50
Principles of the 996 PDA Detector Optics
4
Optimizing the Signal-to-Noise Ratio
To optimize signal-to-noise ratios, choose an acquisition wavelength range that includes
only the wavelengths of interest and over which the mobile phase absorbs minimally (see
Appendix D, Mobile Phase Absorbance
).
4.4 Computing Absorbance Data Points
The 996 detector calculates absorbance values before transmitting the data to the
Millennium
32
database. To calculate absorbance, the 996 detector:
• Computes the absorbance at each diode using the dark current and reference
spectrum (see
Section 4.4.1, Calculating Absorbance
).
• Averages the absorbances at a particular wavelength as specified in the spectra per
second sample rate and reports the average as a single data point (see
).
• Can apply a filter that acts like an analog filter (see
).
4.4.1 Calculating Absorbance
The 996 detector computes absorbance by subtracting the dark current and reference
spectrum from the acquired spectrum. Absorbance is based on the principles of Beer's
Law.
Beer’s Law
The relationship between the quantity of light of a particular wavelength arriving at the
photodiode and the concentration of the sample passing through the flow cell is described
by the Beer-Lambert Law (commonly called Beer’s Law). Beer’s Law is expressed as
A
=
ε
lc
Where:
A
= absorbance
ε
= molar absorptivity
l
= path length (1.0 cm in the 996 detector normal flow cell)
c
= molar concentration
