Question

how to prepare 24 μL of 5% H3BO3

Answer 1

Is that w/w or w/v? I'll assume w/v.

What sort of equipment is available.
5% means 5g in 100 mL soln.
You want 24 uL; therefore,
5 g x (24E-3/100) = ? g H3BO3 which look like about 1.2 mg H3BO3 and that's easy enough to measure. I don't know how you will measure the 24 uL.