Operating Principle - Thermo Scientific Vanquish VC-D40 Operating Manual

3.2

Operating Principle

Variable Wavelength Detectors (VC-D40, VF-D40)
Operating Manual
The detector is a dual-beam spectrophotometer that is designed for
ultraviolet (UV), visible (VIS) and near-infrared (NIR) absorption
spectrophotometry in combination with HPLC or UHPLC separations.
Photometric detection bases upon the absorption of monochromatic
light. The optical system of the detector provides a deuterium lamp
(UV), and may provide a tungsten lamp (VIS), which is optional for VC-
D40 detectors. A concave mirror (no. 2) focuses the light from the VIS
lamp (no. 1) to the aperture of the UV lamp (no. 3). Another mirror (no.
4) receives the combined light from both light sources and focuses it into
the entrance slit (no. 6). The shutter (motorized filter paddle, no. 5) can
be moved into the light path before the flow cell to protect the flow cell.
The light passes through the entrance slit to the mirror (no.7) and on to
the grating (no. 8), where the light beam is diffracted. The angular
position of the grating determines the selected wavelength that is
focused by the mirror (no.9) and directed to the beamsplitter (no. 10)
and flow cell (no. 13).
The beamsplitter (no. 10) directs part of the light beam to the reference
diode (no. 11). If the flow cell is removed, a safety shutter (no. 12) blocks
the light beam to protect the user from UV radiation. If a flow cell is
installed, the remaining light travels through the flow cell (no. 13), and is
partly absorbed depending on the sample solution in the flow cell. It
travels on to the measurement photodiode (no. 14), where the
remaining light intensity is measured. The absorbance of the analyte is
calculated from the measured intensities based on the law of Lambert-
Beer.
The response of the detector is proportional to the concentration of the
analyte. Simultaneously, the reference beam intensity is measured and
used to compensate any intensity fluctuations of the light source.
The following picture shows the optics of the detector, and illustrates
how the detector operates:
3 • Device Overview
Page 33