CEI-MS-03 — worked example

§6Worked example

§6.1Worked example — a fully conformant mass balance and the label claim it implies

1. A 5 mg vial of a hexapeptide of molecular mass 887 Da is characterised as follows. Chromatographic purity by CEI-MS-01: 98.4 %. Water by Karl Fischer: 5.2 %. Counter-ion, identified as trifluoroacetate by ion chromatography: 11.8 %. Residual acetonitrile: 0.02 %.
2. Peptide content by difference = 100 − 5.2 − 11.8 − 0.02 = 82.98 %, rounded to 83.0 %.
3. Mass of peptide in the vial = 5.00 mg × 0.830 = 4.15 mg.
4. Mass of target sequence = 4.15 mg × 0.984 = 4.08 mg.
5. Mass-balance residual = 100 − (83.0 + 5.2 + 11.8 + 0.02) = 0.0 %, so the declared composition closes.
6. The vial is labelled 5 mg and contains 4.08 mg of the target peptide, an 18 % shortfall against the label. Nothing here is a defect: a trifluoroacetate salt of a basic hexapeptide cannot contain much more peptide than this, and the counter-ion mass is chemically unavoidable. The defect would be a certificate that reported 98.4 % and stopped, leaving a reader to assume 4.92 mg.

§6.2Illustrative mass balance

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.

1. 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
2. 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.