Your sand samples must be free of any material not analyzable by sedimentation in water.
Your sand samples must:  

1. consist of water-insoluble particles only,
2. be within the grain size range of 0.05 – 4.0 mm only.
3. be free of "dirt" (such as organics etc.), salt, etc. 
4. be representative for your bulk sample, i.e. consist of about 20,000 particles.
   This particle number, for quartz and/or calcite density, gives the sample mass
   M = 25 d³ , where
   M — sample mass in grams
   d — the 10-percentile of grain size in millimeters.
   Coarse sand samples should have mass about 5 gram,
   medium sand samples should have mass about 1 gram, 
   fine sand samples should have mass about 0.2 gram. 
   
   Here ist the picture of our 20-sample box suitable for airmailing, outer dimensions 24 by 17 by 5 cm: 
   
   
   
   
5. The sample amount must be never adjusted "by spoon", but by a precision rotational sample splitter. 
   Please send us multiples of 8 to 10 of the above given representative gram masses: we will divide them by our precision rotational sample splitter into the suitable sample sizes. This allows us to repeat the analyses and keep your samples in our back-up archive for the minimum of 1 year.