Co-elution
Definition
Two components leaving the column together and arriving as one peak. It is the main way a purity figure can be honestly measured and still be too high, and it is why an orthogonal method is worth running.
In more detail
Co-elution is the failure mode that makes a purity figure wrong in the flattering direction. Two components leave the column close enough together that the detector records one peak, the integration counts one area, and the impurity is added to the target rather than subtracted from it.
Nothing in the chromatogram necessarily announces it. A co-eluted pair can produce a peak that looks clean, and the only hints are subtle: a slight asymmetry, or a peak that is broader than the others on the same trace.
This is the argument for an orthogonal method. A mass spectrometer placed after the column weighs whatever arrives, so two compounds that could not be separated in time are still distinguishable by mass. Running the same chromatographic method a second time reproduces the result and the error together.
What this means for research sourcing
Purity and identity verification are central to research reproducibility. Every Volta compound ships with >99% HPLC-verified purity and a certificate of analysis.
Related Terms in Quality
Peak Integration
Deciding where a peak starts and stops so its area can be calculated. Where the baseline is drawn changes the number, which is why integration parameters belong in the method rather than in the analyst's judgement on the day.
Reversed-Phase HPLC (RP-HPLC)
The chromatographic mode almost all peptide purity work uses: a non-polar stationary phase with a polar moving phase, separating components by hydrophobicity. The gradient usually runs water and acetonitrile with an acidic modifier.
HPLC-UV
High-performance liquid chromatography with an ultraviolet detector, typically reading at 214 nm where the peptide bond absorbs. It reports how much of the absorbing material is the target peak, which is a purity measurement and not an identity one.
LC-MS
Liquid chromatography coupled to a mass spectrometer, so each separated component is also weighed. It answers identity and purity in one run, which HPLC-UV alone cannot: a UV detector cannot tell two compounds apart if they elute together.
Tandem Mass Spectrometry (MS/MS)
Selecting one ion, fragmenting it, and measuring the fragments. The fragment pattern is far more specific than mass alone, which is why certificates quote transitions rather than a single mass for confirmation.
Electrospray Ionisation (ESI)
The soft ionisation technique that gets peptides into a mass spectrometer intact, by spraying the solution through a charged needle. It tends to produce several charge states of one molecule rather than a single ion.
Mass-to-Charge Ratio (m/z)
What a mass spectrometer actually measures: mass divided by charge, not mass. A 4,000 Da peptide carrying three charges appears near m/z 1,334, which is why a raw spectrum rarely shows the molecular weight directly.
Charge State
How many charges an ion carries in the mass spectrometer. Peptides commonly appear in several at once, and the series of peaks they produce is what allows the true molecular mass to be calculated.