Waters 2998, Overview And Maintenance Manual

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December 5, 2017, 715004753 Rev. B
Page 18
1.3 Resolving spectral data
Together with photodiode spacing, the detector’s 50-µm slit determines the intensity and
bandwidth of the light that strikes the photodiode array. Reducing the bandwidth increases the
resolving power of the detector. As a result, similar spectra are more effectively distinguished.
The grating images the slit onto the photodiode array. The angle of diffraction from the grating
determines the wavelength that strikes a particular photodiode in the array.
The following figure shows an absorbance spectrum of benzene. Note that the wavelength
resolution is sufficient to resolve five principal absorption peaks.
Figure 1–4: Benzene spectrum at different resolutions
1.4 Measuring light at the photodiode array
The 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 a row of 512 photodiodes. Each photodiode acts as a capacitor initially
holding a fixed amount of charge.
Light striking a photodiode discharges the diode. The magnitude of the discharge depends on the
amount of light striking the photodiode.
Absorba
nc
e
nm
230.00 250.00 270.00
1.2 nm
3.6 nm