HPLC purity vs net peptide content

HPLC purity is the integrated area of the target peak as a percentage of total integrated peak area. The arithmetic is trivial. Everything interesting happens before it — in whether the impurities were separated from the main peak in the first place.

The characteristic impurities of solid-phase synthesis are deletion sequences: chains missing one residue because a coupling step failed. A 15-mer missing one residue is chemically very similar to the target and elutes very close to it. Whether it appears as a separate peak or disappears into the shoulder of the main peak is decided by the gradient.

Both figures are honest. The 97.1% is the better measurement, and describes the better-characterised material. Without the chromatogram and the gradient conditions there is no way to tell which kind of number you are being quoted.

What actually moves the number

Net peptide content is most reliably determined by amino acid analysis. The peptide is hydrolysed into its constituent free amino acids, those are quantified against an internal standard, and the resulting peptide mass is divided by the gravimetric weight of the sample.

The standard hydrolysis is 6 M hydrochloric acid at 110 °C for 24 hours. It is also destructive, in ways that are well documented and rarely mentioned by suppliers quoting the result.

That is the United States Pharmacopeia, general chapter <1052>, stating it plainly. A single standard hydrolysis cannot see all twenty. Any certificate presenting an AAA result as a simple, unqualified number has omitted the conditions under which that number holds.

The alternatives, and what each requires

• Quantitative NMR. Non-destructive and indifferent to residue lability, but needs a certified internal reference standard and clean spectra.
• Elemental nitrogen determination. Fast and cheap, but counts every nitrogen in the sample — including nitrogen-containing impurities — so it tends to overstate.
• UV spectrophotometry. Uses the peptide's extinction coefficient at 280 nm. Only works if the sequence contains tryptophan or tyrosine. For a peptide containing neither, this method is simply unavailable — which is why it cannot be the fallback vendors sometimes imply it is.
• Mass balance. Counterion content by ion chromatography plus water by Karl Fischer plus residual ash, subtracted from 100%. Reasonable, but it infers the peptide rather than measuring it.

The sequence determines the analytical difficulty. This is why a supplier's willingness to state the method matters more for some compounds than others.

BPC-157 is a useful worked case. Its sequence — Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val — contains no tryptophan, no cysteine, no asparagine and no glutamine, and its single valine sits at the C-terminus with no adjacent valine or isoleucine. Standard acid hydrolysis handles it cleanly, so an AAA result on BPC-157 is unusually trustworthy.

Net peptide content can also be calculated — take the molecular formula, assume a counterion stoichiometry based on the number of basic sites, assume a typical residual water figure, and derive a percentage. Manufacturers legitimately use this to estimate yields.

It is not a measurement. It is a prediction from assumptions, and the assumption most likely to be wrong is counterion stoichiometry, which varies with purification conditions. A theoretical figure presented on a certificate without that label reads exactly like an analytical result.

A measured content figure names a method — AAA, qNMR, elemental nitrogen. A theoretical figure does not, and often lands on a suspiciously round number. If the certificate reports content but no method, ask which it is. The answer distinguishes a laboratory result from an arithmetic exercise.

This is the situation the two numbers exist to expose. Both certificates are truthful; one product is materially better.

Supplier B's material is better characterised and its peptide mass is known. Supplier A wins the product-page comparison, because a purity percentage is the only number most buyers look at — which is precisely the incentive that keeps net content off certificates.

Is a higher purity percentage always better?

No — and this is the least intuitive thing on the page. Purity is method-dependent. A steep, short gradient co-elutes deletion sequences under the main peak and produces a higher number from a less rigorous separation. Between two certificates, the one reporting slightly lower purity with full method disclosure and a chromatogram describes better-characterised material than the one reporting a higher figure with neither.

Why do certificates report Asx and Glx instead of Asn and Gln?

Because acid hydrolysis deamidates asparagine to aspartic acid and glutamine to glutamic acid. After hydrolysis the two members of each pair are chemically indistinguishable, so results are reported as their sum: Asx for Asp plus Asn, Glx for Glu plus Gln. Distinguishing them requires a separate derivatisation step that most routine analyses skip.

Can amino acid analysis measure every amino acid?

Not in a single standard run. USP <1052> states that the loss of tryptophan, asparagine and glutamine during acid hydrolysis limits quantitation to 17 amino acids. Tryptophan needs alkaline hydrolysis or a protecting agent; cysteine needs a performic acid oxidation step; serine and threonine need a time-course extrapolation; and isoleucine/valine adjacencies need a time course to reach full cleavage.

If both numbers are method-dependent, is either worth anything?

Yes, provided the method travels with the number. A purity figure with its gradient, column and wavelength stated, alongside the chromatogram, is a reproducible result. A content figure with its determination method named is a reproducible result. What has little value is a bare percentage on a product page, because you cannot tell what was done to obtain it.

Which number should be used for concentration calculations?

Net peptide content, applied to the gross vial mass, then multiplied by purity. Using the label mass alone overstates concentration by the entire non-peptide fraction — typically 15–25%, and invisible unless content is reported. The arithmetic is worked through in how to read a certificate of analysis.

• United States Pharmacopeia, general chapter <1052>, Biotechnology-Derived Articles — Amino Acid Analysis. Hydrolysis conditions, residue-specific losses, the 17-amino-acid limitation, and time-course correction for labile and slow-cleaving residues.
• European Pharmacopoeia, general chapter 2.2.56, Amino Acid Analysis — recognised method for peptide identification and content determination.
• Fountoulakis M, Lahm HW. Hydrolysis and amino acid composition analysis of proteins. Journal of Chromatography A, 1998 — review of hydrolysis conditions and recovery behaviour.
• Standard references on reverse-phase chromatography of peptides, for gradient slope, resolution and the impurity profile characteristic of solid-phase synthesis.