Peptide powder holds water. None of 126 supplier records notes a water figure
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Key findings
- Peptide powder takes up water from the air. A 2019 multi-lab study states that "most peptides are very hygroscopic."
- Karl Fischer titration measures water by a reaction with iodine. ICH Q6A prefers it over loss on drying because it is specific for water.
- In five freeze-dried USP reference standard lots, water measured 1.38 to 2.79 percent of weight by coulometric Karl Fischer titration.
- A purity figure by HPLC leaves water out. A mass balance counts it.
- Of 126 supplier records in this publication's ledger, none notes a certificate that reports a water content figure.
Freeze-dried peptide pulls water out of the air. That water goes on the balance with the peptide. A weighed milligram of powder is part peptide, part salt, and part water.
The test for it is Karl Fischer titration. In this publication's ledger of 126 suppliers, no record notes a certificate that reports its result.
Why the powder holds water
Peptides attract water, and the amount moves with the air around them. A 2019 survey of peptide measurement methods in the Journal of Pharmaceutical and Biomedical Analysis (Li and colleagues, read 2026-10-11) states that "most peptides are very hygroscopic." Hygroscopic means it takes up moisture from the air.
When USP shipped its peptide reference standards as loose powder, the paper notes, users had to weigh them under controlled humidity and measure the water at the time of use.
One lab in the study, Lab 10, could not get a stable water reading by Karl Fischer. The authors read this as poor humidity control while the standard was being prepared, and they left that lab's assay results out. In the authors' reading, the room's humidity moved the water result.
What Karl Fischer titration measures
Karl Fischer titration measures water through a chemical reaction. A 2023 paper in Lab on a Chip (Pilat and colleagues, read 2026-10-11) describes the method as "based on the reaction of residual water with sulfur dioxide and iodine in an alcohol solution containing a base."
It runs in two forms, coulometric and volumetric, the paper notes. It puts the method's detection limit at tens of micrograms of water for both.
The international drug standard leans the same way. ICH Q6A, the 1999 ICH guideline on specifications for new chemical drug substances (read 2026-10-11 on database.ich.org), lists water content as a test in section 3.3.1 when a substance "is known to be hygroscopic or degraded by moisture." It allows that loss on drying may sometimes be enough. Then it adds: "a detection procedure that is specific for water (e.g., Karl Fischer titration) is preferred."
Q6A says it "may be applicable" to small synthetic peptides and is not sufficient for larger ones. It is a drug-approval standard. Nothing binds a research supplier to it.
How much water the standards held
For drug reference material, measured water is small but not zero. A 2023 paper in Pharmaceutical Research on USP peptide reference standards (McCarthy and colleagues, read 2026-10-11) measured residual moisture in freeze-dried lots by coulometric Karl Fischer titration. The vials were opened under dry nitrogen to keep room air out. Five of the lots it reports appear below.
| Peptide reference standard lot | Residual moisture, percent of weight |
|---|---|
| Bivalirudin | 1.40 |
| Desmopressin acetate | 1.64 |
| Gonadorelin acetate | 2.79 |
| Leuprolide acetate | 1.40 |
| Oxytocin | 1.38 |
These are reference standards, filled and sealed under tight control. They say nothing about a vial sold for research.
Where water fits in what a milligram holds
Water is one of three weights between a weighed milligram and the peptide in it. The others are impurities and the counterion salts weighed with a peptide, and together they set its net peptide content, which purity does not report.
A 2016 set of recommendations in Clinical Chemistry (Hoofnagle and colleagues, read 2026-10-11) defines net peptide content as the actual peptide in a weighed sample. The measurement, it says, "excludes the weight of water and counter ions that exist in all peptides."
The 2023 reference-standards paper shows how the parts add up. It measured water by Karl Fischer titration "as described in USP general chapter <921> Water Determination." Its theoretical worked example subtracts HPLC impurities, acetic acid, TFA, residual solvents and ash to reach 0.93 mg of peptide per mg of material on a dry basis, by mass balance. Then each lab applies its own water reading. At 1.5 percent water, the figure became 0.92 mg per mg. At 2 percent, it became 0.91.
