Waters 2996, Operator'S Manual

Page: 53 / 84

Resolving Spectral Data 31
4
4.2 Resolving Spectral Data
The ability to distinguish similar spectra depends on photodiode spacing and the
bandwidth of the light striking the photodiode. The bandwidth of the light striking the
photodiodes depends on the aperture width.
The aperture width determines:
• Attainable wavelength bandwidth at the photodiode array
• Intensity of the light reaching the photodiode array (optical throughput)
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 2996 Detector
using the 50-µm aperture. In this spectrum, the wavelength resolution is sufficient to
resolve five principal benzene absorption peaks.
Spectrograph
mirror and mask The mirror focuses light transmitted through the flow cell onto the
aperture at the entrance to the spectrographic portion of the optics.
The mirror mask defines the beam of light focused on the spec-
trograph mirror.
Aperture Controls wavelength resolution and intensity of light striking the
photodiodes. The width of the aperture is 50 µm.
Shutter assembly Prevents light from reaching the photodiode array except during
sampling and calibration. For details on the dark current, see
Section 4.4.1, Calculating Absorbance.
Grating Disperses the light into bands of wavelengths and focuses those
wavelength bands onto the plane of the photodiode array.
Second-order
filter Reduces the contribution of second-order reflection of UV light
(less than 370 nm) to the light intensity observed at visible wave-
lengths (greater than 370 nm).
Photodiode array An array of 512 photodiodes arranged linearly. The diode width and
spacing provide a single wavelength resolution of 1.2 nm.
Table 4-1 Optics Assembly Components (Continued)
Component Function