(13e) Refractive index verification via comparative wet and dry laser diffraction measurements

Refractive index is probably the most important parameter in the deconvolution of the measured intensity-angle information obtained in a laser diffraction light scattering experiment. Errors in the assigned values to this parameter provide corresponding errors in the derived values especially with respect to the y- (or %) axis. Use of inappropriate models or refractive indices can lead to (for example) extra peaks being seen in the size distribution and, in many cases, an over-assignation of the fine material actually present in a sample. The extent of this difference from reality is difficult to quantify in simple terms as the size distribution and relative refractive index (RRI) both play a role. This paper briefly outlines the ways that intensity - angle information can be deconvoluted to a particle size distribution before citing 2 examples where a comparison between data obtained in dry and wet diffraction measurements enables rejection of possible, but unreasonable, values and 'homing in' on reasonable and robust properties for the optical parameters especially the imaginary component of the RI. This practical approach relies on the fact that the RRI is considerably higher in a dry measurement (RI of air is ~ 1.00 as opposed to water at 1.33) and thus some robustness of the derived result to optical properties is expected.