Probably, it is good to measure the particle size distribution by yet another method that is more direct, such as electron microscopy (TEM or SEM) and to compare the size distribution profile with those obtained by DLS. Then, the experiment will show you which one is most correct for your system. Anyway, DLS gives the so-called hydrodynamic diameter, which could be slightly larger than the actual size. Also, DLS gives you a kind of idealized distribution curve, and also its difficult to distinguish between particle aggregates and single particles. Probably, most correct is distribution by intensity (although I have heard different opinions), but if you need a value for an average size, the so-called Z-average size is probably close to the real average size. Anyway, I recommend a comparison of the respective distributions with such obtained by a different, more direct, microscopic method.

When measuring using DLS , following Z-average size is the best to compare different sample. Just make sure they all contain the same concentration of particles. Intensity distributions are the best in this case.

Good luck !

Thank you for your response. But I have a question about Z-average. In all measurements method (number, volume, or intensity) the value of Z-average is greater than all measurements done by these methods. Why Z-average is greater than all? and what is the differences between Z-average and other three methods?

Another question is about the application of these method on DDS. in drug delivery systems beside the particle size and size distribution (PdI) which is very important parameters, there are two other parameters which is important too. These two parameters are loading of drug and release rate which is related to volume and size of particle,respectively. Regarding to this fact, particle size measurements based on Volume is important or intensity?

As I know, in DLS, the distribution obtained from a measurement is based on intensity of scattered light. The various distributions (volume, number, intensity) usually can be calculated from each other. If your particles are mono-disperse in size with a quite narrow and symmetric distribution, then the various distributions will differ a little from one another. When the size distribution is quite broad (large Pdi), unsymmetrical (skewed) or even multi-modal, then the various distribution profiles will look more different. For example, the smaller particles in the distribution will appear with higher freq. (% in class) in the number distribution compared to intensity distribution.

Size data in DLS are obtained by various exponential fits of the so-called correlation function (which represents the actual experimental data). Z-average diameter and polydispersity index (pdi) could be obtained by a single exponential fit, while size distribution is obtained by multiple exponential fits (although I am not a specialist in the actual mathematics behind these fits). Therefore, I think it is normal to expect differences in the obtained diameters. anyway, if you provide a size distribution of your samples in a publication, you have to indicate what is this distribution (intensity, number or volume).

Particles for drug delivery I think should be monodisperse in size, with narrow distribution and therefore differences between the various distributions should be small. If otherwise, the very different in size particles would have different properties and interactions with bio-systems and therefore, their biological effects would be difficult to interpret. If no drug release or aggregation of particles takes place during the DLS measurement, then it should be ok. The loaded drug should not absorb the laser light that is used for measurement, because this will decrease the intensity of the scattered light and will diminish the sensitivity of the method.

If your particles are spherical, then I think it generally does not matter which distribution you will give (because these could be calculated from each other if you know some parameters of the system), but I think it is important to provide always data about the z-average size and Pdi. If your particles are of various shapes, then you can provide all distributions (for information) and electron microscopy images for the reader to get more realistic impression about the characteristics of your particles.

I hope these comments help somehow.

Good luck!

Dear Yordanov

Thank you for your valuable information.

As you talked about DDS, I should mention that most of the DDS is monodisperse in size with narrow distribution and almost has spherical shape. Regarding to this fact and with your comment, I think all these three method is valuable for reporting size of particle.

But there is even another question for me that I can't find answer for that. Why Z-average is greater than other three method (volume, number, intensity)?

Indeed, the Z-average is calculated by an equation which is already implemented in the machine, which is off course based on the diffusion of the particles. This equation is a kind of default. In some DLS machines it is possible to change the diffusion model by which the calculation is done. Honestly , I won't recommend to change it.

For more information regarding the use of DLS to characterize DDS, advantages, disadvantages, and release studies you are invited to have a look on the attached publication.

Good luck !

Generally, d(intensity) > d(volume) > d(number). I obtain similar tendencies in my experiments. Because, z-average is calculated from intensity measurements, one may expect it to be close to d(intensity) and therefore it is larger than d(volume) and d(number). As I know, one needs to know refractive indexes for calculation of d(volume) and d(number). If, for example such indexes are not correctly estimated, one may obtain a quite large deviations in different size distributions even for monodisperse particles.

Z- average gives the overall average size of the particles you have in your system. When you measure a monodisperse sample (associated with one peak in distribution and a very small pdI) then the value of the z-average is close to the size of the main peak observed in the intensity distribution graph. The peak mean is generally smaller than the z-average, I think because when obtaining a distribution graph the built in models allow the mathematical fits to take the possibility of having more than one population which results in removing the contribution of other populations and potential impurities from the major peak mean. One may consider this a great option, however, you should realize that doing this you are making more assumptions and rely too much on the mathematical models rather than direct experimental observations. 

In general my recommendation would be to use the z-average as far as possible because then you make the least number of assumptions in assesing the size of particles. In case you have other populations in your pool of particles (polydispersity) then I would use other methods (such as NTA) alongside DLS to get a better picture of the particle size distribution as DLS results are less reliable for polydisperse samples (For more information on the limitations of DLS see Filipe et. al. in Pharmaceutical Research May 2010, Volume 27, Issue 5, pp 796-810).