01Define what counts as the target

The BIPM CCQM-K115 comparison assigned a human C-peptide mass fraction using a balance that considered peptide-related impurities, water, TFA, other ions and volatile organics. It provides a primary example of why a peptide material’s composition extends beyond one chromatographic percentage.BIPM — Final Report CCQM-K115: peptide purity determination, synthetic human C-peptide (opens in a new tab)

Before doing arithmetic, define whether the target quantity is peptide alone, a specified salt form or another explicitly described chemical entity. Counterions cannot be both part of the target definition and separately subtracted from it without double counting.

All contributions also need a common denominator. A percentage of dry material and a percentage of material as received are not directly additive. Nor is a UV area percentage automatically a mass fraction.

The question is therefore not simply whether the displayed numbers add up. It is whether they describe non-overlapping components of the same defined material.

02Work through an illustrative subtraction

For an original simplified example, define the target as peptide excluding counterions and express every measured non-target contribution in mg per g of material as received.

The listed contributions sum to 100 mg/g. Subtracting them from 1,000 mg/g gives a target estimate of 900 mg/g, or 90.0% by mass. This is an illustrative calculation, not an observed Novum composition.

The result is only as complete as its component accounting. An unmeasured non-target contribution would be included in the remainder and would overestimate the target under this simple subtraction model.

03Look for omissions and double counting

A common pitfall is to subtract total related impurities and then subtract each impurity again. Another is to include the same counterion contribution under both a specific ion result and a broader inorganic total.

A result below a method’s quantification limit also needs an explicit treatment. It should not silently become an exact zero merely because zero makes the balance easier to calculate.

In CCQM-K115, the report discusses how underestimated TFA and overestimated peptide-related impurities affected different mass-balance assignments. This real comparison illustrates that the component measurements, not just the final equation, control the result.BIPM — Final Report CCQM-K115: peptide purity determination, synthetic human C-peptide (opens in a new tab)

Agreement with 100% can also arise from compensating errors if independent target and non-target measurements are summed. A positive error in one contribution can conceal a negative error in another; apparent closure is therefore a consistency check rather than automatic validation.

04Read uncertainty and cross-checks alongside the result

The uncertainty of the target estimate reflects uncertainty in the quantities used to obtain it. Reporting many decimal places does not remove uncertain water, counterion or impurity measurements. Shared preparation or calibration effects may also connect their errors.

A complementary quantitative method can provide a useful comparison when its own selectivity and corrections are understood. Agreement is more informative when the methods do not simply share the same unexamined assumption.

An interpretable report states the target definition, component methods, reporting basis, treatment of unresolved contributions and uncertainty. Keep these with the mass-fraction value rather than reducing the assessment to a purity headline.