Each lab used its own water reading "to account for differences in humidity during testing."
The same paper says USP moved many peptide standards to freeze-dried vials "to eliminate the need for the user to determine counter-ions and residual moisture prior to use." Even the standards body treats water as something to measure, or to design out.
That is the stake. A purity figure by HPLC is a share of what the detector saw, and the 2023 paper measured water with a separate test. Someone who weighs powder and treats it as all peptide has counted the water, the salt and the impurities as peptide too.
What 126 supplier records show
This publication keeps a record on 126 suppliers, built from dated notes on each storefront and its certificates, written between 2026-08-17 and 2026-09-25. On 2026-10-11, every note was searched for terms that name a water measurement: Karl Fischer, KF, water content, moisture content, residual water, loss on drying, water activity, coulometric, volumetric titration, hygroscopic and moisture. Each was matched as a whole word in any capitals. Eight of the 126 records matched. Each match was read in context.
Five are storefront text, such as storage or packaging copy that mentions moisture. One is a testing lab's accredited scope that covers moisture in cannabis products, not peptides.
Two touch on testing. A SideChain Analytics certificate in Luxara Labs' library, noted 2026-08-25, says its purity figure "bounds peptide-related impurities only, not excipients, counter-ions, or residual water." The note records no water figure. A note on Vici Peptides dated 2026-08-26 records that SteriGenix Analytical, a contract lab, lists a Metrohm Karl Fischer titrator among its instruments. The note does not record a water result.
A second search looked for the word water anywhere, skipping uses within 30 characters of diluent words such as BAC, bacteriostatic, sterile or reconstitution. Six records matched. They held a diluent product name, chromatography solvent mixtures, manufacturing descriptions, the cannabis lab scope and the Luxara Labs certificate. None was a water result.
So none of the 126 records notes a certificate that reports a water content figure.
Limitations and open questions
The ledger holds notes on what storefronts and certificates said when read. It is not a copy of every certificate, so a water line a note left out would be missed. The count describes the ledger, not the market.
The water values above come from five USP reference standard lots. No source read for this piece measured water in a peptide sold for research, and this piece gives no typical figure.
The USP chapter on water determination is cited as the 2023 paper names it. The USP text itself was not read. The detection limits quoted from the 2023 Lab on a Chip paper were stated for solvents, not peptide powder.
The water is on the balance every time. Whether it reaches the certificate depends on someone running the titration.
Sources
- Li C, Bhavaraju S, Thibeault MP, et al. "Survey of Peptide Quantification Methods and Comparison of Their Reproducibility: A Case Study Using Oxytocin." Journal of Pharmaceutical and Biomedical Analysis, 2019;166:105. Read 2026-10-11. PMC6507411
- Pilat F, Schwarz B, Baumgartner B, et al. "Beyond Karl Fischer titration: a monolithic quantum cascade sensor for monitoring residual water concentration in solvents." Lab on a Chip, 2023;23:1816. Read 2026-10-11. PMC10045895
- ICH Q6A, "Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products: Chemical Substances," Step 4 version dated 6 October 1999. Read 2026-10-11 on database.ich.org (named, not linked).
- McCarthy D, Han Y, Carrick K, et al. "Reference Standards to Support Quality of Synthetic Peptide Therapeutics." Pharmaceutical Research, 2023;40(6):1317-1328. Read 2026-10-11. PMC10338602
- Hoofnagle AN, Whiteaker JR, Carr SA, et al. "Recommendations for the generation, quantification, storage and handling of peptides used for mass spectrometry-based assays." Clinical Chemistry, 2016;62(1):48-69. Read 2026-10-11. PMC4830481
- This publication's vendor ledger, ledger/vendors.json: 126 supplier records, searched 2026-10-11. Each vendor observation above carries the date its note was written. Vendors are named here, never linked.
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