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). Thus                            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

The Diode Array Detector The diode array detector, although offering detection over a range of UV wavelength, functions in an entirely different way from that of the dispersive instrument. A diagram of a diode array detector is shown in figure 20. Figure 20. The Diode Array Detector

printer, will provide an adsorption curve relating adsorption to wavelength (or frequency). The adsorption curve is known as the adsorption spectrum and its shape will be characteristic for the substance being examined and be determined by the transitions that occur at the scanned wavelengths.   The Diode Array UV/Visible Spectrometer   As already explained, the diode array spectrometer functions in an entirely different way from the dispersive instrument. A diagram of a diode array detector is shown in figure 12. Light from a deuterium lamp is collimated by an achromatic lens system so that the total light passes through the sensor cell onto a holographic grating. In this way the sample is subjected to light of all wavelengths generated by the lamp.       Figure 12. The Diode Array Spectrometer

dispersed light from the grating is then allowed to fall onto a diode array. The array may contain many hundreds of diodes and the output from each diode is regularly and frequently sampled by a computer and stored on a hard disc. At the end of the chromatographic separation, the output from any diode can be selected and a chromatogram produced using the UV wavelength that was falling on that particular diode. The dispersed light from the grating is then allowed to fall onto a diode array. The array may contain many hundreds of diodes and the output from each diode is regularly and frequently sampled by a computer and stored on a hard disc. At the end of the chromatographic separation, the output from any diode can be selected and a chromatogram produced using the UV wavelength that was falling on that particular diode.   Most instruments will permit the monitoring of at least one diode in real time so that the chromatogram can be followed as the separation develops. This system

the chromatographic development to be arrested otherwise the column eluent must be passed to waste during the scanning process with the possible loss of sample.   The diode array spectrometer operates in quite a different manner. Light of all wavelengths generated by the deuterium lamp is transmitted through the cell and the transmitted light is then dispersed over an array of diodes. In this way, the absorption at small discrete groups of wavelengths is continuously monitored at each diode. However, the light falling on a discrete diode may not be solely that transmitted through the cell from the source, but may contain light resulting from fluorescence excited by light of a shorter wavelength. In this case, the situation is exacerbated by the fact that the cell contents are exposed to all the light emitted from the lamp and so fluorescence is more likely. In general, this means that under some circumstances measurement of transmitted light may also contain fluorescent light

A diagram of a diode array detector is shown in figure 30. Light from the broad emission source 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 onto a diode array. The array may contain many hundreds of diodes and the output from each diode is regularly sampled by a computer and stored on a hard disc. At the end of the run, the output from any diode can be selected and a chromatogram produced using the UV wavelength that was falling on that particular diode. Figure 30. The Diode Array Detector   During chromatographic development, the output of one diode is recorded in real time producing a real time chromatogram. It is seen that by