A multiprocessor design for a photodiode array direct reader

Abstract

The photodiode array is a linear solid state imaging device. As a spectrochemical detector it allows the simultaneous measurement of spectral information from several resolution elements as opposed to the single resolution element measurements of photomultiplier tubes. In particular, when a number of short segment photodiode arrays are used to replace photomultiplier tubes in a direct reader, its spectral measurement power is greatly enhanced, allowing it to record simultaneously both analyte spectral line intensity and the background intensity in the immediate vicinity of the line. Access to this information about the background against which the intensity of a spectral line is measured, is a major advantage a photodiode array direct reader has over the more conventional photomultiplier based design. Furthermore, this additional background information is available for several analyte lines in a multielement measurement situation. A single microprocessor or microcomputer is incapable of supporting several photodiode arrays, all of which require high speed signal conversion and control operations. Furthermore, any successful computer system for a spectrometer based on several photodiode arrays, each of which not only generates spectra consisting of several data points but several such spectra per analytical measurement, requires highly developed data manipulation capabilities. The design and construction of a photodiode array signal acquisition and control system based around a network of single board computers is described. The hardware and software developed and the underlying design concept for a networked system of signal acquisition, control, and high-level data processing computers for a photodiode array direct reader is described. The ability of the system to acquire spectra simultaneously on all channels at integration times as low as 10 milliseconds is demonstrated. This contrasts with a single processor based system which was limited to sequential monitoring of individual photodiode array channels with minimum integration times of 0.1 second. The performance of the system is shown to be greatly superior to a single computer based design. The reproducibility and quality of the spectra generated show significant improvement. Detection limits compare well with the original photodiode array direct reader system and to photomultiplier based direct readers. Finally, innovative techniques for the correction of spectral interferences using simulated interference spectra are explored.