Aliquot
In short An aliquot is a measured portion of a solution. When working with peptides, the finished solution is divided into small portions so that the same container does not have to be opened again for every withdrawal.
The volume of an aliquot is the amount required divided by the concentration. At five micrograms per microlitre, 250 micrograms corresponds to exactly 50 microlitres.
Aliquoting protects the rest of the solution from exposure to air and from repeated thawing. If the solution is to be frozen, divide it first so that each portion thaws only once.
The practical reason for dividing is the number of punctures. Every time the septum of a vial is pierced, air and possible microbes get into the container and the solution is put under additional strain. Leave the whole amount in one container and you pierce it again at every withdrawal; divide it straight after preparation and each portion is disturbed exactly once.
The second consideration is freezing. Repeated freezing and thawing takes its toll on the molecule, which is why it makes sense to divide the solution before freezing it: each portion is then thawed only once.
How large a portion should sensibly be depends on which volume can still be measured accurately. Very small volumes are prone to error, so a lower concentration with a larger withdrawal volume is often the more accurate choice. Both are set when the solvent is added, as described under reconstitution.