GC residual solvents

Why would a peptide powder contain solvent?

Peptides are made and purified using organic solvents. The European Medicines Agency's draft guideline on synthetic peptides notes that solid phase synthesis requires extensive washing of the resin with solvents, and expects the depletion of every reagent and solvent used to be addressed by data or risk analysis. It lists residual solvents among the typical specification tests for a peptide active substance (EMA draft guideline on synthetic peptides, 2023). Purification adds more: reversed-phase HPLC of peptides commonly uses acetonitrile in the mobile phase (Mant and colleagues, Methods in Molecular Biology, 2007).

How does headspace gas chromatography work?

A weighed portion of the sample is dissolved or suspended in a diluent inside a sealed vial and heated. Volatile solvents move into the gas above the liquid (the headspace) until they reach equilibrium, and a portion of that gas, not the liquid, goes into the gas chromatograph, so the non-volatile peptide never enters the column.

A carrier gas sweeps the vapour through a long, narrow column inside an oven, and different solvents reach a detector, such as a flame ionisation detector (FID), at different times. A 2025 validation study of residual solvents in a drug raw material calls gas chromatography the preferred method for residual solvents and headspace sampling the most used in pharmacopoeias and the literature. Its set-up is typical: a 20 mL headspace vial, dimethyl sulfoxide as diluent (chosen over water, the pharmacopoeial choice), 30 minutes of equilibration at 100 °C, and FID detection. The authors first tried procedure A of USP general chapter <467> and found it could not adequately quantify one of their six solvents, triethylamine, which is why they developed their own method (de Camargo and colleagues, Journal of Separation Science, 2025).

ICH Q3C itself says residual solvents are typically determined by chromatographic techniques such as gas chromatography, that harmonised pharmacopoeial procedures should be used if feasible, and that the impact of solvent volatility should be considered in validation (ICH Q3C(R9), 2024).

What are ICH Q3C solvent classes?

ICH Q3C sorts solvents into three classes by risk. Class 1 solvents are to be avoided: known or strongly suspected human carcinogens and environmental hazards. Class 2 solvents are to be limited, each with its own permitted daily exposure and concentration limit. Class 3 solvents have low toxic potential and permitted daily exposures of 50 mg or more per day, which under the guideline's Option 1 corresponds to 5000 ppm (0.5%).

Ethanol, acetone, acetic acid and dimethyl sulfoxide are among the Class 3 solvents. Trifluoroacetic acid appears in a separate table of solvents for which no adequate toxicological data was found, where ICH Q3C asks manufacturers to justify residual levels. In a peptide, trifluoroacetate usually remains as a counter-ion bound to the peptide rather than as free solvent, which is why it is measured separately; see counter-ion content.

When is a gas chromatography test not needed?

ICH Q3C allows one shortcut: if only Class 3 solvents are present, a non-specific method such as loss on drying may be used, if properly validated, and its example supplier statement reads that only Class 3 solvents are likely to be present and loss on drying is less than 0.5%. Class 1 solvents that are likely to be present, and Class 2 or Class 3 solvents above their Option 1 limit or 0.5%, should be identified and quantified, which in practice means chromatography.

How should you read a "not detected" or "complies" result?

A residual solvent method looks for a list of solvents chosen in advance, calibrated with standards of each one. "Not detected" means none of the listed solvents gave a peak above the method's detection limit, not that the sample is free of every solvent. ICH Q3C ties the list to the process: solvents used in the final manufacturing step, and those used earlier and not removed consistently by a validated process, are the ones "likely to be present".

"Complies" is a comparison, not a measurement. It should sit next to the limit used for each solvent, so you can see whether the laboratory applied the ICH Q3C Option 1 concentration or a limit calculated another way.

Does Tested.ph offer residual solvent testing?

No, and it is not planned. The standard package covers identity, purity and amount, with endotoxin as an option, performed by the partner laboratory when testing opens. The COA reading guide covers where these lines usually sit.