CEI-MS-10 — Identification and quantification of counter-ions in synthetic peptide salts

§1Scope

§1.1Scope

Applies to all synthetic peptides supplied as salts, which in practice means all of them. Establishes that counter-ion identity and content are part of the specification and not a residue to be disregarded.

§2Principle

A peptide with basic residues is isolated from reverse-phase purification as a salt of the mobile-phase acid, most commonly trifluoroacetate or acetate. The counter-ion associates approximately stoichiometrically with each protonatable site, so its mass fraction is determined by the number of basic residues and the peptide’s molecular mass. For small polybasic peptides this fraction is large. An acidic peptide with no basic residues carries a cation rather than an anion, and a trifluoroacetate determination on such a peptide is meaningless.^[1,2]

§3Apparatus and reagents

§3.1Apparatus

• Ion chromatograph with suppressed conductivity detection for anions
• Capillary electrophoresis as an alternative for trifluoroacetate
• ¹⁹F nuclear magnetic resonance spectrometer as an orthogonal method for trifluoroacetate
• Ion chromatograph with cation-exchange column for sodium and ammonium

§3.2Reagents and reference materials

• Anion-exchange eluent, typically carbonate or hydroxide gradient
• Trifluoroacetate and acetate standards
• Cation standards for sodium, potassium and ammonium
• A fluorine-containing internal standard for the NMR method

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

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