The Atomic Emission Detector AED

Most of the detectors described above can be used selectively for several elements but this is normally carried out one element at a time, although the FPD, for example, can determine at least two elements (S and P) simultaneously if desired. One of the advantages of the AED is that theoretically it can detect a large number of elements simultaneously with high sensitivity and selectivity. In practice the number determined simultaneously is usually limited to four and, since one of these is normally a carbon channel, only three other elements can be determined.

In the AED, or microwave plasma detector, the compounds are subjected to a high energy microwave plasma so that the emitted spectra are line spectra which are of an extremely narrow wavelength, often only 0.1 A wide. This results in (potentially) a much higher selectivity and sensitivity.

The AED had a long history of development. It first appeared commercially in the early 1970s. This detector was operated with a microwave plasma in pure helium at low pressure (10-100 mm) and made use of a nitrogen purge to remove carbon deposits from the wall of the silica discharge tube at the outlet of the column. The emission spectra were resolved with a conventional 4m optical spectrometer which resulted in a rather large and unwieldy apparatus which was also sensitive to movement, requiring fairly frequent realignment of the optics. One of the claimed advantages of this detector was that is was possible to obtain the carbon, hydrogen and other element contents of an unknown molecule and so calculate an empirical formula. In practice it was found to give an empirical formula for low molecular weight compounds that was accurate to about + 10% and the equipment was never really seriously used for this purpose. Sensitivity and selectivity varied considerably from one element to another. In favourable cases such as the halogens, picogram per second levels could be detected with a selectivity over carbon of about 103 but nitrogen and oxygen gave much poorer performance mainly due to the large background signal.

A new version was made commercially available in the 1980s. This version differs from the original in two main respects - firstly the microwave discharge takes place in a cavity at atmospheric pressure and secondly the optical spectrometer is replaced by a photodiode array. The result of these modifications is a detector that is much smaller and easier to operate although the actual performance in terms of sensitivity and selectivity is not greatly different from the original designs. The relative compactness and ease of operation have led to a resurgence of interest in this detector but it is not cheap and as GC-MS systems come down in price they represent strong competition. The definitive account of the various forms of microwave plasma detector is given by de Wit and Beens (see Further Reading).

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Solar Panel Basics

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