How Is a Peptide Identified by Mass Spectrometry?

The identity line on a certificate is usually short: a technique, a theoretical mass, an observed mass, and the word "conforms". Behind it is a spectrum, and the spectrum is often attached. This article describes what that spectrum shows and how to read the peaks that are not the one expected.

Two common techniques

Most peptide identity testing uses one of two ionization methods. Electrospray ionization, usually coupled to liquid chromatography, produces ions carrying several charges and is the more common of the two [1]. MALDI, generally paired with a time-of-flight analyzer, produces mainly singly charged ions and gives a simpler picture. Both measure mass-to-charge ratio. A certificate that names the technique tells the reader which kind of spectrum to expect.

Average mass or monoisotopic mass?

Reference tables quote two different masses for the same molecule, and confusing them produces phantom discrepancies of about one dalton. The monoisotopic mass counts every atom as its lightest isotope and corresponds to the first peak of an isotope cluster; the average mass weights all isotopes by natural abundance and is what a low-resolution instrument effectively reports. For BPC-157, the average mass is near 1419.5 daltons while the monoisotopic mass is near 1418.7. An identity line should say which one its theoretical value is; a reader comparing an observed mass against the wrong reference will see an error that is not there.

Charge states

In an electrospray spectrum the same molecule appears as a series of peaks, one for each number of protons it has picked up. A peptide of monoisotopic mass 1418.7 appears near 1419.7 with one charge, 710.4 with two and 473.9 with three. These are not three substances. Each peak, converted back, gives the same molecular mass, and agreement across the series is itself a confirmation [1].

Isotope pattern

At sufficient resolution each of those peaks is a small cluster. Carbon-13 occurs naturally at about one percent, so a molecule with 62 carbons frequently contains one or two heavy atoms, producing peaks one mass unit apart for a singly charged ion and half a unit apart for a doubly charged ion. The spacing therefore reveals the charge, and the first peak of the cluster is the monoisotopic mass.

Adducts

Peaks slightly heavier than expected are often the same molecule carrying sodium or potassium in place of a proton, about 22 and 38 units above the protonated ion. They come from glassware, buffers and salts and are ordinary. They are not impurities in the peptide sense.

Peaks that do carry information

A peak 16 units above the target indicates oxidation. A peak lighter than the target by the mass of one amino acid residue indicates a deletion sequence. A peak 18 units lighter can indicate loss of water or formation of a cyclic side product. Small amounts of these are consistent with a high-purity material; the chromatogram, not the mass spectrum, is what quantifies them, because peak height in a mass spectrum depends on how readily each species ionizes and is not a measure of amount [2].

How close must observed and theoretical agree?

Within the accuracy of the instrument, which the report's own decimal places usually reveal. An observed mass quoted to one decimal that lands within a few tenths of a dalton of the right reference value is agreement; a discrepancy of one full dalton suggests the wrong reference mass was used for comparison, and a discrepancy of tens of daltons or more is a different molecule. Modification arithmetic is diagnostic here too: a persistent difference equal to the mass of one residue, one water, or one oxygen is not noise but chemistry, and it names its own suspect.

What a complete identity line contains

The technique, the theoretical mass with an indication of whether it is average or monoisotopic, the observed mass, and the lot number of the sample. "Conforms" with no observed value asks the reader to accept a conclusion without the measurement.

On FLASK records

Where the laboratory report for a lot includes an identity result, it is published under the lot number with the observed mass and the technique, and the laboratory report is attached.

Frequently asked questions

Are several peaks in an electrospray spectrum several substances?

No. The same molecule appears once per charge state, and each peak converts back to the same molecular mass.

Does the mass spectrum quantify impurities?

No. Peak height depends on how readily each species ionizes. The chromatogram quantifies them.

What should a complete identity line contain?

The technique, the theoretical mass marked as average or monoisotopic, the observed mass and the lot number of the sample.

Why does the certificate's mass differ from a database value by one dalton?

Almost always because one is monoisotopic and the other is average mass. Check which reference the report states before reading it as a discrepancy.

Can mass spectrometry alone certify a peptide?

No. It confirms identity - the right molecule - while purity and content come from the chromatogram and a content assay. The three answer different questions and a complete record carries all three.