Liquid Chromatography - The Multi-Wavelength Dispersive Detector 1
By the use of very small sensing cells and electronic systems with very small time constants, the fixed wavelength detector can be designed to give a very fast response at high sensitivity and very low dispersion and for this reason it can be used for high speed separations. An example of the rapid separation of a two component mixture (8) is shown in figure 18. The reason for separating benzene and benzyl alcohol in 2.6 seconds remains (to say the least) obscure and figure 1 is obviously and example of "Chromatography Show Biz". Nevertheless, it does demonstrate that columns can be designed and detectors developed that can provide extremely fast analyses.
The Multi-Wavelength Detectors
Multi-Wavelength UV detectors utilize a single (perhaps more accurately a narrow range) of wavelengths to detect the solute. Most multi wavelength UV detectors can also provide a UV spectrum of the eluted solute if appropriately arranged. There are two types of multi-wavelength detectors the dispersion detector that monitors the eluent at one wavelength only and the diode array detector that monitors the eluted solute over a range of wavelengths simultaneously. The former passes the light from a broad emission light source through a monochrometer, selects a specific wavelength and allows it to pass through the detecting cell. The second, also uses a broad emission light source, but all the light is allowed to pass through the sensing cell and subsequently the light is dispersed by means of a holographic grating and the dispersed light allowed to fall on an array of diodes.
The Multi-Wavelength Dispersive Detector
A diagram of the Multi-Wavelength Dispersive Detector is shown in figure 19. Light from a broad wavelength source such as a deuterium or xenon discharge lamp is collimated by two curved mirrors onto a holographic diffraction grating. The dispersed light is focused by means of a curved mirror, on to a plane mirror and light of specific wavelength selected by appropriately positioning the angle of the plane mirror. Light of the selected wavelength is then focused by means of a lens through the flow cell and consequently, through the column eluent. The exit beam from the cell is focused by another lens onto a photo cell which gives a response that is some function of the intensity of the transmitted light. The detector is usually fitted with a scanning facility that, by arresting the flow of mobile phase, allows the spectrum of the solute contained in the cell to be obtained.