Characterization of standard reference material 1690 by modulated dynamic light scattering

We have measured the number average diameter (NAD) and the standard deviation of the diameter distribution (SDDD) of spherical particles from a sample of Standard Reference Material 1690 (SRM 1690) using the method of Modulated Dynamic Light Scattering to determine the diameter of single test particles by examining their Brownian motion as they sediment freely in dry air. Standard Reference Material 1690 is a water-suspension of 1 mm nominal diameter polystyrene spheres and has been characterized by National Institute of Standards and Technology (NIST) personnel as a primary reference standard for the calibration of particle-size-measuring instruments. We have obtained for SRM 1690 particles the values NAD = 0.9103 +- 0.0021 mm and SDDD= 0.0090 mm. The NIST-certified values for SRM 1690 are NAD = 0.895 +- 0.008 mm and SDDD= 0.0095 +- 0.0012 mm. The SDDD value from this work agrees with the NIST-certified value within the stated limits of uncertainty, but the NAD value is 1.7% larger than the NIST-certified value. We suggest that the discrepancy is due to the presence of nonspherical particles in the test suspensions measured by NIST.