Residual solvents test for peptides: ICH Q3C classes, limits and headspace GC

What a residual solvents test measures

A residual solvents test measures organic solvents left in a powder after it was made. ICH Q3C, the international guideline on the subject, defines them as volatile organic chemicals used or produced in manufacture that practical techniques do not remove completely. The test names each solvent it looked for and reports its amount in µg per g of sample, the same as parts per million (ppm).

It cannot tell you which peptide is in the vial or how many mg it holds. An HPLC purity or LC-MS identity result does not report solvents either, so the test is ordered on its own.

The EMA guideline on synthetic peptides, in force since 1 June 2026, lists residual solvents in the specification of a peptide active substance. Research peptides have no specification, so the buyer decides whether to test.

Where residual solvents in peptides come from

A 2020 industrial review in RSC Advances calls solid-phase peptide synthesis (SPPS) “generally the method of choice” for making peptides chemically. Each step uses its own solvents:

Step	Solvents and reagents	ICH Q3C status
Loading, coupling and washing on the resin	DMF, NMP, dichloromethane (DCM)	Class 2: DMF 880 ppm, NMP 530 ppm, DCM 600 ppm
Fmoc removal	Piperidine, usually 20%, in DMF or NMP	Piperidine is on no Q3C list
Cleavage from the resin	TFA with water and scavengers	TFA: Table 4, no permitted daily exposure
Precipitation	Diethyl ether, diisopropyl ether (DIPE), MTBE	Diethyl ether and MTBE: Class 3. DIPE: Table 4
Purification	Reversed-phase HPLC with acetonitrile and water	Acetonitrile: Class 2, 410 ppm
Salt exchange and freeze-drying	Acids for the final salt form, for example acetic acid	Acetic acid: Class 3

How much is left depends on the process and the drying of each batch. Q3C counts a solvent as “likely to be present” if the final step used it, or if a validated process does not remove it consistently. For a peptide, the last solvents in the process are those of purification and salt exchange, such as acetonitrile and acetic acid. Earlier ones, such as DMF or ether, count too unless the process removes them reliably.

ICH Q3C classes and limits

ICH Q3C(R9), dated 24 January 2024, sorts solvents into three classes, plus a fourth table:

• Class 1, to be avoided. Known or strongly suspected human carcinogens and environmental hazards, such as benzene (2 ppm).
• Class 2, to be limited. Each has a permitted daily exposure (PDE) and a concentration limit. Besides those in the table above: methanol 3000 ppm, tetrahydrofuran 720 ppm, toluene 890 ppm.
• Class 3, low toxic potential. Up to 5000 ppm (0.5%) is acceptable without justification. Examples: acetic acid, ethanol, acetone, diethyl ether, MTBE.
• Table 4, no adequate toxicological data. No PDE could be set. TFA and isopropyl ether are here.

The Class 2 ppm limits assume a product mass of 10 g given daily. Q3C was written for medicines, so for a research peptide these values are a reference point, not an official limit. Any limit on a research peptide report was chosen by the client, the lab or a seller, and the report should say which.

The numbers also show scale. At its limit, DMF is 0.088% of the powder’s weight; a Class 3 solvent at 5000 ppm is 0.5%. Solvents near these limits do not explain a large gap between the label and the mg of peptide in a vial.

How the test works: headspace GC and USP <467>

ICH Q3C says solvents are typically measured by chromatographic techniques such as gas chromatography (GC), with harmonised pharmacopoeia procedures where feasible. Loss on drying, which cannot tell water from solvents, is allowed only when Class 3 solvents alone are present.

A 2015 method paper in the Journal of Pharmaceutical Analysis describes static headspace GC. The sample is heated in a sealed vial until the solvent in it and the gas above it reach equilibrium. A portion of that gas goes into the chromatograph; the peptide stays behind. A flame ionisation detector (FID) is the most common detector, and GC-MS can confirm which solvent a peak is.

USP chapter <467> has three procedures for Class 1 and Class 2 solvents, summarised in a 2010 paper by authors including USP staff: A, an identification and limit test on a G43 column; B, a confirmation on a G16 polyethylene glycol column that separates differently; and C, the quantitative test. Three points follow:

• A limit test is not a number. “Complies with USP <467>” after Procedure A means the solvent was below its limit. Ask for µg/g if you want to compare batches.
• The diluent matters. Water-soluble samples are dissolved in water; water-insoluble ones in DMF or DMSO. A method that dissolves the sample in DMF cannot also measure DMF in it.
• Volatility matters. The 2015 paper notes that a water-based diluent keeps the vial below 100 °C, which limits how much of each solvent reaches the gas. Its authors used DMSO, which allows higher temperatures.

