Mass spectrometry proteomics

In one sentence

Mass spectrometry proteomics identifies and measures protein-derived molecules through their mass-to-charge and fragmentation signatures, with conclusions limited by sample preparation, identification and quantification rules.

The intuition

Think of recognizing a book from distinctive sentences after its pages have been cut apart. Protein-derived peptides can identify their source, but shared sentences may belong to several editions. The analogy stops at measurement: the instrument detects charged molecules and their fragments, while software evaluates possible identities and abundance.

How it works

In common bottom-up proteomics, proteins are extracted and cut into peptides, short amino-acid chains, using an enzyme. Liquid chromatography (LC) separates the peptide mixture. A mass spectrometer measures ions by mass-to-charge ratio, and tandem mass spectrometry (MS/MS) fragments selected ions to obtain additional sequence evidence.

Software compares observed patterns with candidate peptide sequences and applies identification rules. Protein inference combines peptide evidence: a peptide unique to a protein can be more informative than one shared by several proteins or forms. Bottom-up evidence does not automatically show that an intact, functional protein molecule was present.

Quantification can use relative ion intensities or labels comparing samples. A targeted assay measures selected peptides with predefined performance checks. Calibrated standards can support stated amounts, but extraction, digestion and sample-matrix effects still need validation. A peptide standard alone does not guarantee recovery of every original protein molecule.

An estimated false discovery rate (FDR) describes the expected proportion of incorrect identifications in a defined accepted set under the method's assumptions. Peptide, spectrum and protein levels require their own reporting. An acceptable set-level FDR does not guarantee every individual identification.

Why it matters in cancer

Protein measurements can test whether an RNA-level difference has a corresponding product-level signal. Integrated breast-cancer proteogenomic work demonstrates that transcript, protein and phosphorylation measurements add different information.

A bulk extract mixes malignant cells and neighbors and loses their original coordinates. Protein abundance per mass of extract cannot become surface receptors per cancer cell. Immunohistochemistry can add localization; immunopeptidomics instead studies peptides recovered from antigen-presentation complexes. Those naturally presented peptides are different evidence from peptides made by laboratory digestion.

Assay card

Common confusions

• Peptide versus intact protein: digestion removes information about complete molecular structure.
• Not detected versus absent: the molecule may be below sensitivity or outside the measured set.
• Relative ratio versus absolute amount: ratios need a denominator; amounts require calibration.

Try it

A fictional bulk assay reports twice as much receptor-derived peptide per microgram of extract. Does it show twice as many accessible receptors on each cancer cell?

Answer: No. Cell mixture, location and the quantified peptide's relation to receptor forms remain unresolved. A suitable cell-specific surface measurement addresses a different question.

Related concepts

• Phosphoproteomics: measuring a specific kind of protein modification.
• Immunopeptidomics: measuring recovered antigen-presentation peptides.

Sources

Source check: October 9, 2026. Expert and learner review remain pending. Examples are fictional.

• Carr et al., fit-for-purpose targeted peptide measurements (2014).
• Elias and Gygi, target-decoy identification confidence (2007).
• Mertins et al., breast-cancer proteogenomics (2016).
• Sprung et al., method-specific FFPE and frozen-tissue proteomic comparison (2009).

Used in

• Ask whether protein is present, placed and active
• Understand multi-omics, one question at a time