# Filter out the SiO2 ultrafine particles from water solution to measure their total weight and size distribution?

The black soil sample mostly contains the organic matter, chalk and quartz. It is necessary to measure the relative weight of SiO2 particles that are 2.5 μm or less.

To do that the organic matter and chalk were dissolved by additional chemicals and then the sample was filtered through the paper filter and dried. Then the dried part of the sample was weighted and Mastersizer 3000E laser diffraction particle size analysis instrument was used to measure the sizes of particles in it.

The measured relative number of particles with size from 2.5 μm to 0.6 μm was 3.6% But Mastersizer 3000E did not find any particles smaller then 0.6 μm perhaps because they went through the paper filter into the filtered out solution.

The question is how to catch that particles that are smaller then 0.6 μm in this case ? Which filter or process can be used to collect and return them to that dried part of the sample ?

There are plently of commercial membrane filters which can readily filter out particles less than 0.6 $$\mu$$m. All you have to do is to buy the right one. Simplest and most common filters are HPLC filter for solvents. They can easily block > 0.2 $$\mu$$m particles. Search "Filter membrane, Nylon 47 mm, pore size 0.20 µm". They are not very expensive.
Note that you cannot quantitatively scrape out very small particles on nylon filters. Also pore size definition is "The validated retention/removal cut off for a membrane. Note that a 0.22 $$\mu$$m membrane does not have 0.22 $$\mu$$m “holes” but rather a pore structure validated to remove all particles or microbes above its stated pore size."