Purity, potency and net peptide content are three different numbers

A certificate reporting 99.4% purity and a certificate reporting net peptide content 78.2% are not contradicting each other. They answer different questions. Chromatographic purity is a relative measure — of the material the detector saw, what share was the target peak. Net peptide content is an absolute measure — of the total mass in the vial, what share is peptide at all. Identity and potency are a third question: whether the molecule present is the labeled one, and how much active substance a unit holds against a reference standard. A lot can be 99% pure by HPLC and still be well under 99% peptide by mass, because the balance — counterion, residual water, inorganic salts — is invisible to the purity measurement. Knowing which number you are reading is what makes a price-per-milligram comparison mean anything.

Three numbers, three different questions

Dimension	Chromatographic purity	Net peptide content	Identity and potency
Question answered	Of the material detected, what fraction is the target peak?	Of the total mass in the vial, what fraction is peptide?	Is this the labeled molecule, and how much active substance per unit?
Typical method	RP-HPLC with UV detection, percentage of total peak area (USP <621>)	Amino acid analysis (USP <1052>), nitrogen determination (USP <461>), or quantitative NMR	LC-MS or high-resolution MS; assay against a reference standard
Blind to	Counterions, water, inorganic salts — anything not resolved as a peak	Which impurities are present, and their distribution	Bulk composition of the powder
Reported as	% area	% w/w	Mass match; % of label claim

None of the three substitutes for another. ICH Q6A treats identification, assay and impurities as separate specification elements for this reason.

Chromatographic purity is a relative measure

Reversed-phase HPLC separates a sample's components and a UV detector records absorbance as each elutes. Purity is calculated by integrating every peak above the reporting threshold and expressing the main peak as a percentage of that total area. The mechanics are covered in HPLC purity testing explained.

The critical word is total. The denominator is not the mass of the sample but the summed area of the peaks the detector resolved. Anything that does not produce an integrable peak never enters the calculation:

• Trifluoroacetate and acetate counterions. The counterion dissociates on injection and elutes unretained at the solvent front, outside the integration window — not a resolvable peak, and conventionally not counted.
• Water. Lyophilized powder is hygroscopic and carries residual moisture. Water is chromatographically silent.
• Inorganic salts. Sodium, chloride, residual buffer components — none produce an integrable peak.

A sample that is 60% peptide and 40% counterion plus water, with only trace synthesis by-products, will report as a very clean chromatogram. The purity figure is honest; it simply is not describing vial mass. This is the most common misreading of a research-compound certificate.

What net peptide content measures, and how it is determined

Net peptide content, sometimes called peptide content assay, is the mass fraction of the solid that is actually peptide, expressed as % w/w. It is determined by methods that measure the peptide directly rather than by difference.

Amino acid analysis

The reference approach, described in USP <1052>. The sample is hydrolyzed under strong acid at elevated temperature and the liberated amino acids are separated, derivatized and quantified against calibrated standards. Measured amino acid moles back-calculate to peptide mass against the sequence's theoretical residue composition. The method has limitations a competent lab designs around: tryptophan is largely destroyed by acid hydrolysis, serine and threonine degrade partially, and sterically hindered bonds such as Ile-Ile or Val-Val hydrolyze slowly. Quantification is therefore anchored to stable residues such as alanine, glycine, leucine and phenylalanine rather than to every residue in the sequence.

Nitrogen determination

Total nitrogen is measured by Kjeldahl or combustion analysis (USP <461>) and converted to peptide mass using the sequence's theoretical nitrogen content. It is faster and cheaper than amino acid analysis, but counts all nitrogen. Residual ammonium salts, amide solvents or nitrogenous scavengers inflate the result. Trifluoroacetate contains no nitrogen, so it does not distort the number — but the method also never sees the counterion load it is implicitly correcting for.

Quantitative NMR and orthogonal checks

Quantitative proton NMR against a certified internal standard gives absolute content without hydrolysis. Fluorine-19 NMR or ion chromatography quantifies trifluoroacetate directly. Water is measured separately by Karl Fischer titrimetry (USP <921>), and residual solvents fall under ICH Q3C. Complete characterization is these methods together, not any one of them.

