46
Principles of the 996 PDA Detector Optics
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)
Flow cell
assembly
Houses the segment of the flow path (containing eluent and
sample) through which the polychromatic light beam passes. This
arrangement of optical components, with the flow cell positioned
between the lamp and the grating, is commonly called reversed
optics.
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
spectrograph 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
350 nm) to the light intensity observed at visible wavelengths
(greater than 350 nm).
Photodiode array
An array of 512 diodes 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
