What is Z-average and polydispersity index (PDI)?
Z-average is the intensity-weighted mean hydrodynamic size of the ensemble collection of particles measured by DLS.
The Z-average is a hydrodynamic size that is determined by an ISO method, i.e., the cumulants method. For the cumulants method, the correlation function is solved by the following expression:
where A is the intercept of the correlation function, which represents the signal-noise ratio and Γ is the average decay rate, which can be substituted into the equation Γ =q2D to calculate the average diffusion coefficient D of all particles. D is then put into the Stokes-Einstein equation to produce the average particle size DH , namely the Z-average mean hydrodynamic diameter.
For dynamic light scattering, the Z-average is used to present the average particle size, which is the intensity-weighted average size. PDI (Polydispersity Index), defined by PDI=µ2/Γ , which indicates the width of the distribution.
As a rule of thumb, the following PDIs are shown for different samples.
|Mono-dispersed Sample||0 - 0.05|
|Narrow-dispersed Sample||0.05 – 0.08|
|Moderate-dispersed Sample||0.08 – 0.7|
|Wide-dispersed Sample||> 0.7|
DLS is a technique suitable for samples whose particle size distributions are relatively narrow. For samples with PDI greater than 0.7, DLS measurement might not be applicable.