Why the non-peptide mass depends on the sequence

Preparative reversed-phase purification almost always uses trifluoroacetic acid as the ion-pairing modifier, so peptides are isolated as TFA salts. Each protonatable basic site — an arginine, lysine or histidine side chain, or a free N-terminal amine — can carry one trifluoroacetate, adding roughly 114 Da of non-peptide mass.

The counterion burden is therefore a function of two things: how basic the sequence is, and how large it is. Four examples from the Merit catalogue, as illustrative arithmetic only — real values come from the lot certificate:

• Selank (TKPRPGP, 751.9 Da) has an arginine, a lysine and a free N-terminus on a very small backbone. Three TFA equivalents would put the peptide at roughly 69% of the salt mass, before any water is counted.
• Epitalon (AEDG, 390.4 Da) has no basic side chains; only its free N-terminus can take up a counterion. But at 390 Da, a single equivalent is already about 23% of the salt mass — very small peptides are disproportionately sensitive to everything that is not peptide.
• BPC-157 (GEPPPGKPADDAGLV, 1419.6 Da) is net acidic — two basic sites against four carboxyls — landing near 86% peptide on the same arithmetic.
• Thymosin Alpha-1 (28 residues, 3108.3 Da, N-terminally acetylated so there is no free alpha-amine) has four lysines on a large, strongly acidic backbone — roughly 87%.

The consequence is that there is no universal correction factor. A supplier's typical net peptide content for a small basic heptapeptide tells you nothing about their 28-residue acidic peptide; applying one number across a catalogue will be wrong in both directions.

Identity and potency are a third, separate question

HPLC tells you how much of the detected material is the main peak, not that the main peak is the molecule on the label. A misidentified or mis-synthesized sequence can produce a flawless chromatogram, which is why identity is confirmed by mass spectrometry — observed mass against theoretical mass for the sequence, covered in mass spectrometry for compound identity verification.

Potency in the pharmacopeial sense is different again: a content determination against a characterized reference standard, run by a validated stability-indicating method with a calibration curve and reported as a percentage of label claim. It is not area normalization. Research-compound certificates frequently print an area-percent purity figure under a heading reading "potency." The two are not interchangeable, and a certificate offering only one has not answered the other.

Why this breaks price-per-milligram comparisons

Two vials, both labeled 10 mg, at similar prices. Supplier A labels by gross lyophilized mass — whatever weight went into the vial. Supplier B labels by net peptide content, compensating for counterion and moisture so the stated peptide mass is present. If supplier A's material assays at 72% peptide, that vial holds roughly 7.2 mg of peptide against supplier B's 10 mg. On a per-milligram-of-peptide basis, supplier A's material is about 39% more expensive than the sticker suggests.

Neither convention is dishonest — peptide reference materials are routinely sold by gross weight with a stated content value, and that is perfectly rigorous. The failure is comparing two vials as though the labels mean the same thing when neither certificate claimed they did. A certificate that names its convention has told you something material.

What to look for on a certificate

1. Which convention the label uses. Gross mass or net peptide — stated explicitly, not inferred.
2. Whether a content method is named. Amino acid analysis, nitrogen determination or quantitative NMR — "purity 99%" alone is not a content determination.
3. Whether water is quantified. Karl Fischer per USP <921>; residual moisture is real mass and moves with storage conditions.
4. Whether the counterion is identified and measured. Trifluoroacetate versus acetate matters independently of the mass accounting.
5. Whether identity was confirmed orthogonally. A mass spectrum, not just a retention time.
6. Whether the chromatogram itself is published. The trace shows integration windows and impurity distribution; the headline number shows neither.

Every Merit lot ships with its HPLC chromatogram rather than a summary figure, and each certificate is published against the lot number on the vial, so you can verify any lot's certificate independently of the shipment. How to read a Certificate of Analysis walks the document section by section. All materials are supplied for laboratory research use only.

The takeaway is narrow: purity describes the population of molecules the detector saw, net peptide content describes the contents of the vial, potency describes measured active substance against a reference. Three questions, three methods, three numbers. When two certificates disagree, check first whether they are even measuring the same thing — and when they agree, check whether the agreement is real or an artifact of both reporting the easiest number to produce.