Net peptide content: why a 10 mg peptide vial does not hold 10 mg of peptide

Net peptide content in one line

Net peptide content (NPC) is the share of a peptide powder’s weight that is peptide. Bachem defines it as the fraction of peptidic material, meaning the requested peptide plus peptide impurities, relative to counter-ions and residual water. Its formula is plain: net weight equals gross weight times NPC divided by 100.

The gap is not small. Bachem notes that research peptides are typically above 95% pure by HPLC, while the peptide content of the solid may range from 70 to 90%. A freeze-dried cake that weighs 10 mg can hold 7 to 9 mg of peptide, before you even consider purity.

What else is in the powder

A lyophilised peptide is a salt, and salts bring company. These are the parts of a typical vial’s powder, and the tests that measure each one.

Part of the powder	Where it comes from	How it is measured
Target peptide	Synthesis and purification	Content per vial against a reference standard
Peptide impurities	Side products of synthesis, degradation	HPLC purity (% area)
Counter-ions (TFA, acetate)	Purification buffers; they pair with charged groups on the peptide	Counter-ion test by HPLC or ion chromatography
Water	The powder absorbs moisture from air	Karl Fischer titration
Residual solvents	Synthesis and purification	Gas chromatography

Counter-ions and water make up most of the non-peptide weight. Peptides carry positive charges on basic amino acids, and each charge pairs with a negative counter-ion. Bachem notes that peptides with many basic amino acids, especially arginine, can have a relatively low peptide content for this reason. The EMA synthetic peptide guideline asks for the counter-ion to be defined and limited. Our guides on TFA in peptides and water content cover these two parts.

Peptide purity vs peptide content: three numbers that get mixed up

Number	Unit	What it measures	What it ignores
HPLC purity	% of UV peak area	Target’s share of what the UV detector sees	Water, counter-ions, mg
Net peptide content	% of powder weight	All peptidic material in the powder	Which peptide; impurities count as peptide
Target peptide content	mg per vial (free base)	mg of the target peptide itself	Nothing, if the basis and standard are stated

Purity and net peptide content are both percentages, which is why they get confused. They measure different things. Water and counter-ions are not counted in a purity result, so a 98% pure powder can be only 80% peptide by weight. Our guide on what purity means covers the purity side.

The EMA guideline sets the basis regulators use. Assays by liquid chromatography are expressed on a “counter-ion free, anhydrous” basis, and strength is the “mass of peptide base (not including salt or counter-ion)”. That is the free-base basis.

How to calculate peptide content

Three calculations cover most cases.

From gross weight, NPC and purity. Target peptide (mg) = gross weight × NPC/100 × purity/100. Bachem’s worked example uses a bradykinin acetate salt: 5 mg gross weight, 85.0% NPC and 98.7% HPLC purity give about 4.195 mg of the target peptide. The same percentages applied to a 10 mg fill would give about 8.4 mg.

By mass balance. A paper by USP authors on reference standards assigns peptide content by subtracting everything that is not peptide. Its formula multiplies two fractions: (100 minus the sum of HPLC impurities) / 100, times (100 minus acetate, TFA, residual solvents and residue on ignition, in % w/w) / 100. The result is on an anhydrous basis, so water is measured and corrected separately. The paper’s worked example gives 0.99 × 0.94 = 0.93 mg of peptide (free base) per mg.

By assay against a reference standard. This is how most labs measure mg per vial. The lab dissolves the vial’s contents, injects them alongside a reference standard of known content, and reads the concentration from a calibration line. Concentration times volume times dilution gives mg per vial. The result is only as good as the reference standard’s assigned content and basis.

Why so many COAs read above the label

Labels do not say which basis “10 mg” means. It can mean 10 mg of powder or 10 mg of peptide. Labs differ too.

Peptigrity compares measured mg with the label on every certificate. It explains that some labs report the gross mass of the powder, including salts and moisture, while others report net peptide only. So a gross result measured against a net label can read far above the label for an ordinary vial. In September 2026, about 40% of single-compound certificates that reported both a label and a measured figure were above 110% of the label. Peptigrity never colours a result above 110% as good for this reason.

The reverse also happens. A vial filled with 10 mg of powder and tested for net peptide will often read under the label, even when nothing is wrong. Our guide on underdosed and overfilled vials explains how to read a low or high result.

Why labs report different mg for the same vial

Peptide Critic compared results from five labs, and the seller’s own report, on one retatrutide batch. Their mass results ran from 10.18 to 12.80 mg, a spread of about 26%. Differences in basis and reference standard can explain much of a spread like that. Labs publish their approaches in different places, and the differences are clear:

Lab	How it describes its mg result
Janoshik	Reports net peptide, without counter-ions; characterises its own reference material by CHNS elemental analysis and NMR (Krysia’s Substack, August 2026)
Vanguard Laboratory	Buys certified reference standards; uses ion chromatography to identify counter-ions (same source)
BTLabs	External reference standard with a calibration curve; states “mg per vial = mass of vial contents × HPLC purity fraction”; stated uncertainty ±2–3% (Finnrick questionnaire, January 2026)
Krause Analytical	Calibration with reference materials and a check standard each run; check-standard acceptance of ±10%, which it treats as the lower limit of error; reports mg recovered with a % of label claim (questionnaire, March 2026)

Each approach is stated openly, and each answers a slightly different question. That is the point: two honest labs can print different mg for the same vial when their bases differ. Compare results only when you know both bases. Our guide on why labs disagree goes further, and measurement uncertainty covers how much spread a single lab’s result carries.

What a good content result states

A content result you can rely on states six things:

1. The basis: free base, salt form, total peptide, or gross powder.
2. The reference standard: its source, lot and assigned content.
3. The calibration range, with the sample inside it.
4. The uncertainty, with its coverage factor (k).
5. The per-vial values, when more than one vial was tested.
6. The % of label claim, marked as using the client’s label.

Most reports leave some of this out. In HK Analytical’s September 2026 review of public COAs from 25 labs and one rating site’s certificate wrapper, 21 of 26 printed mg per vial. Only 1 clearly stated what the mg meant. Only 1 printed an expanded uncertainty with its coverage factor, and only 1 showed a calibration curve with its range.

How HK Analytical reports content

HK Analytical’s content per vial test reports the mg of the target peptide in the vial, as free base, measured against a reference standard, with the uncertainty and the % of the label claim. If no qualified reference standard exists, the value is marked “estimated”. Every number comes with its raw data.

The same order can add counter-ion (TFA, acetate), water content (Karl Fischer) and purity (HPLC), which together show where the rest of the powder’s weight goes. The example report shows a content result with its basis and standard.