Waters 2996 - Page 59

Computing Absorbance Data Points 37
4
Dark Current
Photodiodes lose charge over time even when they are not exposed to light. The amount of
charge lost is called dark current.
At the start of a chromatographic run, the 2996 Detector closes the shutter to take a dark
current reading for each diode. The shutter closes after the exposure time is calculated and
stays closed for the same interval as the exposure time.
The detector subtracts the dark current values from the current values recorded during
absorbance measurements for both the sample and the reference spectra.
Reference Spectrum
Immediately after the dark current measurement and before any components are eluted,
the 2996 Detector records a reference spectrum. The reference spectrum is a measure of
lamp intensity and mobile phase absorbance over the interval specified in the exposure
time taken with the shutter open.
Note: For best results, the reference spectrum should be representative of the initial
mobile phase.
Note: For extremely long exposure times, the dark current and reference spectrum
readings may take several seconds to finish.
Absorbance
The 2996 Detector calculates the absorbance for each diode at the end of each exposure
time using the following equation:
where:
S = obtained during sample analysis
D = obtained during the dark current test
R = obtained from the reference spectrum
n = diode number
Absorbance
n
Sn Dn–()
Rn Dn–()
-------------------------
log=