What is Aliquot?

An aliquot is a measured portion withdrawn from a larger volume. In a peptide lab the word usually describes what happens straight after reconstitution: the stock is split into small tubes, each holding enough for one experiment, so the rest of the material stays frozen until it is needed. Each tube is thawed once, used, and whatever is left is discarded.

Why split a stock

• One thaw per portion. A stock thawed for every experiment accumulates freeze–thaw cycles; aliquots cap that number at one.
• Fewer entries into the material. Every pipette entry is a chance to introduce microbes or proteases. Sterile technique lowers the risk per entry; aliquoting lowers the number of entries.
• Traceability. Numbered tubes let an odd result be tied back to a specific portion and its storage history.

Worked example

A 5 mg vial of BPC-157 (molecular weight 1419.5 g/mol) dissolved in 2.5 mL gives 2.0 mg/mL, about 1.41 mM. Suppose each assay day consumes 80 µg.

The 50 µL format fits. Some liquid always stays on the vial wall and in tips, so the real count lands a few tubes short of 50. These figures also treat the 5 mg as pure peptide; where molar accuracy matters, correct for net peptide content first. The reconstitution calculator handles the volume arithmetic and the molarity calculator the conversion to molar units.

Tube and volume choice change what you recover

Peptides adsorb to container surfaces, and the loss is proportionally larger in small volumes. When cerebrospinal fluid was stored in polypropylene, amyloid-β 1–42 measured from 0.5 mL aliquots was higher than from 0.1 mL aliquots of the same fluid; of the low-retention tubes compared, 1.5 mL Eppendorf LoBind tubes adsorbed the least ^[1]. A separate study of eight radiolabelled endocrine peptides, including ghrelin and GLP-1, found large differences in recovery between glass and plastic surfaces, lower recovery after siliconising, and better recovery when 1% bovine serum albumin was added ^[2]. Its authors concluded that the container has to be chosen peptide by peptide ^[2].

Common misunderstandings

• "Weigh out the powder in portions instead." Splitting a few milligrams of hygroscopic lyophilised cake on a standard balance is far less accurate than dissolving the whole vial and dividing by volume.
• "Refreezing the leftover is harmless." It turns that tube back into a multi-thaw stock, the situation aliquoting exists to avoid.
• "Smaller is always safer." Very small aliquots lose a larger fraction to the tube wall ^[1], and pipetting and evaporation errors grow as volume shrinks.
• "The diluent no longer matters once frozen." Bacteriostatic water contains benzyl alcohol so that a multi-entry vial resists microbial growth. A set of single-use frozen aliquots does not need that, and in a cell assay the preservative is one more component in the well.

Label every tube with compound, concentration, diluent, date and tube number, and log it. The peptide storage guide covers temperatures and hold times for dissolved material; the reconstitution guide covers the steps before the split.