Mass spectrometry measures the mass-to-charge ratio of ionized molecules. In peptide work it answers a question that no purity number can answer on its own: is this the right molecule? A chromatogram tells you how much of one thing is present. A mass spectrum tells you what that thing weighs, and therefore what it probably is.

What the instrument measures

A mass spectrometer does three things in sequence. It turns neutral molecules into ions, it separates those ions according to their mass-to-charge ratio, and it counts them. The output is a spectrum: intensity plotted against m/z. Nothing in that process weighs a molecule directly. What it measures is how an ion behaves in an electric or magnetic field, and mass is inferred from that behaviour. (Reference: Aebersold & Mann, 2003)

How a peptide becomes an ion

Peptides are large, fragile and non-volatile, which made them very hard to analyse until two ionization methods solved the problem in the late 1980s.

  • Electrospray ionization (ESI). The sample is sprayed from a fine needle held at high voltage. The solvent evaporates from the resulting droplets until bare ions are left in the gas phase. Because it works from solution, ESI couples directly to a liquid chromatograph. (Reference: Fenn et al., 1989)
  • Matrix-assisted laser desorption/ionization (MALDI). The sample is co-crystallised with a light-absorbing matrix and hit with a laser pulse. The matrix absorbs the energy and carries the peptide into the gas phase with it. (Reference: Karas & Hillenkamp, 1988)

The practical difference shows up in the spectrum. MALDI usually produces singly charged ions, so the spectrum is simple to read. ESI produces a series of multiply charged ions of the same molecule, and software combines that series into one mass. A spectrum with several evenly spaced peaks is not several compounds; it is one compound carrying different numbers of protons.

Monoisotopic mass and average mass

A certificate that reports an observed mass is reporting one of two different numbers, and they are not interchangeable.

Monoisotopic mass is calculated using the lightest and most abundant isotope of every element in the formula: carbon-12, hydrogen-1, nitrogen-14, oxygen-16. Average mass weights every element by the natural abundance of all its isotopes. For a small peptide the two figures differ by well under a dalton. As the molecule gets larger the gap widens, because there are more carbon atoms available to carry a heavier isotope. (Reference: Senko et al., 1995)

An observed mass a dalton away from what you expected is not automatically a problem. It may be the same molecule reported on the other convention. It is also why a good certificate says which mass it is quoting.

Tandem mass spectrometry and the sequence

Measuring the whole molecule confirms its mass. It does not confirm the order of the residues, and two different sequences built from the same amino acids weigh exactly the same. Tandem mass spectrometry closes that gap by selecting one ion, breaking it apart, and measuring the pieces.

Peptides tend to break along the amide backbone, producing two families of fragments: one series that keeps the N-terminus and one that keeps the C-terminus. The naming convention for those fragments has been standard since 1984, and the spacing between consecutive fragments corresponds to the residue that was lost. Read across the series and the sequence falls out. (Reference: Roepstorff & Fohlman, 1984) (Reference: Domon & Aebersold, 2006)

What it pairs with, and why

HPLC separates and quantifies but does not name. Two compounds can leave the column at a similar time, and a UV detector reports both as peaks without opinion. Mass spectrometry names but does not, on its own, tell you what share of the sample each component represents. Run together as LC-MS, the two cover each other's blind spot, which is why a complete certificate of analysis carries a result from each. (Reference: Yates et al., 2009)

The configurations you will see named

Three combinations cover most peptide analysis, and a certificate will usually name one of them.

  • MALDI-TOF. Laser ionization paired with a time-of-flight analyzer, which separates ions by how long they take to travel a fixed distance. Fast, tolerant of salts, and well suited to confirming the mass of a purified sample.
  • ESI-MS. Electrospray feeding the analyzer directly from solution. The natural choice when the sample is already in liquid form.
  • LC-MS/MS. Liquid chromatography in front, tandem mass spectrometry behind. The chromatograph separates, the first analyzer selects one component, and the second measures its fragments. This is the configuration that gives separation, mass and sequence evidence in a single run.

None of them is better in the abstract. They answer slightly different questions, and which one was used is part of what a result means.

What mass spectrometry cannot tell you

The limits matter as much as the capabilities, and they are rarely stated.

  • Isomers weigh the same. Leucine and isoleucine have identical residue masses, so a standard measurement cannot distinguish sequences that differ only by swapping one for the other.
  • Stereochemistry is invisible. A D-amino acid and its L counterpart are the same mass. Nothing in the spectrum separates them.
  • Peak height is not concentration. Different molecules ionize with different efficiency, so intensity does not translate directly into how much is present without a reference standard.
  • It sees what gets ionized. Salts, water and counterions that make up part of the weighed powder do not appear as the peptide does, which is one reason mass is not the same question as what else is in the vial.

Where it fits in a batch record

For a synthetic peptide the routine sequence is straightforward. The material is made, it is purified, a chromatographic run establishes how much of it is the target, and a mass measurement establishes that the target is the intended molecule. Put a batch number on both results and the label becomes something you can check. You can look up the batch on a vial at our batch verification page. (Reference: Aebersold & Mann, 2016)

Check this against a real batch

Every compound in the catalog lists its formula, molecular weight and CAS number, and every batch we ship has a certificate you can look up by number.

Frequently asked questions

What does mass spectrometry measure in peptides?

It measures the mass-to-charge ratio of ionized peptide molecules. From that, the molecular mass is inferred and compared with the mass calculated from the intended sequence, which is what confirms identity.

What is the difference between MALDI and ESI for peptides?

MALDI ionizes the peptide from a solid matrix hit by a laser and usually gives singly charged ions, so the spectrum is simple. ESI ionizes from solution and gives a series of multiply charged ions, which software combines into one mass. ESI couples directly to liquid chromatography; MALDI does not.

What is the difference between monoisotopic and average mass?

Monoisotopic mass uses the lightest abundant isotope of each element. Average mass weights every element by the natural abundance of all its isotopes. The two differ by well under a dalton for small peptides and diverge as the molecule gets larger, so a certificate should state which one it reports.

Can mass spectrometry confirm a peptide sequence?

Tandem mass spectrometry can. It selects one ion, fragments it along the backbone, and measures the pieces. The spacing between consecutive fragments corresponds to the residue lost, so the sequence can be read across the series.

What can mass spectrometry not detect?

It cannot distinguish isomers of equal mass, such as leucine and isoleucine, and it cannot see stereochemistry. Peak intensity also does not translate directly into concentration, because different molecules ionize with different efficiency.

Why is purity without a mass measurement incomplete?

Chromatographic purity reports how much of the detected material is one main component. It does not establish that the component is the correct molecule. Pairing a purity result with an identity measurement is what makes a certificate complete.

References

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