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Optical System
Component
Tungsten Lamp
Visible Focus Lens
Deuterium Lamp
Source Lens
Flow Cell
Spectrograph Lens
Filter Paddle
Slit
Grating
Photodiode Array
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Function
The light source for visible and near-infrared wavelengths (380 to
800 nm). The tungsten lamp is focused through an opening in the
internal structure of the deuterium lamp; this allows the lamps to
share the optical axis to the source lens.
Focuses the visible light from the tungsten lamp to the through-
hole of the deuterium lamp.
The light source for UV wavelengths (190 to 380 nm).
Receives the light from the tungsten and deuterium lamps and
focuses it so that the beam passes through the flow cell.
The measurement site for sample absorbance. The cell has a flat
window on each side. Thermo Fisher Scientific offers five cells
for use with the Dionex PDA: standard cells (in both PEEK and
316 stainless steel), semi-micro cells (in both PEEK and 316
stainless steel), and semi-preparative cells (in PEEK only).
Receives the light from the flow cell and focuses it onto the slit.
The motorized filter paddle in the spectrograph has three
positions:
•
Open = Light passes, unobstructed, along the light path when
the holmium oxide filter is moved out of the light path.
•
Blocked = No light passes along the light path; used to
measure the dark signal without turning off the lamps.
•
Holmium = Places the holmium oxide filter in the light path;
used to verify wavelength accuracy.
The width of the slit (equivalent to 1 nm) optimizes the optical
resolution.
Diffracts the light beam into its component wavelengths and
directs the light onto the photodiode array.
A series of 1024 photosensitive elements. Each element measures
a narrow band of the spectrum. A spectrum is obtained by
measuring the light intensity of each wavelength and reporting the
results over the selected wavelength range.
2 • Description
15
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