01Identify why material was pooled

EPA’s composite-sampling guidance discusses physical combination of samples, reduced analytical workload and the information lost when individual units are not measured separately. Its environmental examples illustrate a general sampling trade-off rather than a peptide testing standard.US EPA — Composite Sampling (opens in a new tab)

The question matters. Estimating an average target content differs from establishing that each selected vial lies within a range. A method designed for the first task should not quietly be used to claim the second.

Pooling is not intrinsically weak or strong. Its usefulness depends on whether the retained information answers the intended question.

02Work through what an average can hide

For an original simplified example, three vials contain 8, 10 and 12 mg of a target peptide. Complete quantitative combination gives 30 mg in total and an average of 10 mg per contributing vial.

Three different vials containing 10, 10 and 10 mg give the same total and average. The pooled amount cannot distinguish the variable set from the uniform set. This assumes complete recovery and a suitable quantitative method; it is not an observed product result.

The preparation also determines the weighting. Equal aliquots from solutions prepared on different volume bases do not necessarily contribute equal fractions of their original vials. Read what was actually combined before interpreting a per-vial average.

An unweighted average of unequal contributions can answer the wrong question even when the final analytical measurement is precise.

03Consider an unusual unit in a larger pool

EPA identifies dilution as a limitation when a high-value sample is combined with lower-value material. It also notes that combining samples can change their integrity or introduce a need for adequate homogenisation.US EPA — Composite Sampling (opens in a new tab)

In an original idealised example, one liquid aliquot contains an impurity at 5 units/mL and four equal-volume aliquots contain none. The combined concentration is 1 unit/mL if volumes add and the impurity is conserved.

If the method cannot reliably quantify 1 unit/mL, the pool may not provide a quantitative account of the original high aliquot. The relevant sensitivity is therefore the sensitivity after pooling, not merely for an individual sample.

The example does not prescribe pool size or an acceptance limit. Actual performance depends on the analyte, preparation, matrix and analytical method.

04Describe the composite so another reader can interpret it

Record how many units contributed, how they were selected, what fraction of each entered the pool and whether results are reported for the pool or converted to another basis. Keep the calculation and any recovery assumptions visible.

Multiple independently formed composites can provide more information than repeated injections of one pool. The latter primarily assess analytical repetition of the same combined material, not variation between the original units.

If the goal may later require locating an unusual unit, the design needs a way to preserve that possibility, such as suitable individual retained material. Once all units have been irreversibly combined, their original individual amounts cannot be reconstructed from the pooled number alone.