Waters ACQUITY UPLC Operator's, Overview And Maintenance Manual page 23

For certain combinations of sampling rates, wavelength ranges, or filter-time
constants, the Auto Exposure time setting does not always optimize
performance. If this is the case, you can set the exposure time manually, in the
instrument editor.
Using Exposure Time
Use the Exposure Time function to manually set the length of time the
photodiodes are exposed to light before they are read. The supported range is
5 to 500 msec.
Changing exposure times within a set of samples can cause changes in
Tip:
baseline noise.
Note that increasing exposure time can saturate the photodiodes and cause
the detector to lose signal at certain wavelengths. To avoid signal loss, select
an exposure time-value that provides settings for an optimum signal-to-noise
ratio over the wavelength range of your analysis (see the next section,
"Optimizing the signal-to-noise ratio").
Optimizing the signal-to-noise ratio
To optimize signal-to-noise ratios, choose an acquisition wavelength range
that includes only the wavelengths of interest. It is also important that the
range be one in which the mobile phase absorbs only minimally (see "Mobile
phase absorbance" on page 105). You can also improve the signal-to-noise
ratio by increasing the spectral resolution value. For example, you can choose
to operate at 3.6 nm instead of at 1.2 nm resolution.
Optimizing filter constants
The filtering constant you select affects the peak intensity. To increase
sensitivity, decrease the filtering (filter-time) constant.
Selecting the appropriate sampling rate
A sufficient number of points must fall across a peak to define its shape. Thus,
at very low sampling rates, the definition between peaks is lost. Empower
uses the index of the data point closest to the end time, minus the index of the
data point closest to the start time, to calculate the Points Across Peak value
for each integrated peak in the chromatogram.
The Points Across Peak value appears in the Peaks table, at the bottom of
Tip:
the Review Main window. If the Points Across Peak field is not visible,
Operating principles of the light-guiding flow cell 11