Halogen Lamp
Lens and Window Material in Spectrometers
Cuvettes
Luminescence
Photoluminescence
Fluorescence
Phosphorescence
Bioluminescence
Radioluminescence
Electroluminescence
Fluorescence Reagents
Spectrum
Diffraction Grating
Interferogram
Fourier Transform IR Spectrometer
FT-IR
Halide Disks
Mull Samples
Film Samples for IR Spectroscopy
Light Pipes
Attenuated Total Reflectance Spectroscopy
Multiple Internal Reflectance
External Reflectance
Specular Reflectance
Diffuse Reflectance
Photoacoustic Spectroscopy
Beam Splitter
Raman Scattering
Rayleigh Scattering
Raman Spectroscopy
Atomic Spectroscopy
Atomic Emission Spectroscopy
Atomic Absorption Spectroscopy
The Inductively Coupled Plasma Torch
The Helium Plasma Torch
Emission Spectrometer
Atomic Absorption Spectrometry
Flame Atomic Absorption Spectrometer
Flame AA
Hollow Cathode Lamp
Electrothermal Atomization
Graphite Furnace
L’vov Platform
Electron Paramagnetic Resonance
Zeeman Effect
Continuous Wave
Electron Paramagnetic Resonance
Pulsed EPR
Electron Spin Echo
Multple Resonance Spectroscopy
Magnetic Resonance Spectroscopy
NMR
Precessing
Nucleus Spin Decoupling in NMR
Superconducting Magnets
NMR Microcells
Electron Impact Ionisation
Chemical Ionization
Inductively Coupled Plasma Ionization
Secondary Ion Mass Spectrometry
Fast Atom Bombardment
Plasma Desorption Mass Spectrometry
Laser Desorption Mass Spectrometry
Matrix Assisted Desorption mass Spectrometry
Field Desorption Ionization
Thermospray Ionization
Electrospray Ionization
Atmospheric Pressure Ionization
Particle Beam Interface
Permeable Membrane Interface
Sector Mass Spectrometer
Quadrupole Mass Spectrometer
Ion Trap Mass Spectrometer
Time of Flight Mass Spectrometer
Optical RotationCircular Dichroism
Circularly Polarized Light
Verdet Constant
Faraday Effect
 

Raman Spectroscopy Instrumentation

Basically, the components of a Raman spectrometer comprise a light source, some collimating optics, a dispersing element such as a diffraction grating and a light sensor. The major problem associated with Raman spectroscopy is the very weak intensity of the scattered light, which virtually demands an extremely high-energy light source such as a laser. It follows, that the spectrometer must also have very high discrimination against internal scattered or stray light and the detecting device must have a very high sensitivity.

Originally there were basically two types of laser source for Raman Spectroscopy the Continuous Wave source and the pulsed gas ion laser source. Raman spectrometers originally utilized continuous wave argon or krypton gas lasers. The gas used was important as the Raman Scattering cross section varies as the fourth power of the light frequency. Thus, spectra obtained at 514.5 nm from an argon laser was from 2.3 to 4.5 times more intense than spectra obtained at 632.6 nm from a He-Ne laser operating at the same power. For additional power in the red region of the Visible Spectrum, dye lasers (pumped with a high power argon laser) were also employed to complement the basic gas lasers. Pulsed lasers in Raman spectrometers are not so common. The frequency doubled Nd/YAG laser with a fundamental at 1.06mm was used because it was powerful, stable and reliable. Although it can be used in the continuous mode it was more often used in the pulsed mode with pulse widths of 10ns and a peak power output of 500kW. Employing potassium tri-hydrogen phosphate and potassium di-hydrogen phosphate outputs of 532.0, 354.6 and 266,0 nm could be obtained. A synchronously pumped dye laser can also be used as a pulsed output system. Pulsed lasers were mostly employed for UV Raman measurements.

Today diode lasers are commonly employed to produce excitation units in Raman Spectroscopy and diode lasers having powers of 30 to 300 mW are readily available providing light at 785 and 830 nm. Tuneable lasers providing radiation between 670nm to 1100 nm using Ti sapphire lasers are also readily available. The great advantage of the diode laser is that it is monochromatic. The power used for the lasers, however, must be carefully controlled so as not to degrade the sample.

If the laser is not a solid state device it is usually focused on the sample using a long focal length lens that is anti-reflection coated for the laser line of interest. The spot size at the sample can be adjusted by changing the focal length of the lens. To simplify the collection process, the spot size is made as small as possible (if a solid state laser is used this, of course is unnecessary). In addition to the focusing lens, a polarization rotator and a line filter are usually incorporated. When used, the laser line filter is either a narrow band pass interference filter or a prism monochromator. The filter must have a high transmission at the selected laser line and high rejection at other wavelengths (more than 99.9% rejection).

The 90o and 180o Lens Collection Systems

The collection system for the Raman scattered light comprises of two lenses the first a very short focal length with a low f number to collect the largest possible solid angle and the second focuses the collected light onto the entrance slit of a monchromator. Most instruments operate with a 90o or 180o scattering geometry. The 90o system is the easiest to operate as the since the illumination axis and the collection axis are apart in space. The 90o system can be modified to the 180o system by placing a small mirror prism on the face of the first lens. The two systems are shown diagrammatically in figure 5.

The monochromator must disperse the scattered light across a slit for sequential presentation to the detector or across a solid-state diode array. The standard dispersing device in modern Raman spectrometers is the double monochromator or the dual grating system. The disadvantage of this system is the transmission loss and, as a consequence, the Diode Array system is now the more popular.

The single channel sensor system was originally the photomultiplier tube that must be designed to have an extended red response and a very low dark count. Direct current amplitude detection is now hardly ever used and photon counting systems now have a large dynamic range and can, thus, deal with a wide range of scattered light intensities.



Figure 5. The 90oand 180oLens Collection Systems

The multi-channel sensor system utilizes a Diode Array with a signal intensifier coupled to it. The only disadvantage of this system is that the resolution depends on the number of diodes and thus, the size of the diodes. However, with modern solid-state technology, the diodes can be made very narrow and, thus, the resolution fairly high.

 

About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.

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