Waters 2475, Operator'S Manual, Page: 31 / 228

Auto-optimize gain 1-17
Optical components degrade slowly over time. In conventional fluorescence
detectors, response increases by incrementing the PMT gain. However, the
response of the sample varies with energy throughput. If excitation energy is
degraded, peak response is degraded. If excitation intensity diminishes, peak
response decreases and noise increases.
During normal operation, lamps are commonly replaced when the reference
energy at specific wavelength settings falls below a user-set threshold relative
to initial values. The useful lamp life depends on your requirements for noise
performance.
Tip:
You should inspect the detector’s general condition when you replace
lamps.
Predicting when the detector’s performance degrades to an unacceptable level
based solely on reference energy is unsatisfactory. Each user’s analyses will
require different levels of sensitivity. Checking reference energy alone to
evaluate performance assumes that every lamp has the same longevity,
degradation patterns, and spectral output characteristics. To reduce this
uncertainty, Waters designed the detector to operate as independently of lamp
output as possible. After the unit has verified the calibration of the
monochromator, the instrument evaluates the energy levels in a number of
characteristic regions across the spectrum. The integration time of the
front-end electronics is adjusted to maximize the signal within these regions.
The intent is to maintain a high signal-to-noise ratio and operate with a clean
signal. In this way, the instrument’s sensitivity to lamp energy is virtually
eliminated as a major contributor to performance.
Ultimately, the detector’s performance is a function of each unique application
requirements. Signal-to-noise measurements are the best way to evaluate
performance and set the boundaries for acceptable operational sensitivity
limits.
The 2475 detector source lamp is warranted to light and pass startup
diagnostic tests for 2000 hours or 1 year from the date of purchase, whichever
comes first. The detector’s on-board diagnostics allow you to record lamp
usage and report the lamp serial number.
Auto-optimize gain
Proper PMT gain setting maximizes the signal on the internal
analog-to-digital converter without exceeding its potential limit. If you specify
too high a gain, the fluorescence emissions overload the signal collection