CEI-MS-08 — worked example

§6Worked example

§6.1Worked example — the blank is larger than the finding

1. A pentapeptide designed to contain D-2-naphthylalanine at position 3 and D-phenylalanine at position 4 is analysed by chiral amino-acid analysis. The raw determination gives 3.1 % L-phenylalanine and 2.8 % L-2-naphthylalanine.
2. A control peptide of confirmed all-D configuration at those positions, run through the identical hydrolysis, gives 2.4 % L-phenylalanine and 2.2 % L-2-naphthylalanine.
3. Corrected values: 3.1 − 2.4 = 0.7 % L-Phe and 2.8 − 2.2 = 0.6 % L-2-Nal.
4. Reported without the blank, the sample appears to contain roughly 3 % of the wrong enantiomer at two positions. Reported with the blank, it contains under 1 % at each, and the majority of the apparent finding was generated by the analysis rather than by the synthesis.
5. A laboratory that omits the blank will systematically over-report racemisation, and a laboratory that omits both the blank and the analysis — which is the universal practice in the supply channel the Institute assesses — will report nothing at all, which is worse.

§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. 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
2. 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

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