-

1. Particle Size Result
Interpretation
Elaheh Entezar-Almahdi, Pharm.D
Ph.D Student of Pharmaceutics

2. 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

3. Number vs. Volume Distribution

4. Number vs. Volume Distribution (Cont’D)

5. Transforming Results
Results 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.

6. Example (TiO2)
Mean (nm) Width (nm)
Intensity 87 32
Volume 65 25
Number 50 14

7. Question:
So Which is the Correct Size?
Answer:
All three are correct!!!

8. Z-Average Size
The 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.

9. 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.

10. References:
• Malvern Instrument (Technical notes)
• HORIBA Scientific Guidebook