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Mass Spectrometry

Measures mass-to-charge ratio to confirm a compound's molecular weight — the usual evidence for identity rather than purity.

Mass spectrometry ionises a sample and measures the mass-to-charge ratio of the resulting ions. For a peptide, an observed mass matching the theoretical mass calculated from its sequence is the standard evidence that the intended molecule is present.

Peptides are usually run by electrospray ionisation, which tends to produce multiply charged ions — the same molecule appearing at several m/z values according to how many protons it carries. The neutral mass is reconstructed from that series, so the raw spectrum rarely shows a single peak at the molecular weight.

Identity and purity are separate questions answered by separate methods. Mass spectrometry addresses what the compound is; HPLC addresses how much of the sample is that compound. Note also that mass alone cannot distinguish two peptides built from the same residues in a different order — same composition, same mass, different molecule — which is why sequence is the underlying question.

Topic: Lab Practice

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Guides that cover this

Charged Aerosol and ELSD Detection for Peptides Without a Chromophore

Nebulisation-based detectors answer a physical question rather than an optical one: how much non-volatile mass reached the detector. What that buys when a sequence has no tryptophan or tyrosine, why the response is a power law, how far mobile-phase composition moves it and what inverse-gradient compensation corrects, and the precise claim a relative mass percent from CAD or ELSD will and will not support.

Kaiser and chloranil: reading a free amine on resin

The ninhydrin test answers a narrower question than the one you are asking: it reports that a reacting primary amine was present, not how much, and it gives no diagnostic blue when the terminus is a secondary amine. Where the detection floor sits, what produces a wrong answer, and which quantitative monitors replace it.

Selenocysteine and Selenoester Ligation: Faster Peptide Junctions

Selenocysteine has a side-chain pKa of 5.2, so the selenolate dominates at neutral pH. Paired with a peptide selenoester it gives ligations that finish in minutes at junctions where a thioester stalls, and a deselenisation step that leaves cysteine thiols alone.

Container Closure Integrity Testing for Peptide Vials

A container closure integrity result is a statement about a leak path, not about sterility. This guide traces how the maximum allowable leakage limit was derived from helium leak rate and microbial ingress data, compares helium, vacuum decay, high-voltage leak detection and laser headspace analysis on what each actually measures, and works through why positive controls, residual seal force and low temperature complicate every number - ending on the dried peptide, where a breach can leave the routine release attributes unchanged.

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