Peptide Purity vs. Net Peptide Content

Purity by HPLC is an area-percent: the area under the main chromatographic peak divided by the total integrated peak area. The denominator is everything the UV detector saw — and at the 214 nm or 220 nm used for peptides, what the detector sees is amide bonds.

That is the crux. Trifluoroacetate, acetate, chloride, sodium and water have no meaningful absorbance at that wavelength. They pass through the column and register nothing. They are not counted as impurities because, as far as the measurement is concerned, they are not there. Purity is therefore a statement about the peptide-derived material in the sample: of everything peptide-like that was detected, this fraction was the target sequence.

Net peptide content answers a physical question instead: of the total mass in the vial, what percentage is peptide? Typical values for lyophilised research material run from roughly 70% to 90%, and the remainder is accounted for by three things.

• Counter-ions. A peptide with basic residues carries positive charges at the pH of purification, and those charges are balanced by anions that lyophilise with the solid.
• Residual water. Freeze-drying removes most but not all water, and lyophilised peptide is hygroscopic. A few percent by mass is normal.
• Residual salts. Inorganic salt carried through from synthesis or purification.

Because content is a mass fraction rather than a chromatographic ratio, it cannot be measured by HPLC. It is determined by amino acid analysis — complete acid hydrolysis of the sample followed by quantification of the liberated amino acids — or by quantitative nitrogen determination. For sequences containing tryptophan, tyrosine or cystine, UV absorbance at 280 nm against a calculated extinction coefficient is a faster alternative; for a sequence with no aromatic residues that method simply does not work.

The counter-ion contribution is large enough to be worth calculating rather than estimating.

Reversed-phase purification is normally run with trifluoroacetic acid as the ion-pairing modifier, so the peptide is isolated as its trifluoroacetate salt. A trifluoroacetate ion has a formula mass of 113.02 g/mol. A peptide carrying three positive charges at the working pH will lyophilise with three of them.

For a peptide of molecular weight 1,000 g/mol with three trifluoroacetate counter-ions:

• Peptide mass: 1,000 g/mol
• Counter-ion mass: 3 × 113.02 = 339.06 g/mol
• Salt mass total: 1,339.06 g/mol
• Peptide fraction: 1,000 / 1,339.06 = 74.7%

Before residual water is even considered, a quarter of the solid is counter-ion. Acetate exchange helps — acetate has a formula mass of 59.04 g/mol, so the same peptide as an acetate salt works out at 1,000 / 1,177.12 = 85.0% — which is one reason the salt form is worth knowing. The effect scales with charge density, so short, highly basic sequences are the most affected: a 900 g/mol peptide carrying three positive charges is over 27% counter-ion by mass as the trifluoroacetate salt.

Take a vial labelled 5 mg, with a certificate reporting ≥99% purity by HPLC and net peptide content of 82%.

• Total solid in the vial: 5.0 mg
• Peptide present: 5.0 × 0.82 = 4.1 mg
• Non-peptide mass: 0.9 mg of counter-ion, salt and water

Bring that vial to 2 mL of solvent and the nominal concentration is 2.5 mg/mL. The actual peptide concentration is 4.1 mg / 2 mL = 2.05 mg/mL. The nominal figure is high by 18%, and no amount of care with the pipette will fix it, because the error was in the assumption that labelled mass equals peptide mass.

In molar terms the gap is the same size and often matters more. For a peptide of 1,419.5 g/mol, 4.1 mg is 4.1 mg / 1,419.5 g/mol = 2.89 µmol, giving 1.44 mM in 2 mL — against the 1.76 mM you would calculate from the label.

Which number you should use depends on how the supplier fills. Some fill to gross mass: 5 mg of solid, of which 4.1 mg is peptide. Others fill peptide-corrected: enough solid that 5 mg of peptide is present, so roughly 6.1 mg goes into the vial. Both practices are legitimate and they differ by 22%. The question — is the stated quantity gross or peptide-corrected? — is one line in an email and it is worth sending.

It would be inconsistent to write all of the above and then be vague about our own number, so: Quill Peptides material is supplied at ≥99% purity, determined by reversed-phase HPLC with identity confirmed by mass spectrometry at an independent third-party laboratory, reported against each batch.

By the definitions on this page, that is a purity claim. It says the peptide fraction is essentially all the target sequence, and that the target sequence is the molecule it is supposed to be. It is not a net peptide content claim, and it should not be read as one — the counter-ion and residual-water mass described above is present in high-purity material exactly as it is in any other lyophilised peptide. Anyone calculating a precise molar concentration should be working from a content figure and a salt form, and should ask for both. That applies to this supplier on the same terms as any other.

• Purity — how much of the detected peptide material is the target sequence. Measured by HPLC. Blind to salts and water.
• Content — how much of the vial's mass is peptide. Measured by amino acid analysis or nitrogen determination. Accounts for everything.
• A high purity figure does not imply a high content figure, and a certificate reporting only purity has left the quantity in the vial undefined.
• Ask for the salt form. It moves the number by ten points or more.

The certificate is where both figures should appear; how to read a peptide COA covers the rest of the document, and HPLC vs. mass spectrometry explains why purity and identity are themselves two separate results.

• How to Read a Peptide Certificate of Analysis
• HPLC vs. Mass Spectrometry
• Peptide Solubility Guide
• Peptide Glossary