PerkinElmer 200 UV/VIS Series User Manual page 166

Principles of Operation Application Notes
Rise time, on the other hand, is a non-mathematical description that is used for both single
and double pole filters. If the input signal is suddenly changed (a step change), the detector
requires a short period of time to register this change in its output signal. The rise time is
defined as the time it takes the output signal to rise from 10% of its final value to 90% of its
final value (see Figure 8-4).
A larger rise time will allow the recorder to average the noise frequencies, and, subse-
quently, the baseline will contain much less short term noise. However, larger rise times
may have the following effects on peaks:
• Peak shape will become asymmetric.
• The peak maximum will be shifted.
• The peak height will be reduced.
The integral peak area, however, will be unaffected. Since peak area will remain the same,
the peak width must increase. An increase in peak width can lead to poor resolution of
peaks. It is, therefore, important to choose an appropriate rise time that keeps the baseline
noise to a minimum without compromising the resolution of individual peaks. The following
rule should minimize noise without too much loss of peak resolution.
The rise time should be approximately 20% of the peak width at half height of the
narrowest peak of interest.
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