Benefits of Photodiode Array Detection over conventional Scanning detection

Benefits of Photodiode Array Detection over conventional Scanning detection
photodiode_array
Photodiode array detection schematic (Image Courtesy : http://www.chem.agilent.com/)

A Photodiode Array Detector is a microprocessor controlled multi-channel detector that permits simultaneous access to spectral data for several wavelengths simultaneously. In comparison the conventional UV-Visible detector has only a single channel detector. This unique feature provides several benefits to the spectroscopist which are highlighted in the article.

Simultaneous multi-wavelength measurement

In a conventional spectrophotometer a single wavelength is recorded at any given point of time. On the other hand all wavelengths can be measured simultaneously in the diode array detector and this feature is a great time saver when several wavelengths are to be monitored simultaneously.

Wavelength precision

The required wavelength is selected on a conventional spectrophotometer either manually or using a stepper motor. On the other hand in the photodiode array data is acquired at each wavelength simultaneously. This eliminates repeatability errors that result from mechanical wear of moving parts.

High sensitivity

Diode array systems have fewer optical surfaces as a result of which the light throughput is high and results in improved sensitivities. An added benefit is time averaging feature to get several fold improvement of spectral sensitivity.

Minimal Stray Light

A photodiode array spectrophotometer has reverse optics design which minimizes stray light which is a common interfering component in conventional spectrophotometers. In case of photodiode array system the sample is placed before the polychromator (reverse optics) whereas in case of conventional spectrophotometer sample is positioned after the monochromator.. As the measurements are unaffected by stray light observations can be made with the sample chamber open without interference from outside light. This feature also permits analysis on a wider range of sample sizes and use of special sampling accessories

Ruggedness

The reliability and ruggedness is higher for photodiode array detector due to absence of moving parts and mechanical simplicity.This also eliminates virtually need for maintenance or re-calibration. As photodiode array detector is a solid-state device it is more reliable and secure than the photomultiplier tube. A polychromator gives consistent performance as the light dispersion element is locked in its position whereas in case of conventional spectrophotometer scanning requires movement of the grating inside the monochromator.

The benefits highlighted extend applications to real-time measurements in process stream analysis, detection in HPLC and other flow injection techniques as well as reaction monitoring kinetic studies.

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