Primary standard for the number concentration of liquid-borne particles in the 10 to 20 µm diameter range

Abstract

The national primary standard for the number concentration of liquid-borne particles in the 10 to 20 µm diameter range has been developed at the National Institute of Advanced Industrial Science and Technology (AIST), Japan. The standard consists of a total number counting type flow cytometer (T-FCM) and an electronic balance. The T-FCM is a commercial flow cytometer modified so that the total number of particles in an aqueous suspension sampled in a test tube can be counted, and the electronic balance is used to determine the mass of the suspension. This standard is intended to be used for calibrating commercial standard suspensions of monodisperse polystyrene latex (PSL) particles. The measurand in the calibration is the mass-based number concentration (the particle number in a unit mass of a suspension), and the calibration capability covers the concentration range from 5 × 102 to 2 × 106 particles g−1. When the concentration of the suspension is higher than 2 × 103 particles g−1, the suspension is first diluted to about 1 × 103 particles g−1 to suppress the coincidence loss in particle counting by the T-FCM. The validity of the calibration with the T-FCM was examined by comparison with an independent method in which a scanning electron microscope (SEM) was used to determine the number concentration of particles deposited on a silicon wafer. For a suspension of 10 µm PSL particles with a concentration of approximately 1 × 106 particles g−1, the concentration values determined by the T-FCM and SEM methods were 1.042 × 106 and 1.035 × 106 particles g−1, respectively: The difference was less than 0.7%. The relative expanded uncertainty of the measurement by the T-FCM method with the coverage factor k = 2 was 4.4%.