CEI-MS-04 — worked example

§6Worked example

§6.1Worked example — the effect of water on the label claim

1. A 10 mg vial of a 43-residue peptide gives Karl Fischer determinations of 7.8 %, 8.1 % and 7.9 % on 12.4, 11.8 and 13.1 mg samples. Mean water content 7.9 %, relative standard deviation 1.9 %.
2. Water mass in the vial = 10.0 mg × 0.079 = 0.79 mg.
3. If the certificate also reports counter-ion at 9.4 % and purity at 96.2 %, peptide content by difference = 100 − 7.9 − 9.4 = 82.7 %, giving 8.27 mg of peptide and 7.96 mg of target sequence.
4. Had water not been determined, and had the reader assumed a dry powder, the implied peptide mass would have been 10.0 × (1 − 0.094) × 0.962 = 8.72 mg — an overstatement of 9.5 %. For a compound dosed in the low milligram range this is not a rounding difference, and it is entirely avoidable by performing a determination that consumes fifteen milligrams of material.

§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 ⟨921⟩ Water Determination. 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 Q6B Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products. ICH Harmonised Tripartite Guideline 1999;Step 4 version. 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.