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What Karl Fischer water content means on a peptide COA
Karl Fischer water content on a peptide COA is the water share of the powder by mass. How USP authors subtract it, and what vendor ranges rest on.
Karl Fischer water content on a peptide COA is the percentage of the powder’s mass that is water, measured by a titration specific to water. It is not part of the HPLC purity figure. It is a separate term: subtracted, with counterions and other non-peptide species, to get from gross powder to peptide.
This is an evergreen explainer, with no news peg. This desk read every source below on 4 October 2026.
What does Karl Fischer water content mean on a peptide COA?
Alphex, a UK research peptide supplier, says in a note published 2 June 2026 that residual water stays bound to the peptide and its counter-ions after lyophilisation. Karl Fischer titration measures that water specifically: iodine reacts with water in the presence of sulfur dioxide and a base, and the iodine consumed or generated gives the water content.
The result is a mass fraction of the powder. HPLC purity is a share of UV-detectable peptide signal, and water is invisible to it. That is why a 99% purity figure is not 99% peptide by weight.
Where water sits in a mass-balance purity assignment
Wang and colleagues at China’s National Institute of Metrology assigned purity to a synthetic oxytocin by mass balance (J Pharm Biomed Anal, 2022). The abstract names a method for each term:
| Term | Method in the oxytocin study |
|---|---|
| Water | Coulometric Karl Fischer titration |
| Non-volatile counterions | Ion chromatography |
| Inorganic elements | ICP-MS |
| Volatile organic compounds | Headspace GC-MS |
| 21 structurally related impurities, 0.05 to 15.65 mg/g | LC-high-resolution tandem MS |
The assigned value was 796.5 mg/g, with an uncertainty of 6.5 mg/g (k = 2), and the method was verified through an international key comparison coordinated by the BIPM and the NIM. That 796.5 is the purity. The abstract gives no water percentage.
How is the water figure subtracted?
A paper by authors at the United States Pharmacopeial Convention and the UK’s MHRA (Pharmaceutical Research, 2023) sets out the arithmetic. Bulk water was determined by Karl Fischer titration as described in USP general chapter <921> Water Determination, and water-corrected purity was calculated as purity × (100.0 − %water) / 100. For mass balance, the authors write, all species other than the native peptide, impurities and counter ions included, should be identified and measured. Acetic acid was analysed separately under USP <503> Acetic Acid in Peptides.
The paper’s Table VIII, which it labels theoretical, shows two labs at a purity of 0.93 mg/mg: 1.5% water gives 0.92, and 2% water gives 0.91. The authors determine water content at the time of preparation for the HPLC analysis, to ensure it is accurate. For lyophilised vials, residual moisture was measured destructively by coulometric Karl Fischer, with vials opened under dry nitrogen to minimise moisture ingress.
What do the vendor “typical” ranges rest on?
Alphex gives typical residual moisture of 2-8% by mass for a well-dried peptide, with hygroscopic peptides and TFA salts at the higher end. American Peptides’ COA guide, dated 26 April 2026, says well-characterised COAs may include water by Karl Fischer, “typically <10%”, and its worked example lists 4.2% water beside 6.1% TFA. That is an illustration, not product data. Of the two vendor guides read, neither cites a source for its range. Alphex also says most research peptide COAs, its own included, do not report water. The TFA vs acetate post covers the counterion line that sits beside it.
Frequently asked questions
Does higher purity mean lower water content?
No. HPLC purity is a share of peptide-related UV signal and cannot see water; Karl Fischer titrates water. The two are independent lines. In a theoretical example in a 2023 Pharmaceutical Research paper by USP and MHRA authors, two labs report the same 0.93 mg/mg purity, but water of 1.5% and 2% gives water-corrected purities of 0.92 and 0.91.
Coulometric vs volumetric Karl Fischer: which format for peptides?
Both titrate water with iodine. Alphex’s note says the volumetric method measures the iodine consumed, while the coulometric method generates iodine electrochemically and suits small samples. The two peptide studies this desk read both name coulometric Karl Fischer: the oxytocin purity assignment, and the USP paper’s residual-moisture testing of lyophilised vials.
Can I request Karl Fischer for a batch?
That is for the vendor or testing lab to answer. Alphex says Karl Fischer consumes the sample, needs milligrams of material for a good result on a low-moisture solid, and requires an instrument most research peptide testing does not include by default. A COA that does not report water gives a reader no water figure to subtract.
A water line is one term in an account, and this desk reads it as one; how to read a peptide COA places it among the other lines. Research peptides are supplied for laboratory research use only. Nothing here is medical advice, no compound discussed is intended for human consumption, and this desk does not publish dosing.