Ion Chromatography - The Multi-Wavelength Detector

The multi-wavelength detector, however, can compensate for the lower light sensitivity by selecting a wavelength that has the maximum extinction coefficient for the solutes of interest. The average specifications for commercially available fixed wavelength UV detectors are as follows:-

Typical Specifications for a Fixed Wavelength UV Detector

Sensitivity (toluene)

5x 10-8 g/ml

Linear Dynamic Range

5 x 10-8 to 5 x 10-4 g/ml

Response Index

0.98 - 1.02

The Multi-Wavelength Detector

Multi-Wavelength UV detectors select a single, or more precisely, narrow range of wavelengths with which to detect the solute. Most multi-wavelength UV detectors can also provide a UV spectrum of the eluted solute if so desired. There are two types of multi-wavelength detector, the dispersion detector that monitors the eluent at one wavelength only and the diode array detector that monitors solute absorption simultaneously over a range of wavelengths. The former passes the light from a broad emission light source (such as that produced by a deuterium lamp) through a monochrometer that selects a specific wavelength and directs it through the detecting cell. The latter, also uses a broad emission light source, but light of all wavelengths is passed through the sensing cell and subsequently dispersed by means of a holographic grating onto a diode array.

The Diode Array Detector

A diagram of a diode array detector is shown in figure 24.

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Figure 24. The Diode Array Detector

As the diode array detector has virtually replaced all other types of multi-wavelength detectors it will described here. Light provided by a broad emission source (e.g a deuterium lamp) is collimated by an achromatic lens system to focus the light through the detector cell onto a holographic grating. As a result the sample is subjected to light of all wavelengths generated by the lamp.

The dispersed light from the grating falls on to the diode array that may contain several hundred diodes and the output from each diode is regularly sampled by the computer and the data stored on a hard disc. At the end of the separation, the output from any specific diode can be selected and a chromatogram produced that relates the absorption of light of that UV wavelength that fell on that particular diode against time. Most instruments allow a least one diode to be monitored in real time so that a chromatogram can be produced as the separation develops.