For the Mastersizer, first find the small volume cell, not the 2L "swimming pool" :-). You need about 1 drop of a 10% sample, so that's about 5microL of solid, depending on the size and the polydispersity. Or you can organize your own small volume recirculation by plumbing your own system with two syringes for a back and forth movement. For the Coulter Counter it depends on the particle size, which sets the orifice you need. If you need a large orifice you will use a large volume of their overpriced dispersant, but the sample is very dilute. You need to do a few trials with a not precious sample to see how it works out.

The first question to ask is what is the benefit of doing a particle size analysis on such a small amount of sample?  We don't know what the sample is, the form it is in, and how much sample mass is actually available. What critical quality attributes are governed by the size distribution?  You don’t say whether you require an ‘as is’ measurement (including agglomerates) or a dispersed sample size.  What is important to you and why? See the 4Q’s attached.

Next question to ask is to what precision to you require the analysis and at which point in the distribution (x10, x50, x90, x95?). If each particle were identical then only one particle would be need to represent the entire distribution.  Indeed a single 1 mm particle will provide enough scattering for a particle size distribution.  The wider the distribution then the more sample mass that’s needed to represent the higher points in the distribution; the tighter the accuracy requirements then the larger is the minimum sample mass needed to represent the entire sample (twice the precision requires 4 times the number of sample particles).  See:

December 14th, 2010 Sampling for particle size analysis - estimation of standard error

https://www.brainshark.com/malvern/vu?pi=zH8zgQtOvz5chz0&tx=preview

https://www.brainshark.com/malvern/vu?pi=zGBzlGgVGz5chz0&tx=preview

You state that you want to recover your sample, so this implies a wet analysis followed by decantation or filtration (possibly with a further purification stage such as recrystallization). Wet analyses are conducted typically in low concentrations (0.001 – 0.1 volume%) with large sized samples provided enough scattering signal for a meaningful measurement but small particles scatter weakly and optical concentration being important to optimize.

Now, you do not say what Mastersizer you’re using.  With a MS2000, the smallest volume accessory is 15 mL and for the MS3000 it’s 7 mL of total diluted sample.  With small accessories, cleanliness is next to godliness and care is needed with sample preparation (e.g. dilution).  A single drop of 10% by volume sample could prove too much in such a small accessory and large or dense material needs to be fully suspended  The largest accessory is actually 800 – 1000 mL in the MS2000 (not 2L, although for wide distribution samples this could be advantageous with larger sample masses).  I’ve measured 2 – 5 mg of sample successfully in the past, but this depends on the sample type, the precision required at the top end of the distribution, and other factors as indicated above.

@ Janet Smith Your profile indicates that you may be looking at soils and sediment samples. Here dispersion is usually effected with optimum level of phosphate as stabilizer when dispersed.  Sample size range is usually wide with such samples and thus you have to ask yourself is the sample representative and how you'll divide up a sample.  Perhaps the only way to get good data is to measure the entire amount of material you have if the distribution is broad.  The clay fraction (if present) is critical - hard to disperse without fracture and sets the optical properties. See also:

February 9th, 2012 Size and shape characterization in soils and sediments

https://www.brainshark.com/malvern/vu?pi=241199769&text=M021507&r3f1=

@ Hannan Lagarry    Not possible if the total supply of material is 10 - 20 mg (as it could be for the pharmaceutical industry)......