The easiest way to add very small quantities of a substance is to weigh out a multiple quantity and dissolve it in a defined volume. Appropriate proportions of this stock solution are then added.

Example 1

Dissolve 100 mg of potassium iodide (KI) in 100 ml of water.
Each ml then contains exactly 1 mg of KI. Using a 1 ml injection syringe, quantities up to the lower limit of 0.05 mg can then be measured without further ado.

Alternatively, the substance can be ground in a mortar in several stages with a neutral compound for the intended purpose.

Example 2

100 mg cyanocobalamin is first thoroughly triturated with 400 mg ascorbic acid. Then a further 4.5 g of ascorbic acid is added and again thoroughly triturated. This is then repeated with a further 5 g of ascorbic acid.
The result is 10 g of a mixture, mainly of ascorbic acid, containing 1 % cyanocobalamin.