You have to be much more specific:

What type of particles

What is their size

And what  size do you mean? Most particles are not spherical

Dear Harry Brink 

Well, let us consider particles of some transition metals oxides prepared by sol-gel method and processed to nano-powder. It is not necessary that all particles within this powder are spherical. 

Thanks 

The fact is that automated instruments give you a size as if the particles are spheres and it appears that your 100 nm error limit for nanopowders is very large ?!

I reread your question and I think you mean what techniques are available that can accurately determine the size of particles that are smaller than 100 nm.

Do you want to buy such instruments?

Do you have experience with a technique?

Should it be an on-line technique?

How often would you use it?

Hi,

I am studying nanoparticles from combustion process. These nanoparticles have a typical size about 100 nm and are not spherical. These nanoparticles follow a fractal shape. We determined their size with an SMPS.

The size is expressed in terms of mobility diameter Dm in function of dN/dlog(Dp).

@Alexandra

Indeed and actually we used an SMPS to measure the size of nanoparticles formed in the liwuid pahse by nebulizing it at very low concentrations so that only one particle is present in the droplets

BTW you get the mobility size of the particles

NB: we were the inventors of the DMPS that preceded the SMPS (published in 1983)

Dynamic light scattering can measure sizes from a few nm to about 1 micron, provided the particles can be dispersed in a liquid.