← Knowledge · Updated 6 November 2026 · 5 min read
HPLC and mass spectrometry (MS) answer different questions, which is why peptide analysis usually combines them.
RP-HPLC
Reversed-phase HPLC separates components by hydrophobicity. Detection is typically in the UV range around 214 to 220 nm, where the peptide bond absorbs. Purity is stated as the main peak’s share of the total area.
A single peak does not prove identity. Impurities can co-elute in the same peak, and salts or water are not visible in this measurement.
Column, mobile phases and gradient
A C18 reversed-phase column is the standard for peptides. Mobile phase A is water, mobile phase B acetonitrile, both usually with 0.1% trifluoroacetic acid (TFA). TFA acts as an ion-pairing agent: it binds to basic residues and gives sharper peaks. During the run the share of acetonitrile rises, often over 20 to 40 minutes, for example from 5 to 65%. More hydrophobic peptides only leave the column at a higher acetonitrile share.
For coupling with mass spectrometry, TFA is often replaced by formic acid. TFA strongly suppresses the electrospray signal. That is why chromatograms from a plain HPLC test and from an LC-MS run can look different.
Reading a chromatogram
- Retention time: when a substance leaves the column. More hydrophobic peptides come out later.
- Main peak: the target peptide. Its area gives the purity in area percent.
- Side peaks: other peptidic substances. Small peaks right next to the main peak are often closely related by-products.
- Baseline and integration: where the software starts and ends each peak affects the result. An attached chromatogram makes the figure checkable.
Why two labs can measure different purities
HPLC purity is not a fixed property but depends on the method. A different column, a shallower gradient or another wavelength can reveal impurities that were hidden under the main peak before. Integration, meaning where a peak starts and ends, also affects the figure. Values of 98.1% and 97.4% from two labs therefore don’t necessarily contradict each other.
Orthogonal methods help for a robust result: two separations working on different principles, such as reversed phase and ion exchange. If purity is high in both, a hidden impurity is unlikely. A diode array detector can also check whether the UV spectrum stays the same across the whole main peak.
Mass spectrometry
MS, often ESI-MS or MALDI-TOF, measures the mass-to-charge ratio. A measured mass that matches the calculated mass supports identity. Isomers with the same mass, such as peptides with a swapped sequence, can’t be told apart this way alone. That takes fragmentation (MS/MS).
Average and monoisotopic mass
Mass figures can refer to two values. The monoisotopic mass is calculated from the most common isotope of each element. The average mass allows for the natural isotope ratio and is slightly higher. For small peptides and high-resolution instruments, the monoisotopic mass is usually evaluated, for larger peptides often the average. A certificate should state which is meant. Otherwise a difference of about one dalton can wrongly look like a deviation.
High-resolution instruments such as Orbitrap or modern TOF systems measure mass to within a few ppm. That makes even small changes, such as deamidation at +1 Da, clearly detectable.
MS/MS: confirming the sequence
Mass alone doesn’t tell you the order of the amino acids. For that, a peptide ion is deliberately broken apart in the mass spectrometer, usually by collisions with a gas. The fragments form mainly at the peptide bonds and make up two series, the b and y ions. The mass gap between consecutive fragments matches one amino acid each time. This lets you read the sequence step by step. An MS/MS check takes more effort and is not on every certificate, but it rules out confusion with sequence-swapped peptides of the same mass.
LC-MS: both methods in one run
In LC-MS the liquid chromatograph feeds straight into the mass spectrometer. Each peak in the chromatogram gets its own mass. This shows whether the main peak is the target peptide and what the side peaks are. LC-MS is now the usual route for an identity check.
Typical impurities and their mass
| Impurity | How it shows in MS |
|---|---|
| Deletion sequence (one amino acid missing) | Mass lower by that amino acid residue |
| Truncated sequence | Clearly lower mass |
| Oxidised methionine | +16 Da |
| Deamidation of asparagine or glutamine | +1 Da |
| Protecting group not fully removed | Mass higher by the protecting group |
HPLC as a measure of amount
With a reference standard of known amount, HPLC can also measure how much peptide is present. The sample’s peak area is compared with the standard’s. This is how Alpenpeptid’s earlier samples were evaluated: the measured amount in the vial came from comparison with a reference chromatogram. Accuracy depends on the quality and peptide content of the standard.
What neither method provides
Neither method gives the peptide content of the weighed material. That takes methods such as amino acid analysis or nitrogen determination.
HPLC and MS also do not cover sterility, endotoxins or heavy metals, which need their own tests. What a certificate should state is in How to read a peptide certificate of analysis. How Alpenpeptid’s earlier samples were measured is under Methods.
Common questions
Is HPLC purity enough without a mass spectrum? No. HPLC shows how much of the main peak is present, but not whether the main peak is the right peptide. That takes mass spectrometry.
Why do I see several signals for one peptide in the spectrum? In electrospray ionisation a peptide picks up different numbers of protons. Each charge state shows as its own signal. Together they give the same mass.
Which wavelength is right for purity? 214 to 220 nm is usual for peptides, because the peptide bond absorbs there. A measurement at 280 nm alone misses impurities without aromatic residues and can make purity look too high.
What does “LC-MS confirmed” mean on a certificate? That the mass of the main peak was measured by LC-MS and matches the calculated mass. It is proof of identity, but not yet a full sequence confirmation.
Can I check a chromatogram’s figure myself? With the peak table, yes. Divide the main peak’s area by the sum of all areas. If the result differs from the stated purity, ask which peaks were excluded from the evaluation, such as solvent or system peaks.
For laboratory research use only. Not for use in humans or animals.
