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This page describes instruments and general analytical approaches; it is not medical advice.
Who uses it: proteomics and core-facility staff, lab managers specifying an instrument, and students learning how a protein sequence is read from a mass spectrometer. After reading, you should be able to name the three routes to a sequence (bottom-up, top-down, de novo), say where ion mobility fits, and list what to specify when buying an instrument or outsourcing the work.
What protein sequencing by mass spec means
Protein sequencing by mass spectrometry means reading amino acid sequence from tandem mass spectra (MS/MS): ions are selected, fragmented, and the fragment masses are interpreted as sequence. It is done on peptides (bottom-up) or on whole proteins (top-down), and the interpretation either searches a sequence database or reconstructs the sequence from the spectra alone (de novo). For the instrument itself, see what a mass spectrometer is.
Bottom-up vs top-down vs de novo
These three terms describe different things: bottom-up and top-down describe what goes into the instrument, while de novo describes how the spectra are interpreted. A top-down run can be interpreted de novo, and so can a bottom-up one.
| Route | What is analysed | What the literature says |
|---|---|---|
| Bottom-up | Peptide-level MS/MS data | Peptide-level MS/MS data, the more heavily studied setting for de novo tools, in contrast to intact-protein top-down data. |
| Top-down | Intact proteins | “Opened new horizons in the analysis of intact proteins”; retrieving a complete protein sequence from top-down MS/MS spectra “still remains a distant goal”. |
| De novo | Spectra interpreted without a sequence database | Called “the only way to determine the sequence of proteins from organisms with unknown genomes”. |
The practical reading of that table: if the organism’s genome is known, a database search is an option; if it is not, de novo interpretation is the route the source names. And if the goal is a complete sequence from intact-protein spectra, the same source describes this as still a distant goal, so a project that needs a complete sequence should plan for complementary data rather than assume one top-down run delivers it. (Source: PMC5372227, checked 2026-11-03.)
Fragmentation modes
The top-down datasets in the PMC5372227 study used CID, HCD and ETD spectra, and earlier work used the complementary CAD and ECD. When you specify an instrument or a core-facility service, these are the fragmentation modes to ask about by name, since different modes produce different fragment ion types.
How ion mobility (IMS-MS) fits in
Ion mobility spectrometry-mass spectrometry (IMS-MS, also written ion mobility spectrometry mass spectrometry) adds a separation step before or between mass analysis. Ions move through a neutral buffer gas and separate by mobility as well as m/z, so ions of the same mass and charge but different shape travel at different rates. This gives “an extra level of analytical separation” and can distinguish species of the same mass (Eldrid and Thalassinos, Biochem Soc Trans 2020;48(6):2457-2466, checked 2026-11-03).
| Ion mobility type | How it separates | Key difference |
|---|---|---|
| DTIMS (drift tube) | Constant electric field | Extended structures take longer than compact ones |
| TWIMS (travelling wave) | Travelling wave through stacked ring ion guides | Separation by a moving wave rather than a constant field |
| TIMS (trapped) | A field opposes a gas flow to trap ions | Ions are released by gradually lowering the field |
Ion mobility also yields the collision cross section (CCS), described by Eldrid and Thalassinos as “a descriptor closely related to its three-dimensional structure”, calculable from reduced mobility with the Mason-Schamp equation. The same authors note that tandem ion mobility can give “increased sensitivity and resolution” and may benefit bottom-up proteomics. Ion mobility therefore adds shape-based information to a sequencing workflow; it does not by itself replace fragmentation.
Mass spectrometer applications
Sequencing is one application of a mass spectrometer. For how instrument classes differ, see QTOF vs triple quadrupole, GC-MS vs LC-MS and choosing an ionization source. For single-molecule mass measurement without fragmentation, see mass photometry. For the enzymes used to produce peptides, see trypsin vs chymotrypsin.
What to specify when buying or outsourcing
- Which route: peptide-level (bottom-up), intact-protein (top-down), or both.
- Which fragmentation modes the instrument offers (CID, HCD, ETD, and CAD or ECD where relevant).
- Whether an ion mobility option is included, and which type (DTIMS, TWIMS or TIMS).
- Whether interpretation is database-driven or de novo, which depends on whether the organism’s genome is known.
- For a core facility, whether the group is a member of a professional body such as ABRF.
These are questions to put to a supplier or service provider.
American Society for Mass Spectrometry
The American Society for Mass Spectrometry (ASMS) was formed in 1969 “to promote and disseminate knowledge of mass spectrometry and allied topics”. Its about page (checked 2026-11-03) reports an annual conference attended by more than 6,500 scientists and over 8,500 scientists as members. It is the professional-society reference for the field.
Common mistakes
- Treating bottom-up/top-down and de novo as three alternatives; the first two describe input, the third describes interpretation.
- Expecting a complete protein sequence from a single top-down experiment; the source describes this as a distant goal.
- Assuming ion mobility replaces fragmentation; it adds a separation by shape.
FAQ
What is protein sequencing by mass spectrometry?
It is reading amino acid sequence from tandem mass spectra of peptides or intact proteins, interpreted by database search or de novo.
What is ion mobility spectrometry-mass spectrometry?
It combines a gas-phase mobility separation, based on ion shape and charge, with mass analysis, adding a separation of same-mass species.
What can de novo sequencing do that database search cannot?
It determines sequence without a database, which the PMC5372227 study calls the only way for organisms with unknown genomes.
What are the applications of a mass spectrometer?
Protein sequencing is one; the linked guides cover other instrument classes and uses.
Sources
- De Novo Sequencing of Top-Down Tandem Mass Spectra: A Next Step towards Retrieving a Complete Protein Sequence, PMC5372227, checked 2026-11-03.
- Eldrid C, Thalassinos K. Developments in tandem ion mobility mass spectrometry. Biochem Soc Trans 2020;48(6):2457-2466, PMC7752082, checked 2026-11-03.
- American Society for Mass Spectrometry, About ASMS, checked 2026-11-03.
