What HPLC/MS Purity Testing Actually Measures

HPLC separates a sample into components and reports purity as the main peak's share of total peak area. Mass spectrometry measures the main component's molecular mass and confirms it matches the expected peptide. Purity without identity can certify the wrong molecule; identity without purity says nothing about impurities. APX Labs runs both on every lot at an independent laboratory.

HPLC — how much

High-performance liquid chromatography dissolves the sample and pumps it under pressure through a column packed with fine particles. For peptides the standard mode is reversed-phase (RP-HPLC): the particles are hydrophobic, so components that are more hydrophobic stick longer and emerge later. A UV detector at the outlet — typically at 214 or 220 nm, where the peptide bond absorbs — records a signal over time. The result is a chromatogram: a baseline with peaks.

The target peptide is the main peak. Impurities from synthesis — deletion sequences missing a residue, truncated chains, incompletely deprotected or oxidised variants — appear as smaller peaks, usually close to the main one. Purity is the main peak's area divided by the total area of all peaks, expressed as a percentage.

What HPLC cannot do is tell you *what* the main peak is. Two different peptides can elute at the same time.

Mass spectrometry — what

Mass spectrometry ionises the sample and measures the mass-to-charge ratio of the ions. For a peptide the key output is the measured molecular mass of the main component. The COA compares it with the theoretical mass calculated from the expected sequence — 1419.5 g/mol for BPC-157, 5135.9 for tesamorelin — and agreement within instrument tolerance confirms identity. Peptides often appear as multiply-charged ions, so the raw spectrum needs deconvolution; the COA normally reports the deconvoluted mass.

High-resolution MS can go further and quantify co-eluting impurities that HPLC alone would hide inside the main peak [1], and can distinguish impurity profiles between synthesis routes [2].

Why both

• HPLC only: a 99% pure sample of the wrong peptide passes.
• MS only: the right peptide at 80% purity passes.
• Both: the right peptide, and a measured share of it.

That is why a complete peptide COA has an HPLC section and an MS section, and why APX Labs specifies both.

How APX Labs tests

Every lot goes to an independent laboratory for RP-HPLC purity and MS identity before release. The laboratory's report — chromatogram, peak table, purity percentage, observed vs theoretical mass, method conditions, test date, lot number — is the COA that ships in the box. For blended pens each component is reported. The full process is on the testing page; reading the resulting document is covered in how to read a COA.

Analytical testing documents what a sample contains. It says nothing about effects or suitability for any use. APX Labs products are for research use only.

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