Waters 2996, Operator'S Manual

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34 Principles of the 2996 PDA Detector Optics
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3. Sequentially recharges and reads the current required to recharge all the remaining
510 photodiodes
4. Waits approximately 45 msec before beginning the recharge-and-reading sequence
with diode 1 after all diodes have been recharged and read.
You set the exposure time parameter in the General tab of the 2996 PDA Instrument
Method Editor. You can specify either Auto Exposure or Exposure Time. For details, refer
to the Millennium
32
Help.
Note: For best signal-to-noise performance, adjust the wavelength range optimize
autoexposure computations. For details, refer to the Millennium
32
Help .
Using the Auto Exposure Parameter
The Auto Exposure time parameter allows the 2996 Detector optics to calculate the
optimum exposure time needed to recharge the diodes based on lamp energy, lamp
spectrum, mobile phase absorbance, and the chosen wavelength range using a single
Deuterium light source from 190 to 800 nm. To minimize detector noise, Auto Exposure
adjusts the exposure time from 50 to 90% of full scale.
The Auto Exposure time setting ensures that the photodiodes are:
• Not saturating due to overexposure
• Operating above the range of normal, dark current discharge
With auto exposure enabled, the 2996 Detector:
• Calculates exposure time at the start of a run based on maximum light intensity
within the selected wavelength range
• Limits the exposure so that no diode within the given wavelength range discharges
more than 90%
• Provides proper settings for signal-to-noise and dynamic range for each run
The Auto Exposure time setting may not support certain combinations of sampling rates,
wavelength ranges, or filter time-constant settings required for your analysis. If this is the
case, you can set the exposure time manually to adjust the exposure time from experiment
to experiment.
Using the Exposure Time Parameter
The Exposure Time parameter enables you to manually set the length of time the
photodiodes are exposed to light before they are read. The supported range is 10 to
500 msec.