Effect of dilution solvent and injection volume on the analysis of basic hydrophilic therapeutic polypeptide salts with pressurized carbon dioxide mobile phases

•Basic hydrophilic polypeptides eluted with carbon dioxide mobile phases.•Best results obtained with methanesulfonic acid and water in methanol co-solvent.•Strong impact of the nature of counter-ion on chromatographic quality.•Strong impact of dilution solvent and injection volume on peak shapes.•Best results obtained with dilution solvent identical to co-solvent composition.