Comparing Laser Diffraction and Electrozone Counter for Particle Size Analysis

Table 1 gives the measured variability of the off-line electrozone counting PSA compared with that of Malvern Insitec on-line laser diffraction PSA in a toner milling line.

PSA instrument	Measurement technique	On-line?	Sample throughput	Update period / timelag	Standard Deviation for Dv(50) measurement
Coulter Multisizer	Electrozone counting	No	5g every 2 hours	1 measurement every 2h with at least 20mins time lag before result	>=0.1
Malvern Insitec	Ensemble laser diffraction	Yes	20kg/h	1 measurement every 10s (average of 20,000 measurements), real time updates.	0.01 - 0.1 (product dependent)

Table 1: On-line laser diffraction PSA versus off-line electrozone counting PSA

The higher variability of the off-line electrozone method is due in part to the manual sampling and sample preparation, which is performed by several different operators. Malvern Insitec laser diffraction method has proved to be more sensitive to real process changes than off-line measurement, allowing even subtle variations to be corrected.

In Table 2, an example is given of the measured variablity of each PSA technique for one product type (measurements carried out in same conditions).

PSA technique	Points measured	Standard Deviation for Dv(50) measurement
Coulter Multisizer (Off-line electrozone counting)	108	0.19
Malvern Insitec (on-line laser diffraction)	8000	0.07

Table 2: Measurement variability of laser diffraction PSA versus off-line electrozone counting PSA