The limit of quantitation (LOQ) varies by solvent. In the 2015 paper’s method it ran from 0.009 µg/mL for benzene to 28 µg/mL for propyl acetate and 1-butanol.

A licensed peptide shows how a specification splits the work. The EMA assessment report for Liraglutide Stada lists residual solvents by GC, with limits in line with ICH Q3C, and tests DMF separately by HPLC.

How to read a residual solvents result

Item on the report	What to look for	Why it matters
Solvent list	Every solvent tested, by name	The method finds only what it looks for
Result and unit	µg/g (ppm) per solvent, or “less than” the LOQ	A number lets you compare batches
LOQ	One value per solvent	Sensitivity differs by solvent and sample mass
Method	Headspace GC, detector, procedure (USP <467> or in-house), diluent	Shows whether it is a limit test, and whether the diluent hides a solvent
Limit and its source	Who chose any pass limit	A research peptide has no official limit
Sample	Mass of powder tested, and which vial	Less powder means a higher LOQ in µg/g
Raw data	Chromatogram with labelled peaks and the standard run	Shows peaks the method did not name

An unlabelled peak may be a volatile compound outside the calibration; GC-MS can help identify it. Our guide to verifying a peptide COA covers the checks for every test on a report.

TFA, acetate and water are separate tests

TFA and acetic acid appear in ICH Q3C, but in a peptide powder they are usually counter-ions, part of the salt form. The EMA guideline lists counter-ion content and residual TFA as their own specification items. In a paper on USP’s peptide reference standards, acetic acid and TFA were measured by HPLC (USP <503> and <503.1>) and other solvents by GC (USP <467>). A counter-ion test reports TFA and acetate as % of the powder; see TFA in peptides. Water is measured by Karl Fischer titration.

All of these add weight. The USP paper assigns peptide content by subtracting HPLC impurities, acetate, TFA, residual solvents and residue on ignition. In its theoretical example, acetic acid was 5.58% of the powder and residual solvents 0.003%, giving 0.93 mg of peptide (free base) per mg on the anhydrous basis. Your batch will differ; only measurement shows how.

Which peptide labs offer a residual solvents test

From the lab sites we read in September 2026:

Lab	What it lists	Method it states
ACS Peptide Testing Labs (Florida)	Residual solvents	GC-HS, USP <467>
Krause Analytical (Texas)	Add-on “Residual Solvents – USP 467”	USP <467>; headspace GC among its instruments
PharmLabs (California)	Residual solvents for peptides	HS-GC/FID; 250 mg of sample; 5–7 business days
Vanguard Laboratory (Washington)	Residual solvents; TFA analysis listed separately	Residual solvents, including TFA, on its A2LA scope
Liquilabs (Czech Republic)	Residual solvents such as TFA and DMF (Czech page)	Not stated

ACS and Krause name the pharmacopoeia chapter. PharmLabs publishes the sample size and turnaround for each test. Finnrick’s custom orders offer residual solvents as an add-on with Krause. Freedom Diagnostics, TrustPointe Analytics and Chromate list no residual solvents test.

Accreditation is granted per method and per type of sample. Vanguard’s A2LA ISO/IEC 17025 scope lists residual solvents, including trifluoroacetic acid. PharmLabs’ PJLA scope covers its headspace GC solvents method for cannabis and hemp samples, not peptides. We do not hold ISO/IEC 17025 accreditation. Our guide to ISO/IEC 17025 in peptide testing explains how to read a scope.

When a residual solvents test is worth ordering

• Bulk powder before filling. Solvent levels belong to a lot. See bulk peptide powder testing.
• Batch release for a brand. A result per lot, kept with the other tests. See batch testing for vendors.
• A maker’s certificate that claims to meet Q3C. Check which solvents it lists, and whether it gives numbers or only “complies”.

Solvent tests can need more powder than other tests, so check how many vials to send.

What HK Analytical reports

Our residual solvents test reports named solvents left from synthesis or purification. It cannot tell you the peptide’s identity or amount, so it fits in the same order as identity (LC-MS), purity (HPLC) and content per vial. Counter-ion (TFA, acetate) and water content (Karl Fischer) can be added in any mix per sample. Every number comes with its raw data, in a signed record you can check on /verify. When you request a quote, ask which solvents the test covers and the limit of quantitation for each. The same question works for any lab.