Topics - Diode Array
The Diode Array
A diode array consists of a number of photosensitive diodes place side by side and insulated from one another in the form of a multi-layer sandwich. Each diode may be only a few thousands of an inch thick and the output from each diode can be scanned, stored and subsequently processed by a computer in a number of different ways. The common use of a diode array is to monitor light that has passed through a liquid sensor cell as in a multi-wavelength liquid chromatography detector. The light source is usually polychromatic (e.g. light from a deuterium lamp) and after passing through the cell, the light is dispersed by a quartz prism or a diffraction grating onto the surface of the diode array. Thus, each diode will receive light of a slightly different wavelength to that received by its neighbor. Those wavelengths most useful in liquid chromatography range from about 210 nm to 330 nm (i.e. UV light) and, thus, a sufficient number of diodes must be incorporated in the array to (at least) cover this range of wavelengths. Many organic compounds have characteristic spectra in the UV which can be used to help identify the substance passing though the sensor cell. Thus, when a given substance is eluted through the sensor cell, all the outputs from the array can be acquired and the result used to construct an absorption spectra that can be compared with standard spectra for identification purposes. Alternatively, by selecting the appropriate diode, the wavelength of the light at which there is maximum absorption can be selectively monitored to provide maximum detector sensitivity for that substance.