Measuring Light at the Photodiode 47
4
The aperture creates a narrow beam that reflects from the grating to the photodiode array.
The wavelength that strikes a particular diode depends on the angle of reflection from the
grating.
Figure 4-2
shows an absorbance spectrum of benzene obtained from the 996 detector
using the standard 50-µm aperture. In this spectrum, the wavelength resolution is
sufficient to resolve five principal benzene absorption peaks.
Figure 4-2 Benzene Spectrum at 1.2 nm Resolution
4.3 Measuring Light at the Photodiode
The Waters 996 Photodiode Array Detector measures the amount of light striking the
photodiode array to determine the absorbance of the sample in the flow cell.
The array consists of 512 photodiodes arranged in a row. Each photodiode acts as a
capacitor by holding a fixed amount of charge.
Light striking a photodiode discharges the diode (Figure 4-3
). The magnitude of the
discharge depends on the amount of light striking the photodiode.
nm
Absorbance
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