Most certificates of analysis in the research-peptide space show identity, purity, and mass. Those matter, but they are only part of the picture. The fields that vendors most often leave off are the ones that describe water content, biological contamination, and residual chemistry, and their absence quietly changes what the document actually proves.
A COA is a snapshot. If key measurements are missing, the snapshot is incomplete, and readers can over-interpret a clean-looking purity number. Below are the fields worth looking for, why they exist, and what their absence means. Framing follows international references such as the United States Pharmacopeia (USP), the European Pharmacopoeia, and World Health Organization (WHO) guidance.
Karl Fischer titration: how much water is in the vial
Lyophilized (freeze-dried) peptides are hygroscopic. They readily absorb moisture from the air, and residual water remains from the manufacturing process itself. A "10 mg" vial that is 8% water by mass does not contain 10 mg of peptide. Karl Fischer titration is the standard analytical method for measuring water content specifically, and it is far more accurate than a general loss-on-drying figure because it responds to water rather than to any volatile.
Why it matters for research accuracy:
- Net peptide mass. Water content, together with counter-ion and acetate content, determines how much actual peptide is in the vial. This is why a rigorous COA reports "net peptide content" rather than only gross mass.
- Reconstitution math. In a research setting, concentration calculations assume a known quantity of peptide. High or unreported water content skews those calculations.
- Stability. Excess moisture accelerates degradation of many peptides during storage.
Coulometric vs volumetric Karl Fischer
There are two variants. Coulometric Karl Fischer generates the titrant electrochemically and is suited to small water amounts, which makes it the usual choice for milligram-scale peptide vials. Volumetric Karl Fischer adds titrant from a burette and suits larger water quantities. A COA that names the method signals a lab that understands the distinction. Typical acceptable water content for a well-handled lyophilized peptide is often in the low single-digit to high single-digit percent range, though the acceptable limit depends on the specific peptide and its specification.
If a COA reports purity but not water content, the purity figure is expressed relative to the peptide-related material detected, not the total mass in the vial. The two questions "how pure is the peptide?" and "how much peptide is in the vial?" are different, and both deserve an answer.
Endotoxin testing (LAL): the biological-safety field
Purity by HPLC tells you about chemical composition. It says nothing about bacterial endotoxins, which are lipopolysaccharide fragments from the outer membrane of Gram-negative bacteria. Endotoxins are heat-stable, survive many sterilization steps, and provoke strong biological responses even at low levels. For any material used in cell culture or administered by injection in a research context, endotoxin load is a core safety parameter, not an optional extra.
The standard test is the Limulus Amebocyte Lysate (LAL) assay, referenced in USP General Chapter <85> on bacterial endotoxins and in equivalent pharmacopeial chapters internationally. Results are reported in endotoxin units per milliliter or per milligram (EU/mL or EU/mg).
Gel-clot vs kinetic methods
- Gel-clot is a qualitative or semi-quantitative test: the lysate forms a clot above a threshold endotoxin concentration. Simple and robust, but coarse.
- Kinetic chromogenic and kinetic turbidimetric methods are quantitative. They track a color change or increasing turbidity over time and yield an actual EU value, which is more informative for a COA.
A COA that reports an endotoxin result with units and a named method is doing meaningful work. One that omits endotoxin entirely leaves an important safety dimension unaddressed for anyone whose research use involves cell systems or injection.
Residual solvents and the counter-ion
Peptide synthesis and purification use organic solvents and acids. Two categories frequently go unreported:
- Residual solvents — trace acetonitrile and related solvents from HPLC purification. USP General Chapter <467> and the ICH Q3C framework set international limits for these.
- Counter-ion / TFA content. Reverse-phase purification commonly leaves trifluoroacetic acid (TFA) as a counter-ion. TFA can interfere with sensitive cell-based assays, so many researchers prefer acetate-salt material and want the counter-ion identity and level stated. Reporting acetate or TFA content also feeds directly into the net-peptide calculation.
Related substances, appearance, and reconstitution
Beyond the headline purity number, a thorough COA characterizes related substances, the impurities structurally close to the target peptide, such as deletion sequences or oxidation products. It also states appearance (for example, white to off-white lyophilized powder) and behavior on reconstitution (clear, colorless solution, fully dissolved). These simple observational fields catch problems that a single purity percentage can hide.
Absence of a field is not proof of a problem, but it is a gap. A COA missing water content, endotoxin, and counter-ion data gives an incomplete safety and accuracy picture, even when the purity line looks excellent.
A reader's shortlist of "hidden" fields
- Water content by Karl Fischer, with the method named (coulometric or volumetric).
- Net peptide content, not only gross vial mass.
- Bacterial endotoxin by LAL, in EU/mg or EU/mL, with the method named.
- Counter-ion identity and level (TFA or acetate).
- Residual solvents against a recognized limit.
- Related substances, appearance, and reconstitution behavior.
None of these replace independent, accredited testing; they complement it. A COA from a recognized third-party lab that reports these fields tells a far more complete story than a purity-only certificate.