Primary standard for the number concentration of liquid-borne particles: extension of the diameter range from the micrometer to the sub-micrometer level and reduction of the measurement uncertainty

Abstract

The primary measurement standard for the number concentration of liquid-borne particles adopted by the National Metrology Institute of Japan was upgraded to cover particle diameters in the range of 600 nm–10 µm. The main aim was to report the extension of the lower limit of the measurement range in particle diameters from 2 µm to 600 nm, and the reduction of the measurement uncertainty inherent in the previous method using the total-counting-type flow cytometric technique. To extend the measurement range, we adopted a mass-measurement-type optical particle counting method, which consisted of a pair of optical particle counters with a pulse height analyzer, an electronic balance, and a syringe sampler. The relative expanded uncertainties with a coverage factor of k = 2 for the 600 nm and 10 µm suspension measurements were 4.3% and 3.5%, respectively. The validity of the results was confirmed in both cases via comparison with the results obtained using an independent microscopic counting method. With the change in the measurement method, a reduction of the uncertainty was accomplished: e.g., from a 4.4% to a 3.5% relative expanded uncertainty (k = 2) for a 10 µm polystyrene latex particle suspension.