Methodological standard · §6

CEI-MS-21 — worked example

A worked calculation, and an illustrative mass balance showing what a purity figure alone does not establish.

Document identifier: CEI-MS-21/6
Series: Methodological standard
Version: 1.0
Published: 16 Jun 2026
Last reviewed: 16 Jun 2026
Next review: 16 Jun 2029
Identifier: 10.71829/cei.meth.18
Certainty: High
Cycle: 2026 Q2
Category: Stability

§6Worked example

§6.2Illustrative mass balance

Illustrative — not a published result

The figures below are constructed by the Institute to make the arithmetic concrete. They describe no real batch.

Table 2. Illustrative mass balance for a labelled vial.

Quantity	Value	Note
Label claim, mg	9.1	The figure on the vial.
Chromatographic purity, % area	97.06	Relative area of the main peak under a stated method.
Peptide content, % w/w	89.8	Mass fraction that is peptide, against a named reference standard.
Water, % w/w	3.9	By Karl Fischer titration.
Counter-ion, % w/w	3.4	Trifluoroacetate, by ion chromatography.
Mass balance, %	97.1	Sum of the three preceding rows. A certificate that does not permit this sum to be formed is incomplete.
Net peptide in vial, mg	8.172	Label claim multiplied by peptide content.
Target-sequence mass, mg	7.932	Net peptide multiplied by chromatographic purity.
Shortfall against label, mg	1.168	A shortfall of 12.8 % against the labelled mass, none of which is visible from the purity figure alone.

The same calculation can be run on any set of values in the purity-against-content calculator.

References cited on this page

References are numbered in order of first citation in this document. Each superscript in the text links to its entry below.

International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use. ICH Q1A(R2) Stability Testing of New Drug Substances and Products. ICH Harmonised Tripartite Guideline 2003;Step 4 version. identifier not held by the Institute
Manning MC, Chou DK, Murphy BM, Payne RW, Katayama DS. Stability of protein pharmaceuticals: an update. Pharmaceutical Research 2010;27(4):544–575. doi:10.1007/s11095-009-0045-6 · PMID 20143256

Identifiers are reproduced only where the Institute holds them. Where a digital object identifier or PubMed identifier is not shown, the Institute has recorded the journal and year and has not constructed an identifier.

Compound Evidence Institute · CEI-MS-21/6 · https://compoundevidence.com/methods/freeze-thaw/worked-example/ · retrieved 30 July 2026