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Principles of the 996 PDA Detector Optics
4
Reference Spectrum
Immediately after the dark current measurement and before any components are eluted,
the 996 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 minutes to finish.
Absorbance
The 996 detector calculates the absorbance for each diode at the end of each exposure
time using the following equation:
where:
S
= Signal obtained during sample analysis
D
= Signal obtained during the dark test
R
= Signal obtained from the reference spectrum
n
= Diode number
4.4.2 Resolution
The data reported by the 996 detector to the Millennium
32
database can be the average of
a number of data points. After calculating absorbance, the detector averages absorbance
values based on:
• Spectral resolution
• Sample rate
Averaging Spectral Data Based on Resolution
Spectral resolution (or bandwidth) is the wavelength interval (in nanometers) between
data points in an acquired spectrum. The minimal resolution of the 996 detector is 1.2 nm.
Spectral resolution with the 996 detector is always a multiple of 1.2 nm. For example, the
996 detector averages three diodes for each reported wavelength when the spectral
resolution is set in the Millennium
32
software to 3.6 nm.
Absorbance
Sn Dn
–
(
)
Rn
Dn
–
(
)
-------------------------
log
=
