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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
 

Forms of Light Emission

Light emitted from an object as a result of its elevated temperature is called incandescence, however, light emitted from a body by processes other than high temperature emission is called Luminescence. When molecules are excited by electromagnetic radiation to produce Luminescence, the emitted light is called photoluminescence. If the release of electromagnetic energy is immediate, or stops on the removal of the exciting radiation, the substance is said to be fluorescent.If the release of energy is delayed, or persists after the removal of the exciting radiation, then the substance is said to be phosphorescent.

There are other forms of Luminescence. Light emitted from a gas discharge lamp (e.g. a neon lamp) is called Electroluminescence and light emitted during radioactive decay is called radio Luminescence. If light is emitted by a chemical reaction then this process is called chemiLuminescence and if the reaction has a biological origin (e.g. the firefly) then the light emission is called Bioluminescence.

Thus, the study of Fluorescence involves photoluminescence where light is absorbed by a body and re-emitted at a different wavelength, and when the incident light is arrested the emission of light stops.

Now the energy in a quantum of light of frequency ((ν) is given by,

E = h.ν = hc/λ (1)

Where (c) is the velocity of light,

(λ)is the wavelength of the light,

and (h) is Planks constant = 6.62 x 10-27 ergs/sec.

The size of a single quantum of light energy is inconveniently small and so the energy associated with the transition of (N) quanta is used where (N) is Avogadro’s number 6.02 x 1023 (the number of molecules in a gram molecule of the substance). The energy associated with the transition of a gram molecule is called an einstein. Thus, the number of einsteins required to effect a given transition will vary with the frequency of the radiation. The transition energies together with the type of transition in addition to other pertinent data are summarized in table 1.

Table 1 Approximate Values for Quanta Energies to promote certain Reactions and Transitions.

Analytical Fluorescence spectroscopy is largely confined to UV and visible regions of the Spectrum (although Raman emission might also be considered a form of Fluorescence or vice versa). The energy associated with this region is quite high and amounts to about 100 kilogram calories per einstein. Such energies, on absorption, may be sufficiently high to initiate chemical reaction or cause molecular break down (e.g. the effect of sunlight on certain dyestuffs).

When a molecule adsorbs a quantum of light, an energy transition occurs in the molecular orientation of the atom or molecule, and the wavelength at which this happens will be determined by the nature of the particular transition. The energy is usually dissipated and the electron eventually returns to its ground state. However, if the excess energy of the molecule in the higher energy state is not dissipated rapidly by collisions with other molecules, or by other means, the electron will return to the ground state with the emission of electromagnetic radiation in the form of Fluorescence. As some energy is inevitably lost before the emission occurs, the emitted fluorescent light is always of a longer wavelength than that absorbed on excitation (i.e. the quantum of light has less energy). Excellent discussions on the theoretical basis of Fluorescence have been given by Guilbault [3], Udenfriend [4] and Rhys Williams [5].

In the absorption of light and the emission of Fluorescence light the quantum efficiency

(υE) is defined as,

υE = (einsteins emitted)/(einsteins absorbed)
= (number of quanta emitted)/(number of quanta absorbed)
Clearly, this ratio can never exceed unity.

The fluorescent signal (IF) (that is the fluorescent light emitted) is given by

(2)


where (IF)

(φ)

is the emitted fluorescent light

is quantum yield (the ratio of the number of photons emitted to the number of photons absorbed),

(Io)

is the intensity of the incident light,

(c)

is the concentration of the solute,

(k)

is the molar absorbance,

and (l)

is the path length of the cell.




 

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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