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Can someone describe to me the difference between absorbance and
fluorescence and the reason of what I would choose one over the other ?
Many thanks,
Morgan
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In general terms, an absorbance measurement is a measurement of the
fraction of light that is absorbed by the analyte e.g. a transition to a
higher energy state in the molecule. For fluoresence, there is a process
after absorption in which collisional deactivation reduces the energy to
the lowest vibrational energy state of the excited single state followed
by loss of energy by emmision of light (the fluorescent light).
Fluoresence is usually more sensitive that absorbance (depending upon the
efficiency of the fluoresence process) but is also more dependent upon
solvent composition as well as other instrumental paramenters. A book
like Modern Optical Methods of Analysis would give more details.
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Absorbance occurs when some molecules of any substance absorbs light
energy. When light is absorbed, molecules go to higher state of energy.
Some chemical category of molecules when absorb light of higher
energy(lower wave length), they emit a lower energy of radiation (higher
wave length) and this phenomenon is called fluorescence.
Both absorbance and flourescence are useful to a larger extent in the
analysis of Pharmaceuticals or other chemicals. As the emitted energy is
very less in quantity when compared to absorbed energy, we can get very high
sensitive measurements in fluorescence.
Not all substances fluoresceand this makes the fluoresence measurements
more selective, but precision of measurements is less when compared
absorbance measurements. It is always better to use absorbance
measurements when sensitivity and selectivity are not a primary concern.
Madhu Chaluvadi
PharmPC Discussion List Archive Index page
David Bourne (david@boomer.org)