Liquid Chromatography - The Diode Array Detector 2

Light from a broad emission source such as a deuterium lamp is collimated by an achromatic lens system so that the total light passes through the detector cell onto a holographic grating. In this way the sample is subjected to light of all wavelengths generated by the lamp. The dispersed light from the grating is allowed to fall on to a diode array. The array may contain many hundreds of diodes and the output from each diode is regularly sampled by a computer an stored on a hard disc. At the end of the run, the output from any diode can be selected and a chromatogram produced employing the UV wavelength that was falling on that particular diode. Most instruments will permit the monitoring of a least one diode in real time so that the chromatogram can be followed as the separation develops. This system is ideal in that by noting the time of a particular peak, a spectrum of the solute can be obtained by recalling from memory the output of all the diodes at that particular time. This gives directly the spectrum of the solute, i.e., a curve relating adsorption against wavelength.

The performance of both types of multi-wavelength detectors are very similar and typical values for their more important specifications are as follows.

Typical Specifications for a Multi-Wavelength UV Detector

Sensitivity

1 x 10-7g/ml

Linear Dynamic Range

5 x 10-7 to 5 x 10-4 g/ml

Response Index

0.97 - 1.03