Fluorolog-3 v. 2.2 (12 Jul 2002)
Glossary
16-4
dent radiation is constant in energy units (cm
–1
).
Rayleigh scattering
Light scattering from particles whose dimensions are much smaller
than the wavelength of incident light. The scattered light is of the
same energy as the incident light. Rayleigh scatter shows scatter ra-
diation intensity inversely proportional to the 4
th
power of the wave-
length of incident radiation.
Rayleigh-Tyndall
scattering
Combination of Rayleigh and Tyndall scatter. These two scattering
phenomena cannot be separated. If the molecule’s Stokes shift is
small, Rayleigh-Tyndall scatter will limit the ultimate resolution.
Red-sensitive
photomultiplier
A detector that extends fluorescence detection to 850 nm.
Reference photodiode
Detector used to monitor the output of the xenon lamp.
Resolution
The ability to separate two closely spaced peaks. Resolution can be
improved by decreasing the bandpass and the increment (step size).
Right-angle detection
Collection of fluorescence at 90° to the incident radiation. Right-
angle detection typically is selected for dilute and clear solutions.
Scatter
A combination of Raman, Rayleigh, and Rayleigh-Tyndall scatter-
ing, which can distort fluorescence spectra with respect to intensities
and wavelengths.
Signal photomultiplier
Detector used to measure excitation and fluorescence from the sam-
ple. It is operated in the photon-counting mode of detection to pro-
vide high sensitivity. Different detectors cover different wavelength
regions.
Singlet state
The spin-paired ground or excited state. The process of absorption
generally produces the first excited singlet state that emits fluores-
cence, or undergoes intersystem crossing to form a triplet state.
Spectrometer
The component in a fluorometer system that is scanned to provide
the excitation and emission spectra. The spectrometer is chosen for
stray-light rejection, resolution, and throughput.
Stokes shift
Generally, the energy-difference between the absorption peak of
lowest energy and the fluorescence peak of maximum energy.
Synchronous scan
Scan that characterizes the overlap between the excitation and emis-
sion. The excitation and emission spectrometers are scanned at the
same time, with a constant offset specified as either nanometers
(wavelength units) or in cm
–1
(energy units).
Time-base scan
Scan in which the sample signal is monitored while both the excita-
tion and the emission spectrometers remain at fixed wavelengths.
Time-base data are used to monitor enzyme kinetics, dual wave-
length measurements, and determine the reaction rate constant.
Time-resolved emission
scan
Scan in which the emission spectra are acquired at various times af-
ter the excitation pulse. Provides insight into excited state reactions,
charge-transfer-complex formation, solvent dipolar relaxation and
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