A missing solvent line on a COA means no reported result, not no solvent

ON THIS PAGE

1. Key findings
2. Which solvents peptide synthesis uses
3. Why solvents stay in the powder
4. The three Q3C classes
5. The limits for the four peptide solvents
6. How residual solvents are measured
7. What a residual-solvent line on a COA looks like
8. Limitations
9. Sources

Making a synthetic peptide takes a lot of solvent. Some of it can stay in the powder, and the purity number on a certificate does not measure it.

Drug makers work to a written standard for this, the ICH Q3C guidance on residual solvents, which FDA publishes as its own guidance. It sorts solvents into three classes and sets a limit for each one that matters. A certificate of analysis (COA) that carries no residual-solvent line has simply not reported that test. Silence is not a clean result.

Which solvents peptide synthesis uses

Four names come up again and again. A 2023 review by Rossino and colleagues in the journal Molecules (read October 21, 2026) says the main drawback of solid-phase peptide synthesis, from a green-chemistry view, "is the huge volume of organic solvents required." It names N,N-dimethylformamide (DMF), dichloromethane (DCM) and N-methyl-2-pyrrolidone (NMP) as "the most widely used," followed by diethyl ether and tert-butyl methyl ether.

Purification adds more. A 1984 paper by Rivier and colleagues in the Journal of Chromatography (abstract read on Europe PMC, October 21, 2026) described purifying synthetic peptides by reversed-phase HPLC. The mobile phases it lists include 0.1% trifluoroacetic acid and acetonitrile.

The open question is how much of each is still in the vial.

Why solvents stay in the powder

Drying does not get all of it out. FDA's guidance "Q3C Impurities: Residual Solvents" (dated December 1997, read October 21, 2026) defines residual solvents as "organic volatile chemicals that are used or produced in the manufacture of drug substances or excipients, or in the preparation of drug products." The next sentence is the one that matters: "The solvents are not completely removed by practical manufacturing techniques."

The guidance draws the conclusion itself. Because residual solvents add nothing to a product, it says, they "should be removed to the extent possible." It also says testing should be done "when production or purification processes are known to result in the presence of such solvents." Peptide synthesis is one of those processes.

The three Q3C classes

Q3C ranks solvents by how toxic they are. The 1997 guidance gives the three classes in these words:

• Class 1, "Solvents to be avoided": "Known human carcinogens, strongly suspected human carcinogens, and environmental hazards."
• Class 2, "Solvents to be limited": "Nongenotoxic animal carcinogens or possible causative agents of other irreversible toxicity such as neurotoxicity or teratogenicity," and solvents "suspected of other significant but reversible toxicities."
• Class 3, "Solvents with low toxic potential": no health-based exposure limit is needed, and these solvents have permitted daily exposures "of 50 milligrams (mg) or more per day."

Benzene is the plainest Class 1 example. FDA's companion document "Q3C: Tables and List" (Revision 3, June 2017, read October 21, 2026) sets its limit at 2 ppm and lists its concern as "Carcinogen."

The limits for the four peptide solvents

Each Class 2 solvent gets two numbers. The permitted daily exposure (PDE) is a daily amount in milligrams. The concentration limit, in parts per million (ppm), is worked out from the PDE using an assumption written into the guidance. The Class 2 figures below are copied from Table 2 of the 2017 Tables and List document. Ethyl ether is on its Table 3.

The ethyl ether PDE entry comes from section 3.1 of the 1997 guidance, which says Class 3 solvents need no health-based exposure limit and have PDEs "of 50 milligrams (mg) or more per day." Its limit entry comes from the 2017 document's text introducing Table 3: amounts "of 50 mg per day or less (corresponding to 5,000 ppm or 0.5 percent under Option 1) would be acceptable without justification." NMP, the third synthesis solvent the 2023 review names, is also on Table 2, at 5.3 mg/day and 530 ppm.

One more line is worth knowing. Table 4 is headed "Solvents for Which No Adequate Toxicological Data Were Found," and trifluoroacetic acid is on it. Q3C gives it no PDE; manufacturers "should supply justification" for any residue.

These are drug-product numbers. They describe what regulators expect of an approved medicine. No law applies them to a research-use-only peptide, and no research supplier is bound to test against them. They are still a published yardstick to read a research certificate's solvent result against.

How residual solvents are measured

The method is gas chromatography, not the HPLC behind a purity figure. Section 3.4 of the 1997 guidance says: "Residual solvents are typically determined using chromatographic techniques such as gas chromatography." It adds that harmonized procedures "as described in the pharmacopoeias should be used, if feasible."

There is one shortcut. "If only Class 3 solvents are present, a nonspecific method such as loss on drying may be used." Loss on drying tells you how much weight the sample lost when dried. It does not say which solvent left, so it cannot clear DMF or dichloromethane.

An HPLC purity figure answers a separate question: how much of what the detector saw was the target peptide. A powder can read high on that test and still carry solvent the test was never set up to see.

What a residual-solvent line on a COA looks like

A real result names what was looked for. Q3C's reporting section shows the shape. A supplier states which solvents are "likely to be present" and whether each is under its limit, and the guidance says "the supplier would name the Class 2 solvents." Any Class 1 solvent likely to be present "should be identified and quantified."

Q3C defines "likely to be present" as the solvent used in the final manufacturing step, plus earlier ones "not removed consistently by a validated process." In practice, then, a useful line on a peptide certificate would carry:

• the method, such as gas chromatography;
• each solvent tested by name, which for a typical synthesis means DMF, dichloromethane, acetonitrile and an ether;
• a figure in ppm for each, or a stated "below" value with its limit;
• the limit it was judged against, and where that limit came from.

If you are comparing certificates, look for that line before you look at the purity figure. If it is not there, the solvent content of that batch is unknown. That is the honest reading, and it is the only one the document supports.

Limitations

Q3C is guidance for drugs. The 2017 Tables and List document says "FDA's guidance documents do not establish legally enforceable responsibilities," and none of it governs research peptides. The tables quoted come from the 2017 revision; FDA's Q3C(R8) document of December 2021, also read October 21, 2026, added limits for three more solvents, and later changes may exist that this piece did not see. The pharmacopoeial chapter that sets the detailed test method could not be read for this piece, so its procedure is not described here. The 1984 purification paper was read as an abstract only. Nothing here says how often research certificates carry a solvent line, because no source read for this piece counts that.

The next certificate that names its solvents, its method and its ppm figures will be easy to read against these tables. The ones that name nothing leave the question open.

Sources

All read 2026-10-21. Guidance for Industry: Q3C Impurities: Residual Solvents, FDA, December 1997, sections 1, 2, 3.1, 3.4 and 3.5. Q3C: Tables and List, Guidance for Industry, FDA, Revision 3, June 2017, Tables 1 to 4. Q3C(R8) Impurities: Guidance for Residual Solvents, Guidance for Industry, FDA, December 2021. Rossino G, Marchese E, Galli G, et al. Peptides as Therapeutic Agents: Challenges and Opportunities in the Green Transition Era. Molecules. 2023;28(20):7165. Rivier J, McClintock R, Galyean R, Anderson H. Reversed-phase high-performance liquid chromatography: preparative purification of synthetic peptides. J Chromatogr. 1984;288(2):303-328. Abstract read on Europe PMC.