Wolfgang Laemmle, SICO scientific industrial consulting office,
ex. SYMPATEC GmbH, Germany
The most popular and best researched method, using light scattering, is Photon Correlation Spectroscopy PCS including its variations. First PCS instruments, available from the late 1970ies, needed a very robust and simple calculation mode, the ‘2nd Cumulant’ method (ISO 13321).
The PCS method makes use of the Brownian motion, which creates a fluctuation of the scattered light intensity in the visual field of a stationary observer and the fluctuation frequency is directly related to the particle size. But this direct relation is valid only under multiple restrictive conditions.
Only instrumental setup of Photon-Cross-Correlation-Spectroscopy PCCS can eliminate multiple scatter entirely. Two different light beams of identical wave length and intensity are focused to an identical measuring volume and the fluctuation of the scattered light is detected each under 90°. The identical part in both measurements is extracted by cross correlation. Hence independent from concentration only unadulterated primary signals contribute to the evaluation.
Modern particle size measuring instruments provide smooth diagrams, but no reliable indication for the relevance of the presented result. Only comparison of the results correlation function diagram to the raw signal correlation diagram can show meaningful differences and thus detect misinterpretation, if so, a more sophisticated evaluation mode needs to be applied. e.g. NNLS method
High gloss brochures qualify instruments often misleadingly because they disregard above mentioned regards as well in PRINZIPLE, in INSTRUMENTS as in RELEVANCE.