Particle Size Result InterpretationElaheh Entezar-Almahdi, Pharm.D

Ph.D Student of Pharmaceutics

What is a distribution?

Number of particles in the different size bins

Total volumes of particles in the different size bins

How much light is scattered by the particles in the different size bins

Number Distribution

Volume Distribution

Intensity Distribution

Number vs. Volume Distribution

Number vs. Volume Distribution (Cont’D)

Transforming ResultsResults from number based

systems, such as microscopes or image analyzers construct their beginning result as a number distribution.

Results from laser diffraction construct their beginning result as a volume distribution.

Example (TiO2)

Mean (nm) Width (nm)

Intensity 87 32

Volume 65 25

Number 50 14

Question:So Which is the Correct Size?

Answer:All three are correct!!!

Z-Average SizeThe Z-Average size used in DLS is a parameter also known as the cumulant

mean.It is the primary and most stable parameter produced by the technique.The Z-Average mean is the best value to report when used in a quality control

setting as it is defined in ISO 13321 and more recently ISO 22412 which defines this mean as the ‘harmonic intensity averaged particle diameter”.

It should be noted that the Z-average is a hydrodynamic parameter and is therefore only applicable to particles or molecules in solution.

The Z-average size will only be comparable with the size measured by other techniques if the sample is monomodal, spherical or near-spherical in shape, monodisperse, and the sample is prepared in a suitable dispersant.

Polydispersity Index (PDI)

• The Polydispersity Index is dimensionless.• It is scaled such that values ˂ 0.05 are rarely seen other than with

highly monodisprse standards.• Values ˃ 0.7 indicate that the sample has a very broad size distribution

and is probably not suitable for the DLS technique.

References:

• Malvern Instrument (Technical notes)• HORIBA Scientific Guidebook