The calibration of a particle size analyzer can be performed using certificated reference material (CRM) or general reference material (RM). CRM was verified by the qualified or regulatory departments and the data was released with scheduled dates. At present, the absolute measurement method in China is the microscopic method. Its scale is traceable to the length standard. Usually, the scale bar is 10μm and the smallest scale has now reached 416nm.
In addition to accurate size value, a RM should possess the following properties:
(1) The material is stable and no size change within a certain period (e.g., 1 year);
(2) Monodisperse particles, that is, the coefficient of variation for particle size distribution is as small as possible. Usually, the coefficient of variation is ≤3% for grade 1 RM and in the range of 3% ~ 8% for grade II RM;
(3) Good spherical shape, usually requiring roundness of 95% to 98%.