Single particle optical sensing (SPOS) is a highly sensitive particle characterization technique for the measurement of the large particle (> 0.5 μm) tail of a submicron particle size distribution. We have examined the SPOS technique for fumed silica dispersions which have a mass mean particle size of ~140 nm. The reported large particle population varies as a highly non‐linear function of sample concentration and is accompanied by the distortion of the particle size distribution. A similar response is also demonstrated for monodisperse polystyrene latex spheres. The source of spurious particle counts and the distortion of the particle size distribution is shown to be secondary coincidence. Model calculations indicate that the primary contributors to spurious counts are particles near the peak of the size distribution. These particles are substantially smaller than the nominal minimum 0.5 μm diameter. These findings establish the limited range of reliability for SPOS measurements when used to measure a small fraction of the total particle size distribution and establish the need for new particle characterization metrologies.

We have examined the Large Particle Count (LPC) analytical method to see whether there are opportunities to improve both the accuracy and precision in hope of improving the utility of the LPC measurement. We have identified weaknesses in the current method that limit both its accuracy and its precision, and which can introduce count errors in excess of a factor of 10. We propose modifications to the current method which result in both accuracy and precision improvements. We recommend these improvements as absolutely necessary for any experiments designed to test the correlation between LPC and defectivity.