Liquid Chromatography Detectors - The UV Detectors > The Fixed Wavelength UV Detector > Page 42

A diagram of a typical optical system for a fixed wavelength UV detector is shown in figure 25. Light from the UV source is collimated by a suitable lens and passed through both the sample cell and the reference cell and then on to two photo cells  The cells are cylindrical with quartz windows at either end. The reference cell compensates for any absorption that mobile phase might have at the sensing wavelength. The outputs from the two photo cells are passed to a signal modifying amplifier so that output is linearly related to the concentration of solute being detected.



Figure 25. The Fixed Wavelength UV Detector


For reasons already discussed, modern sensor cells have angular conduits that form a 'Z' shape to reduce dispersion. The UV sensor can be sensitive to both flow rate and pressure changes but this instability can be greatly reduced if the sensor is well thermostatted. The fixed wavelength UV detector is one of the most commonly used LC detectors; it is sensitive, linear and relatively inexpensive. Sensitivity (minimum detectable concentration) can be expected to be about 5 x 108 g/ml with a linear dynamic range of about three orders of magnitude for 0.98 < r < 1.02. The separation of some aromatic hydrocarbons by exclusion chromatography on a very high efficiency column (efficiency ca 250,000 theoretical plates) monitored by a fixed wavelength detector is shown in figure 26.