Methodological standard · §6

CEI-MS-10 — worked example

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

Document identifier: CEI-MS-10/6
Series: Methodological standard
Version: 1.2
Published: 31 May 2024
Last reviewed: 31 Jul 2025
Next review: 31 Jul 2028
Identifier: 10.71829/cei.meth.9
Certainty: High
Cycle: 2024 Q2
Category: Content

§6Worked example

§6.1Worked example — the theoretical maximum for a small polybasic peptide

Consider a tripeptide of molecular mass 342.44 Da with two protonatable sites: one lysine side-chain amine and the N-terminal amine.
Trifluoroacetic acid has a molecular mass of 114.02 Da. Two trifluoroacetate counter-ions add 2 × 114.02 = 228.04 Da.
Total salt mass = 342.44 + 228.04 = 570.48 Da.
Counter-ion mass fraction = 228.04 ÷ 570.48 = 39.97 %, that is approximately 40 % of the mass of the di-trifluoroacetate salt is counter-ion.
Maximum possible peptide content for this salt form is therefore about 60 %.
A certificate for this peptide reporting 99 % purity and, separately, an implied peptide content near 100 % is not describing a very pure preparation; it is describing a determination that was not performed. Conversely a peptide content of 62 % for this compound is entirely normal and is not evidence of adulteration.
Exchanging to acetate (60.05 Da) reduces the counter-ion fraction to 2 × 60.05 ÷ (342.44 + 120.10) = 25.9 %, raising maximum peptide content to about 74 %. The salt form therefore changes the label arithmetic substantially and must be stated.

§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	9.4	The figure on the vial.
Chromatographic purity, % area	98.13	Relative area of the main peak under a stated method.
Peptide content, % w/w	84.7	Mass fraction that is peptide, against a named reference standard.
Water, % w/w	6.0	By Karl Fischer titration.
Counter-ion, % w/w	8.5	Trifluoroacetate, by ion chromatography.
Mass balance, %	99.2	Sum of the three preceding rows. A certificate that does not permit this sum to be formed is incomplete.
Net peptide in vial, mg	7.962	Label claim multiplied by peptide content.
Target-sequence mass, mg	7.813	Net peptide multiplied by chromatographic purity.
Shortfall against label, mg	1.587	A shortfall of 16.9 % 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.

United States Pharmacopeial Convention. General Chapter ⟨1225⟩ Validation of Compendial Procedures. United States Pharmacopeia — National Formulary (USP–NF) 2024;USP 2024 Issue 1. identifier not held by the Institute
International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use. ICH Q2(R2) Validation of Analytical Procedures. ICH Harmonised Guideline 2023;Step 4 version, 1 November 2023. identifier not held by the Institute

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-10/6 · https://compoundevidence.com/methods/counter-ion/worked-example/ · retrieved 30 July 2026