Liquid Chromatography Detectors - The UV Detectors > The Diode Array Detector > Page 49
An example of the use of the variable wavelength UV in this way is afforded by the separation of some carboxylic acids that is monitored by UV absorption at 210 nm. The separation is shown in figure 29. The separation of a series of common fatty acids was carried out on a reversed phase column using water buffered with phosphoric acid as the mobile phase.
Multi-wavelength dispersive detectors has proved extremely useful, providing adequate sensitivity, versatility and a linear response. As a result of the need for an optical bench inside the instrument, however, it is somewhat bulky In addition, it has mechanically operated wavelength selection and requires a stop/flow procedure to obtain spectra "on-the-fly". In contrast, the diode array detector has the same advantages but none of the disadvantages.
The Diode Array Detector
The diode array detector also utilizes a deuterium or xenon lamp that emits light over the UV spectrum range. Light from the lamp is focused by means of an achromatic lens through the sample cell and onto a holographic grating. The dispersed light from the grating is arranged to fall on a linear diode array.
The resolution of the detector (Dl) will depend on the number of diodes (n) in the array, and also on the range of wavelengths covered (l2 - l1).
Consequently, the ultimate resolving power of the diode array detector will depend on the semi–conductor manufacturer and on how narrow the individual photo cells can be commercially fabricated.