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The advantages and disadvantages of on‐stream energy dispersive analysers for X‐ray fluorescence and diffraction have been examined. These include factors such as ease of sample presentation, selection of exciting X‐ray energy, stability, count rate performance and microphony. Finally a number of applications and feasibility studies have been described including work for the cement, mineral processing, extractive metallurgy and related industries.

OF CONTENTS Introduction X-ray excitation X-ray tubes Electron and proton excitation Radioisotope sources Detectors Electronics Monochromators Pulse height selection Semi-conductor detectors X-ray filters Effect of sample on X-ray intensity Inter-element effects Heterogeneity effects Sample surface Standards Methods of overcoming heterogeneity Methods of overcoming inter-element effects On-stream analysis Limits of detection Measurement of X-rays Energy selection Quantitative analysis Counting strategyWHEN electrons are ejected from the inner orbitals of an atom their replacement results in the emission of X-rays1 Those caused by filling vacancies in the innermost orbital are called K X-rays, those by filling vacancies in the next orbital L X-rays, and so on. Each element emits X-rays at certain characteristic energies that are equal to the energies of the corresponding electron transitions.2 Thus if a substance is suitably excited so that electrons are ejected, the elements composing it can be determined and, as the X-ray intensity at each characteristic energy level depends on the concentration of the element emitting it, both qualitative and quantitative analysis become possible (Fig. 1). If X-rays from a tube or radioisotope are used for excitation the technique is known as X-ray fluorescence analysis. However, as techniques involving electron2 and other particle excitation perform functions similar to those in which X-ray excitation is used, they have been included in this review paper. The existing range of commercial instruments permits the detection of all elements except hydrogen and helium. If sealed X-ray tube excitation is used the range of elements detected is generally limited to those with atomic numbers higher than oxygen because of the difficulties associated with the excitation of the characteristic X-rays from the lighter elements. Most elements can be determined at concentrations varying from a few parts per million to major amounts, although the limit of detection deteriorates with low atomic numbers.The simplest type of instrument for X-ray fluorescence analysis is the portable unit3 with radioisotope sources, which is used for mineral prospecting or shop-floor testing. In these instruments use is normally made of X-ray filters to select the required characteristic For details see Summaries in advertisement pages. Reprints of this paper will be available shortly.

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THE measurement of back-scattered fluorescent X-rays from a block containing a suitable element, placed on the side of the slurry flow remote from the X-ray source has been described.' It was used as a means of correcting for variations in the fluorescent X-ray intensity from the slurry, caused by changes in the composition and content of the solid component of the slurry. If this method is used for on-stream analysis without considering the effect of changes in particle size, further errors could occur. It can be predicted quite easily from basic X-ray absorption theory2 that this back-scattered fluorescent X-ray intensity should increase as the particle size in a slurry of constant Composition increases. However, it has also been shown3 that the fluorescent X-ray intensity from particles in an aqueous slurry will decrease as the particle size in a slurry of constant composition increases.Percentage of solids in slurry Fig. 1. Effect of change in particle size on back-scattered cadmium I<, intensity : curve A, particles from 100 to 150 mesh; curve B, particles less than 300 mesh Percentage of iron in slurry Fig. 2. Effect of change in particle size on iron I<, intensity from the slurry: curve A, particles from 100 to 150 mesh; curve B, particles less than 300 mesh I t was decided to test these predictions experimentally, with a Phillips PW 1540 X-ray spectrograph adapted to take an inverted slurry presenter. It was found that the fluorescent intensity from the reference block-in this case made of cadmium-increased with increasing particle size in the haematite -water slurry (see Fig. l ) , while the fluorescent iron Ka intensity from the particles in the slurry decreased (see Fig. 2). Similar effects would be expected with all elements capable of being determined on-stream by X-ray fluorescence analysis.