April, 19661 SHORT PAPERS 289 The Effect of Particle Size on Back-scattered X-ray Correction Methods in On-stream X-ray Fluorescence Analysis BY K. G. CARR-BRION (Warren Spring Laboratory, Stevenage, Herts.) 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.290 SHORT PAPERS [Analyst, VOl. 91 A decrease in the size of the X-ray absorbing particles will therefore cause a marked increase in the apparent solid content (see Fig. 1). This solid content is used, either directly or indirectly, to obtain the concentration of the element being determined in the solid component, since the fluorescent X-ray intensity is a function of the concentration of the element in the slurry as a whole.Even if separate means of correcting for real changes in solid content are incorporated, the use of back-scattered radiation will in no way compensate for particle-size effects. The measured value of the X-ray mass-absorption coefficient will accentuate these effects to a degree dependent on the system being examined. Thus if the method is used to correct for variations in the com- position and content of the solid component of the slurry, a correspondingly greater control of particle-size variations is necessary. It should be mentioned that this effect has also been observed in heterogeneous solid samples when back-scattered or transmitted methods of correcting for matrix effects are used,4 but that under these circumstances, sufficiently close control of particle size makes it possible to obtain accurate quantitative results. REFERENCES 1. 2. Croke, J., and Deichert, R. W., Norelco R e p . , 1964, 11, 116. Liebhafsky, H. A., Pfeiffer, H. G., Winslow, E. H., and Zemany, P. D., i n “X-ray Absorption and Emission in Analytical Chemistry,” John Wiley & Sons Inc., New York and London, 1960, p. 14. Claisse, F., and Samson, C . , A d v . i n X-ray Analysis, 1962, 5, 336. Carr-Brion, K. G., Analyst, 1965, 90, 9. 3. 4. Received July 12th, 1965