Ask a laboratory how it knows a synthetic peptide is the right molecule and the answer is almost always mass spectrometry. The technique weighs molecules with remarkable precision, and a certificate of analysis usually reports its result in a single line: expected mass, observed mass, match. That line is worth understanding, because it says something specific and leaves other questions open.

Weighing molecules by turning them into ions

A mass spectrometer cannot weigh a neutral molecule directly. It first gives the molecule an electric charge, then measures how the resulting ion moves through electric or magnetic fields. What the instrument actually reports is the mass-to-charge ratio, written m/z.

Two ways of charging peptides dominate.

• Electrospray ionisation (ESI) sprays a solution of the sample through a fine, charged needle. Peptides pick up one or more extra protons as the droplets evaporate, so a single molecule can appear as several ions carrying two, three or more charges. ESI is usually coupled to liquid chromatography, which separates the components of a sample before they reach the detector. Together they are called LC-MS.
• MALDI embeds the sample in a crystalline matrix and fires a laser at it. Most peptides leave with a single extra proton, which gives a simpler spectrum with one main peak per molecule.

Reading a spectrum with several peaks

An ESI spectrum of a single, pure peptide often shows a family of peaks rather than one. Each peak is the same molecule with a different number of protons attached. A peptide of mass M carrying two extra protons appears at roughly (M + 2) divided by 2; with three, at roughly (M + 3) divided by 3. Software works backwards from this ladder, a step called deconvolution, to report a single neutral mass.

Each peak is itself a small cluster. Natural carbon contains about one percent of the heavier carbon-13 isotope, so some copies of the molecule are one unit heavier than others. The spacing between these isotope peaks reveals the charge: one unit apart for a singly charged ion, half a unit for a doubly charged one, and so on.

Two kinds of expected mass

The expected mass on a report is calculated from the sequence, but it can be calculated in two ways. The monoisotopic mass counts only the most common isotope of each element and matches the first peak of the cluster. The average mass weights every isotope by its natural abundance. For small peptides the two differ by a fraction of a unit; for larger ones the gap grows to a unit or more. A report should say which one it uses, or a correct result can look like a mismatch.

How close counts as a match depends on the instrument. High-resolution instruments measure to within a few parts per million, while simpler ones are accurate to a fraction of a mass unit. A sound report states the tolerance it applied.

Mass shifts that tell a story

When the observed mass is off, the size of the difference is often diagnostic.

• About +22 usually means a sodium ion has replaced a proton. It is common, harmless and a feature of the measurement rather than the sample.
• About +16 suggests an oxidised residue, most often methionine.
• About +1 can indicate deamidation, a slow chemical change to asparagine or glutamine.
• The mass of one amino acid missing points to a deletion sequence left over from synthesis.

For a product that contains more than one peptide, each component needs its own match. A multi-component product such as the KLOW peptide blend should show a confirmed mass for every component it lists, usually after liquid chromatography has separated them.

What a matching mass does not prove

A mass match confirms that the molecule has the right elemental composition. It does not, by itself, confirm the order of the amino acids. Rearranging the same residues gives exactly the same mass, and leucine and isoleucine weigh precisely the same. Where order matters, laboratories use tandem mass spectrometry, often written MS/MS. The instrument breaks selected ions along the peptide backbone and measures the fragments, which can be read off one residue at a time.

Mass spectrometry is also a poor judge of quantity. Different molecules ionise with very different efficiency, so the height of a peak does not translate neatly into how much of something is present. That is why identity and purity are measured separately, with chromatography and UV detection handling purity.

Reading the result on a certificate

A useful mass spectrometry result on a certificate of analysis includes:

1. The expected mass, and whether it is monoisotopic or average.
2. The observed mass, with the charge states it was deconvoluted from.
3. The method: ESI or MALDI, and the type of instrument.
4. The spectrum itself, not just the word “conforms”.
5. The lot number, matching the material it describes.

The bottom line

Mass spectrometry answers one question very well: does this sample contain a molecule of the right mass? Read with its expected mass, tolerance and spectrum, it is strong evidence of identity. It is not a sequence read-out unless tandem methods were used, and it is not a measure of purity. Knowing that difference is what turns a single line on a report into information.

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