THE EFFECT OF SUSPENSION MEDIUM REFRACTIVE-INDEX ON THE PARTICLE-SIZE ANALYSIS OF QUARTZ BY LASER DIFFRACTION

The size distributions of four samples of quartz dust, suspended in liquids of different refractive index (RI), were determined using a Malvern 2600 laser particle sizing instrument. As the relative RI, between particle and suspension media, approached unity the instrument over-estimated the mean particle size for the three finest quartz samples. An under-estimated mean particle size was recorded for the largest quartz sample indicating the complex nature of light scattering by small particles under conditions of low relative RI. A regime of relative refractive index is determined within which particle size analysis using laser diffraction, invoking anomalous diffraction theory, becomes prone to large errors and this range is compared with the van de Hulst criterion.