Question

1. Draw a diode array and describe how it operates

explain me how each component works after drawing

Answer 1

Diode arrays are assemblies of individual photo diodes in a linear array. Self-scanned arrays have the read-out electronics included on the chip with the array. When read out, all elements of the array must be read out in series. The array has 1024 elements. Light of all wavelengths falls on the diode-array and is measured simultaneously, that is, data acquisition is done in parallel. speed is the best known advantage of diode-array spectroscopy. Data is acquired in parallel, the detectors are read-out by "electronic scanning", and microprocessors and computers are used to process data; Measurements are made at different wavelengths at the same time. Conventional spectrophotometers can make multi-wavelength measurements but there is a time differential between each measurement. The diagram below illustrates what happens in a diode array spectrophotometer. First, a light source generates light at a specific wavelength or wavelengths. Normally, a UV/Vis spectrophotometers utilize two light sources: a deuterium arc lamp for consistent intensity in the UV range (190 to 380 nm) and a tungsten- halogen lamp for consistent intensity in the visible spectrum (380 to about 800 nm). Some spectrophotometers use xenon flash lamps, which offer decent intensity over the UV and visible regions.

The source light is then directed to a dispersion device that causes different wavelengths of light to be dispersed at different angles. Two common dispersion devices used in UV/Vis spectrophotometers are prisms and holographic gratings. The angle of dispersion with a prism, however, can be nonlinear and sensitive to changes in temperature. Holographic gratings are glass blanks with narrow ruled grooves. The grating itself is usually coated with aluminum to create a reflecting source. Holographic gratings eliminate nonlinear dispersion and are not temperature sensitive. They do require filters, though, since light is reflected in different orders with overlapping wavelengths.

As already mentioned, a special feature of some variable wavelength UV detectors is the ability to perform spectroscopic scanning and precise absorbance readings at a variety of wavelengths while the peak is passing though the flowcell. Diode array adds a new dimension of analytical capability to liquid chromatography because it permits qualitative information to be obtained beyond simple identification by retention time.

There are two major advantages of diode array detection. In the first, it allows for the best wavelength(s) to be selected for actual analysis. This is particularly important when no information is available on molar absorptivities at different wavelengths.

The second major advantage is related to the problem of peak purity. Often, the peak shape in itself does not reveal that it actually corresponds to two (or even more) components. In such a case, absorbance rationing at several wavelengths is particularly helpful in deciding whether the peak represents a single compound or, is in fact, a composite peak.

In absorbance rationing, the absorbance is measured at two or more wavelengths and ratios are calculated for two selected wavelengths. Simultaneous measurement at several wavelengths allows one to calculate the absorbance ratio. Evaluation can be carried out in two ways:

In the first case, the ratios at chosen wavelength are continuously monitored during the analysis: if the compound under the peak is pure, the response will be a square wave function (rectangle),. If the response is not rectangle, the peak is not pure.