Peptide Purity Testing Methods: HPLC, LC-MS, AAA and What Each One Misses

By What Peptides Editorial Team · Updated 2026-09-14 · Part of Peptide Handling Guides: Reconstitution Arithmetic, Storage and Testing

Two numbers on a peptide specification sheet cause more confusion than any others: purity and peptide content. They are different quantities, measured by different methods, and a sample can be 98% pure while still being only 70% peptide by mass. Purity asks what fraction of the detected material is the target molecule. Peptide content asks what fraction of the weighed solid is peptide at all, the rest being water, counter-ions and residual salts. This page sets out the main analytical methods used to answer those questions and, just as importantly, what each one cannot tell you. It is educational content about laboratory analysis of research materials and is not medical advice.

Peptides are analysed differently from small-molecule drugs because they are large, charged, often hygroscopic and usually supplied as salts. A solid that looks dry can hold several percent water by weight, and the trifluoroacetate counter-ion carried over from reverse-phase purification can account for a substantial share of the mass. Understanding which technique measures which of those things is the difference between reading a certificate of analysis and being impressed by one. Every method below has a blind spot, and the blind spots are the reason competent laboratories run more than one.

HPLC: The Workhorse, and Its Limits

Reverse-phase high-performance liquid chromatography separates molecules by hydrophobicity. The stationary phase is a non-polar bonded silica, typically C18, and the mobile phase runs from a polar starting buffer to a less polar organic solvent, usually acetonitrile, over a gradient. More hydrophobic species retain longer and elute later. Detection for peptides is almost always ultraviolet absorbance, and the wavelength matters: the peptide bond absorbs around 214 to 220 nm, so that is where every peptide is visible, while 280 nm detects only aromatic side chains in tryptophan and tyrosine, with a weaker contribution from phenylalanine and from disulfide bonds.

The purity figure quoted is normally area percent: the area of the main peak divided by the total area of all integrated peaks, expressed as a percentage. It is a relative measure with three important limits. It assumes every impurity absorbs at the chosen wavelength with similar intensity, which is often untrue. It ignores anything that does not elute or does not absorb, including water, salts and non-UV-active material. And it depends on the integration settings, so a laboratory that includes tiny peaks in the total and one that excludes them will report different numbers for the same sample. A figure of 98% by area is a statement about a chromatogram, not a statement about the vial.

• 214 nm: peptide bond absorbance, the general-purpose choice
• 220 nm: slightly lower response, less interference from some buffers
• 280 nm: aromatic residues only, useful for proteins and blind to many peptides
• Gradient slope: changes resolution and therefore how many peaks are seen
• Integration threshold: directly changes the reported area percent
• Column chemistry: C18 suits most peptides, C4 and C8 suit very hydrophobic ones

Identity, Content and Water: The Other Methods

Liquid chromatography coupled to mass spectrometry adds a mass measurement to the separation. As each peak elutes, the instrument ionises it and measures the mass-to-charge ratio, which for an intact peptide gives a molecular mass that can be checked against the value calculated from the sequence. Electrospray typically produces a series of multiply charged ions, and the software deconvolutes them into a single mass. This is the standard identity test and it is fast and sensitive. It cannot confirm the order of residues, because two sequences with the same composition share a mass, and it cannot detect an isomer or an isoaspartate rearrangement without additional fragmentation work.

Amino acid analysis answers a different question. The sample is hydrolysed in strong acid, commonly 6 M hydrochloric acid at elevated temperature, which breaks every peptide bond, and the released free amino acids are separated and quantified, often after derivatisation. Comparing the molar ratios checks the composition, and the absolute amounts give the peptide content: how much of the weighed sample was actually peptide. This is one of the few methods that reports an absolute quantity rather than a ratio. Karl Fischer titration measures water specifically, through the reaction of water with iodine in a methanolic system, and reports residual moisture as a percentage; lyophilised peptides typically land in the low single digits.

Why Purity and Peptide Content Are Not the Same Number

Take a vial whose certificate states 98% purity and 70% peptide content. The two figures multiply rather than replace one another. If the vial contains 10 mg of solid, the peptide content says 7 mg of that is peptide, and the purity says 98% of the detected peptide-like material is the target sequence, leaving roughly 6.9 mg of target peptide in the vial. A reader who takes only the 98% and treats the vial as 9.8 mg of peptide has overestimated the material by about 40%. This is the most common arithmetic error in reading peptide documentation, and it is the reason both figures should appear on a specification sheet.

The gap between the two comes from what a lyophilised powder actually is. Water accounts for a few percent, more if the container has been opened repeatedly in humid air. Counter-ions from purification, most often trifluoroacetate from reverse-phase work, can be a large share of the mass for a short peptide with several basic residues. Residual salts and buffer components make up the remainder. None of these appear in a UV chromatogram, which is precisely why peptide content needs a separate measurement. For how this interacts with handling, see why moisture control matters for stored powder, and for the underlying molecule, what counts as a peptide. The wider practical set sits in the peptide guides collection. This page is educational and is not medical advice.

This page is part of the Peptide Handling Guides: Reconstitution Arithmetic, Storage and Testing guide.