Aliquots: concentration and traceability

Two containers derived from the same solution need their own identifiers even when they share an originating batch.

An aliquot is a measured portion separated from a larger solution, the stock solution. It retains the same composition and concentration provided that the stock is homogeneous before division. No particular volume defines an aliquot. What matters is that the portion represents the whole and that the measurement used to separate it is recorded.

In research compound handling, aliquoting commonly follows reconstitution of a lyophilizate. The complete solution is divided into smaller portions, each sized for one use, then closed and stored separately.

Repeatedly freezing and thawing one container promotes aggregation and loss of material in solution. A vial frozen and thawed five times is not equivalent to five aliquots thawed once. In the first case, the remaining stock accumulates another cycle with every use, up to five; in the second, each portion experiences one. Aliquoting turns a stock consumed over several occasions into separate stocks consumed on one occasion each.

Calculate aliquot volume from the amount required by an experiment, including transfer losses. Oversized portions require thawing more than will be used: the remainder must be discarded or refrozen, defeating the purpose. Undersized portions magnify measurement error and the contribution of dead volume, the fraction not recovered because it remains on container walls or in the tip.

It is also useful to retain an unopened portion from the same batch and preparation for repeat testing if a result is inconsistent. Without that reserve, investigating a later discrepancy requires a new vial, which is no longer the same preparation.

An aliquot does not have its own certificate of analysis. Its documentation comes from the source batch, and that connection exists only when recorded. An unlabeled tube is no longer traceable, characterized material, regardless of the quality of its contents.

The minimum label identifies the compound, batch, concentration, diluent and preparation date. Record the concentration calculated using the volume actually transferred, not merely the intended concentration. This easily forgotten difference can explain why two preparations from the same batch disagree.

An aliquot is often confused with a dilution. Aliquoting preserves the concentration of the source solution; dilution lowers it by adding diluent. Dividing a stock among ten tubes creates ten aliquots, not a 1:10 dilution. Nor does aliquot mean vial: the vial is the container, while the aliquot is the measured portion inside it.

Parent identifier, separated volume, any dilution, date and destination container.