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:

Operating principle of the optics
Figure: Operating principle of the optics

No.

Description

1

VIS lamp (tungsten lamp)

2

Mirror (VIS)

3

UV lamp (deuterium lamp)

4

Mirror (combined light)

5

Shutter

6

Entrance slit

7

Mirror

8

Grating

9

Mirror

10

Beamsplitter

11

Reference photodiode

12

Safety shutter

13

Flow cell

14

Measurement photodiode