DAD 6.1L/DAD 2.1L/MWD 2.1L user manual V6700, version 3.2
Optimizing the Detector
31
Bandwidth
The bandwidth defines the total number of wavelengths actually registered
by the photodiode when a specific wavelength is set. For example, a wave-
length set at 254 nm width a bandwidth of 4 nm results in average absorp-
tion of 252–256 nm.
The selection of bandwidth is a balance between sensitivity and selectivity.
Narrow bandwidths increase selectivity, whereas broad bandwidths increase
sensitivity.
Default Bandwidth
By default (see “Default Settings” on page 27), the bandwidth for the signal
wavelength is set to 8 nm and for the reference wavelength the bandwidth
is set to 30 nm.
Spectral Range
When measuring with chromatography software, the spectral range
selected for a given measurement influences the required disc space. The
disc space is necessary to save generated data.
A narrow spectral range reduces disk space. The range, however, should be
wide enough to ensure the detection of all components. Also, the spectral
range always must include the signal wavelength and the reference wave-
length (when applicable).
Time Constant & Data Rate
Response Time
The time constant influences the response time of the detector. The
response time determines how quickly the detector responds to a change in
signal. A good rule of thumb for selection of the time constant is that it
should be no larger than
of the baseline peak width of the first peak of
interest (in seconds). Increasing the time constant allows more averaging of
the signal (also known as digital filtering) and results in less baseline noise.
However, increasing the time constant too much may result in broad peaks,
reduced peak heights and asymmetric peak shapes. Therefore, a compro-
mise has to be found.
Time Constant
Using the time constant a signal smoothing can be achieved. The larger this
value is set, the more the signal will be smoothed. In general, the best time
constant is the reciprocal of the data rate (see table below). If increased sen-
sitivity is desired, or if the baseline noise is interfering with integration, the
time constant should be increased. If resolution is compromised, it should
be decreased.
It is recommended to set the time constant and data rate in relation to peak
width.
Fig. 27
bandwidth
1
10
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