UV detectors

[3] The vast majority of liquid chromatographic systems are equipped with ultraviolet (UV) absorption detectors.

This way they give additional information, which help understand more about the nature of the substances appearing in the chromatogram and allow their identification.

The variable UV-Vis absorbance detector's optical bench is showing how the flow cell is positioned after the optical system, including the monochromator, which typically has a physical slit and a moving grating, so it is illuminated by a selected wavelength, reaching a photo-diode.

The bench of the diode array detector, however, is configured so that the flow cell is positioned before the optical parts, so that the beam containing the entire spectrum is passing through it.

The optical parts consist also with a monochromator and a slit, but with a fixed grating, which disperses the light onto a diode array imaging element.

Variable UV-VIS vs PDA detector
Figure 1: Simplified schemes of the Variable UV-Vis detector compared to PhotoDiode Array detector. In the Variable UV-Vis the entire optical bench is located before the flow cell whereas in the diode array the flow rate is positioned before the main optical bench.