Part Z: Fundamentals and Znstrumentation 39 2.3 DETECTOR SYSTEMS Developments in solid-state electronics have led to the production of a range of electro- optical devices that can be used for photo-detection. Although considerable advances have been made, most of these dcvices suffer from limited wavelcngth coverage, exhibiting rather poor sensitivity at wavelengths below 300 nm. The increasing demand for simultancous or rapid sequential multi-element analysis has placed particular emphasis on imaging type detectors, which can monitor complete spectra.The range of devices is considerable and includcs the silicon vidicon (SV), the silicon-intensified-target vidicon (SIT), the intensified silicon-intensified-target vidicon (ISIT) and the secondary electron-conduction vidicon (SEC).The image dissector tube (ID) can also be used for multi-spectral caverage; it is a rapid sequential device based on a photomultiplier in which segments of the photocathodc can be selected by means of magnetic deflection coils. Winefordner, Caoney and Boutilier (921) have compared the properties of all the above devices in relation to analytical spectroscopy and drew the following conclusion: the SV is not useful for AFS or molecular fluorescence spectrometry, the SEC and ID are potentially useful for AFS particularly with echelle spectrometers and the SIT possesses considerable potential for molecular luminescence spectroscopy in the visible region (> 350nm).An experimental comparison of a U.V. sensitised vidicon and a SIT with photomultiplier tube detection has been prcscnted by Howell and Morrison (149).Studies on the flame emission detection of 23 elements showed40 Analytical Atomic Spectroscopy that the SIT gave equivalent detection power to commonly used photomultiplier tubes in the visible region only. Chiu (1366) has described the use of a slow-scan SEC for the measurement of stellar spectra at high resolution.Several problems were encountered in the use of the SEC tube, namely low target gain (-50), target gain noise, non-linear response and low storage capacity (< 2000 photoelectrons per pixel). In addition considerable varia- tions were found between different tubes, and industry figures showed that only 10% of manufactured tubes yielded good quality dztectors. In spite of these difficulties it was concluded that the SEC was the only vidicon tube capable of low-level light integration with high spatial resolution.A vidicon-based rapid-scan spectrometer capable of producing first derivative and intensity spectra has been described by Pardue et al. (162). The wavelength modulation used to produce the derivative signal was created by the electron-bzam scan pattcm.The vidicon spectrometer was linked to a mini-computer and its performance in atomic emission and molecular absorption was evaluated. Alder and Snook (130) have employed a vidicon detector for the measurement of emission signals from Al, Ni, Mo and Pt in a carbon tubc furnace. The determination of trace metals in Au using vidicon detection has been described by de Villiers et al.(64). Katzenberger, Fassel and Kniseley (1207) have evaluated the use of a silicon-intensified target detector (SIT) for OES with the ICP excitation source. Recent advances in SIT fabrication have brought their performance characteristics closer to those achieved in conventional multichannel polychromators; a comparison of powers of detection, linearity and precision for a modern SIT and for typical polychromators with an ICP source was presented.Pardue and McDowell (947) have investigated the use of a SIT tube as a multi- wavelength detector for LC peaks. Detection limits and performance were found to be similar to those obtainable with currently available commercial systems. Self-scanned linear phorodiode arrays are one of the cheapest and most convenient solid-state detectors for monitoring linear spectra.Sandell and Broadfoot (1 83) have obtained improved sensitivity with such a device by coupling it (via fibre optic) to an image-intensifier tube. In another paper they report (494) the use of a 128-element self- scanned anode array as a detector for a multichannel plate electron multiplier. O’Keefe and Chuang (1089) have studied various self-scanned diode arrays as detectors for FAAS and found that a 1024-element device mounted in the focal plane of a 0.35m Czerny-Turner monochromator provided the most useful performance. Spectral coverage of 55 nm with resolution of 0.2nm was achieved, and yielded good detection for several elements with absorbing lines in the wavelength range 214-589 nm.A similar arrangcment was reported by Horlick (1434), who used the device for studying emission spectra from an ICP. The ability to make simultaneous measurements on different parts of the spectrum was shown to be particularly useful in elucidating the effect of concomitants on line emission. Koirtyohann et al. (1525) have used a 512-element array placed parallel to the exit slit of a stigmatic monochromator to study spatial variations in spectral line emission from an ICP and a N,0/C2H, flame.A device known as the “photosil detector” (PSD) has been used (1309) in a low-noise photon counting system. The PSD is a combination of a conventional photocathode with a solid-state detector. Hieftje (1223) has described a method for measuring sub-ns response times of fast detectors. The technique uses a free running CW laser as a multi-frequency modulated source and a ref. spectrum analyzer to monitor detector response. Detector response times down to 200 ps could be measured. Other references of interest - Automatic photon-counting system: 1394. Comments on signal flicker noise and noise power spectra: 445.Part I : Fundamentals and Instrumentation New vidicon spectrometer design for laser-excited fluorescence : 320. Performance of a vidicon detector at low light levels: 182. Review of multi-channel TV type detectors: 1575. Silicon-photodiode array for AAS and A E S : 601. Vidicon detector for Stark modulation spectroscopy: 1307. 41