HPLC vs Mass Spectrometry: What a Peptide Purity Report Shows

Two analytical techniques dominate peptide quality documentation: high-performance liquid chromatography (HPLC) and mass spectrometry (MS). They answer different questions. HPLC separates a sample into its components and tells you how much of what it detected sits in the main peak. Mass spectrometry measures the mass of the molecules present and tells you whether that mass matches the expected structure. A purity percentage from one is not an identity result from the other, and reading a report well starts with knowing which question it answered.

Two questions a peptide report can answer

Any analysis of a synthetic peptide sample is trying to answer one or both of these:

1. How much of the sample is the main component? This is purity. It is a ratio, normally expressed as a percentage.
2. Is the main component the intended molecule? This is identity. It is a match, or a mismatch, against a known structure.

HPLC is built for the first question. Mass spectrometry is built for the second. Neither fully answers the other's question on its own, which is why the strongest documentation uses both, often coupled in one instrument as LC-MS.

How HPLC works

In reversed-phase HPLC, the standard configuration for peptides, a dissolved sample is pumped through a column packed with fine silica particles bonded to long hydrocarbon chains (commonly C18). The mobile phase starts mostly aqueous and is shifted over the run toward an organic solvent such as acetonitrile, usually with a small amount of an ion-pairing acid such as trifluoroacetic acid. Molecules interact with the hydrophobic stationary phase to different degrees, so they leave the column at different times. The time at which a component leaves is its retention time.

A UV detector at the column outlet records absorbance as each component passes. For peptides the detector is usually set near 210 to 220 nm, where the peptide bond itself absorbs, so nearly every peptide-like species in the sample registers. The result is a chromatogram: a trace of absorbance against time, with a peak for each separated component.

What the purity percentage means

HPLC purity is normally calculated as area percent: the area under the main peak divided by the total area of all integrated peaks. A figure of 99.5% means that 99.5% of the UV-absorbing material the method separated and detected eluted in the main peak. That is a precise statement, and it has precise limits:

• It only counts what absorbs at the chosen wavelength. Water, inorganic salts and many counter-ions are invisible to the detector and are not in the denominator.
• It assumes each peak is one thing. An impurity that leaves the column at the same time as the main component (co-elution) is counted as main component.
• It depends on the method. Column, gradient, temperature and wavelength all change how peaks separate and integrate. Two labs can report different figures for one sample, both correctly.
• It does not identify the main peak. A sample of the wrong peptide, if clean, can show a single large peak and a high purity figure.

How mass spectrometry works

A mass spectrometer converts molecules into gas-phase ions and separates them by mass-to-charge ratio (m/z). For peptides, the two common ionisation methods are electrospray ionisation (ESI), which produces a series of multiply charged ions from a solution, and matrix-assisted laser desorption/ionisation (MALDI), which mostly produces singly charged ions from a sample co-crystallised with a light-absorbing matrix. The instrument records how many ions arrive at each m/z value.

For an ESI spectrum, the series of charge states is deconvoluted back to a single neutral mass. That observed mass is then compared with the theoretical mass calculated from the sequence: monoisotopic mass for high-resolution instruments, average molecular weight for lower-resolution ones. Agreement within the instrument's stated accuracy is evidence that the main component has the expected composition. A mass that is off by a recognisable amount (a missing residue, an oxidised methionine, an incomplete deprotection) points at a specific synthesis error.

What mass spectrometry does not tell you

• It is not a purity measurement. Ion signal depends on how readily each species ionises, so peak heights in a mass spectrum are not proportional to amounts.
• It does not distinguish isomers. Two peptides with the same composition in a different order, or a D-amino acid in place of an L-amino acid, have the same mass. Sequence confirmation needs fragmentation (MS/MS).
• It does not measure content. A correct mass says nothing about how many milligrams are in a vial.

Side by side

LC-MS couples the two: the chromatograph separates the sample and the mass spectrometer reads each peak as it leaves. That lets a lab put a mass on the main peak and on each impurity peak, which is the most complete single picture of a peptide sample. It also costs more, which is why many reports carry one technique, not both.

Purity is not content

A lyophilised peptide is not 100% peptide by mass. Alongside the peptide there is residual water and counter-ions left from purification, typically trifluoroacetate or acetate. None of these show in a UV chromatogram. So a sample can be 99% pure by HPLC while the peptide itself makes up a noticeably smaller share of the powder's mass. Content, the milligrams of material measured in a vial, is a separate measurement, and a good report prints it separately.

Reading a real report

Each report prints the sample name, the measured quantity per vial, a purity figure where the lab reported one, the analysis date and a verify key. It does not print an identity result or name the analytical method. Janoshik reports peptide purity by HPLC; that method is not printed on the report itself. The report states purity; it does not include a separate identity test. Here is how that reads on an ipamorelin 5 mg report from our catalogue:

Not every report carries a purity row. Our NAD+ report prints a measured content and no purity figure. The correct reading is that no purity was reported, and we list it that way rather than supplying a number the report does not contain:

Purity belongs to one lot

Synthesis, purification, lyophilisation and storage all vary from run to run, so purity is a property of a lot, not of a compound. A report on a lot made months ago cannot vouch for a lot made since. That is why a purity figure is only meaningful with a date and a sample reference beside it, and why the measurement should be made by a laboratory independent of the seller: a vendor grading its own material has an obvious conflict of interest, and its result can only be taken on its word.

Red flags in a purity claim

Any one of these should lower your confidence; several together are a clear warning:

Questions to ask about any purity report

• Which technique produced the purity figure, and at what detection wavelength?
• Is there a mass result, and does the observed mass match the theoretical mass for the sequence?
• Is content reported separately from purity, and in what units?
• Can the report be confirmed with the issuing lab using a key or reference number?
• Does the report belong to the lot being sold? See how to verify a certificate of analysis.

Where our reports sit

Batch tested by Janoshik Analytical. Verify any report with its key on Janoshik's site. Our reports state purity and measured content. They do not include a separate identity test, and we do not describe them as if they did. The testing page lists every report on file. Reports you can read against this guide include those on the ipamorelin,epitalon, TB-500 andNAD+ product pages.

Research use only. Purity and content describe a sample's composition, not its suitability for any purpose. Ouralus materials are supplied for laboratory research use only and are not for human or animal use.