High performance capillary electrophoresis - T.E.A.M.

Instrumentation/Operation
light into the capillary. The beam is then dispersed by a
diffraction grating and falls on the photodiode-array. An
array consists of numerous diodes (211, for example), each
of which is dedicated to measuring a narrow-band spectrum.
The number of wavelengths falling on a photodiode is called
the bandwidth.
Importantly, DAD optics can yield detection limits, sensitivity;
and linear detection range that equal or exceed that of
single or multiple wavelength detectors. With respect to
spectral analysis, DAD also has significant advantages over
rapid scanning detectors. Unlike in the scanning design, the
signal-to-noise ratio of spectral data is independent of the
number of wavelengths acquired, the bandwidth of individual
wavelengths is not predetermined, and on-line spectra
are available at all times.
DAD can greatly simplify analysis of electrophoretic data.
When developing a new electrophoretic method there is
usually little or no information regarding the required
detector conditions, and in particular no optimal wavelength.
Using a conventional variable wavelength detector, the
sample must be injected repeatedly, changing the detector
wavelength each time to make sure that all solutes are
detected. With a diode-array a whole wavelength range can
be selected, for example from 190 to 600 nm with a bandwidth
of 400 nm. In a single analysis, all solutes absorbing
within this range will be detected.
Once all peaks have been detected, the diode-array can
be used to determine the wavelength for the absorbance
maximum for all analytes. Appropriate software can calculate
the absorbance maxima automatically (figure 64).
Alternatively, the data can be presented in three-dimensional
form— either as analysis time versus wavelength
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