What Is Mass Spectrometry?

Key facts

01 Definition: What Is Mass Spectrometry?

Mass spectrometry weighs molecules. It does this by giving them an electric charge and then sorting the ions by their mass-to-charge ratio. Every instrument has three parts: an ion source, a mass analyzer and a detector.

Within peptide testing, mass spectrometry handles the identity question: is the main component the intended sequence? HPLC testing takes the purity question. The same technique underpins proteomics, where it identifies thousands of peptides from complex biological samples ^[1].

02 Ionization: How Peptides Become Ions

Neutral molecules can't be steered or measured, so the first job is to give them a charge. Peptides are large and fragile, which is why labs use 'soft' ionization methods that add charge without breaking the molecule apart.

Electrospray ionization (ESI)

ESI sprays the peptide solution from a fine needle held at high voltage. The charged droplets shrink until intact ions escape into the gas phase ^[2]. A peptide usually picks up several protons this way, so one molecule shows up as a series of multiply charged ions. And because ESI runs off a liquid stream, it plugs directly into liquid chromatography, which is what makes LC-MS possible.

Matrix-assisted laser desorption/ionization (MALDI)

MALDI mixes the peptide with a light-absorbing matrix, dries it on a plate and then hits it with a laser pulse that vaporizes and ionizes it ^[3]. Most of the ions come out singly charged, so the spectra are easy to read. MALDI is often paired with a time-of-flight (TOF) analyzer.

03 Mass-to-Charge Ratio (m/z)

The mass-to-charge ratio, m/z, is an ion's mass divided by its number of charges, and it's the quantity a mass spectrometer actually measures. For a peptide carrying z added protons, m/z equals (M + z × 1.00728) divided by z, where M is the neutral mass.

Here's a worked example with BPC-157, whose monoisotopic mass is 1418.70 Da (PubChem CID 9941957). You'd expect these ions:

Reports often carry two different mass values. The monoisotopic mass uses the most abundant isotope of each element, and it's what high-resolution instruments resolve. The average molecular weight (1419.5 g/mol for BPC-157) averages over natural isotopes and is the number used for weighing. Confuse the two and you'll see an apparent 'mismatch' of about one dalton for a peptide this size.

04 Mass Spectrum: Reading the Output

A mass spectrum plots ion abundance (y-axis) against m/z (x-axis). Each peak stands for ions of one m/z value. With a peptide, the peaks that matter are the molecular ions at each charge state and their isotope clusters.

• Charge-state series: peaks at m/z values that fit the same neutral mass at z = 1, 2, 3 and so on ^[2].
• Isotope spacing: peaks within one cluster sit about 1/z apart, which gives you the charge directly.
• Adducts: sodium (+22 Da relative to a proton) or potassium adducts can add extra peaks.
• Deconvoluted mass: the single neutral mass worked out from the series, which the report then compares with theory.

05 Peptide Analysis with Mass Spectrometry

Peptide labs use mass spectrometry for three jobs: confirming molecular mass, confirming sequence, and spotting modifications or impurities that carry a different mass.

Intact mass confirmation

The simplest identity test puts the deconvoluted observed mass next to the theoretical mass. If they agree within the instrument's accuracy, usually stated in daltons or parts per million, that supports identity. How much weight that evidence carries depends on the instrument, the technique, the sample and the workflow.

Tandem MS (MS/MS) and sequence confirmation

MS/MS isolates one precursor ion and breaks it apart, usually along its peptide bonds. The b-ion and y-ion series that result form a ladder, and the spacing between rungs matches individual amino acid residue masses, so you can read the sequence straight off it ^[4]. That lets you tell apart two peptides with the same composition in a different order. Intact mass can't.

Detecting impurities and modifications

Typical synthesis impurities shift the mass in predictable ways. A deletion sequence is lighter by one residue mass. Methionine oxidation adds about 16 Da. A protecting group that wasn't fully removed adds its own mass ^[5]. Spotting these masses points to specific impurity types, which is why LC-MS impurity profiling is so widely used in peptide characterization.

06 Mass Analyzers, Resolution and Mass Accuracy

The mass analyzer sorts ions by m/z, and its type sets how precisely a peptide's mass can be measured. That precision, in turn, decides how much confidence an identity match deserves.

Resolution is about whether two close m/z values show up as separate peaks. High resolution lets an instrument split the isotope peaks of a multiply charged ion and so assign its charge. Mass accuracy is about how close the measured value lands to the true value, often given in parts per million (ppm). For a peptide near 1,400 Da, 5 ppm works out to about 0.007 Da, whereas a low-resolution instrument may only support agreement within a few tenths of a dalton. When a report states the instrument type and the observed vs theoretical mass, a reader can judge whether the match is tight or approximate ^[1][4].

07 Mass Spectrometry vs. HPLC

The two techniques work on different principles and answer different questions. Mass spectrometry measures what a molecule weighs. HPLC separates components and reports how much of each there is, relative to the rest.

On its own, mass spectrometry is a poor purity method: ionization efficiency varies between molecules, and peak heights in a spectrum don't map directly to amounts ^[6]. HPLC, for its part, can't establish identity because it doesn't measure mass. Combine them, especially as LC-MS, and the picture fills in. HPLC vs mass spectrometry has the full comparison, and the certificate of analysis guide shows how both appear on a report.

08 Limitations of Mass Spectrometry

Mass spectrometry alone does not necessarily establish complete purity, concentration, sequence or stability. An intact mass match is one piece of identity evidence, and a full picture of a sample usually pairs it with chromatography; HPLC vs. mass spectrometry compares the two side by side.

REFScientific references

1. Aebersold R, Mann M. Mass spectrometry-based proteomics. Nature. 2003;422(6928):198-207. PubMed 12634793
   review
2. Fenn JB, Mann M, Meng CK, Wong SF, Whitehouse CM. Electrospray ionization for mass spectrometry of large biomolecules. Science. 1989;246(4926):64-71. PubMed 2675315
   methods review
3. Karas M, Hillenkamp F. Laser desorption ionization of proteins with molecular masses exceeding 10,000 daltons. Anal Chem. 1988;60(20):2299-2301. PubMed 3239801
   analytical methods paper
4. Steen H, Mann M. The ABC's (and XYZ's) of peptide sequencing. Nat Rev Mol Cell Biol. 2004;5(9):699-711. PubMed 15340378
   review
5. D'Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30. PubMed 25044089
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6. Pitt JJ. Principles and applications of liquid chromatography-mass spectrometry in clinical biochemistry. Clin Biochem Rev. 2009;30(1):19-34. PubMed 19224008
   review