Mass spectrometry: MALDI-TOF
MALDI-TOF uses a pulsed laser to desorb and ionise molecules mixed with a UV-absorbing matrix. The ions are then separated according to their flight time through a field-free drift tube. The technique produces minimal fragmentation and, for many sample types, requires little or no sample clean-up.
Introduction
MALDI-TOF identifies molecules by measuring their mass. The sample is mixed with a special matrix and placed on a target plate. A laser pulse causes the molecules to become charged and enter the gas phase without significant damage. These ions then travel through a tube, with lighter ions reaching the detector before heavier ones. By measuring the flight time, the instrument calculates the molecular weight of the molecules in the sample.
MALDI-TOF is particularly useful for analysing large molecules such as proteins and is widely used for rapid identification of microorganisms in clinical laboratories.
MALDI-TOF MS is a soft ionisation technique in which a pulsed laser desorbs and ionises analytes co-crystallised with a UV-absorbing matrix. The resulting ions are accelerated through a fixed electric field and separated according to their mass-to-charge ratio during transit through a field-free flight tube, with mass determined from ion flight time.
Both linear and reflectron TOF configurations are used, with reflectron mode providing enhanced mass accuracy and resolution through ion path correction. Advanced TOF/TOF systems enable tandem MS/MS for peptide sequencing and structural characterisation, while MALDI imaging supports spatially resolved molecular analysis of tissues and surfaces.
In MALDI-TOF, ions generated by laser desorption are accelerated to a defined kinetic energy and separated by their flight time through a field-free drift region. Since flight time scales with √(m/z), the mass-to-charge ratio can be calculated directly from the detector arrival time.
Mass resolution is limited by the spread in initial ion energies and velocities produced during desorption. Reflectron TOF systems compensate for this energy spread using an electrostatic mirror, improving peak focusing, mass accuracy and resolution. Delayed extraction further enhances resolution by reducing the effects of initial velocity dispersion.
MALDI-TOF uses a UV-absorbing matrix, typically compounds such as sinapinic acid, α-cyano-4-hydroxycinnamic acid (CHCA) or 2,5-dihydroxybenzoic acid (DHB), to enable efficient ionisation with minimal fragmentation. The matrix co-crystallises with the analyte, absorbs laser energy and promotes ion formation, usually producing singly charged ions through proton transfer.
This soft ionisation process preserves intact molecular ions, making MALDI-TOF well suited to the analysis of biomolecules, synthetic polymers and other fragile compounds that may fragment under harder ionisation techniques.
MALDI-TOF enables the analysis of large biomolecules, including proteins, peptides and nucleic acids, while keeping them largely intact during ionisation. This makes it a valuable tool in proteomics for protein identification and characterisation.
In clinical microbiology, MALDI-TOF identifies bacteria and fungi by generating characteristic protein mass spectra and comparing them with reference databases. This provides species-level identification within minutes and has largely replaced slower biochemical methods in many laboratories.
Key features
Areas of use
MALDI-TOF is used any time someone needs to quickly find out what large molecules are present in a sample, without breaking them apart first:
- Identifying which species of bacteria or fungus is causing an infection
- Confirming the identity and purity of a protein after purification
- Checking the molecular weight distribution of a plastic or polymer
- Screening for counterfeit or contaminated pharmaceutical products
- Mapping where specific molecules sit within a tissue sample
- Clinical Microbiology: Whole-cell profiling for rapid bacterial and fungal species identification from culture isolates, replacing slower biochemical panels.
- Proteomics: Peptide mass fingerprinting, protein identification from gel digests, and MS/MS sequencing of unknown peptides.
- Polymer Science: Determination of molecular weight distribution, end-group analysis and copolymer composition.
- Pharmaceutical QC: Identity confirmation, impurity profiling and counterfeit detection for small-molecule and biologic products.
- MALDI Imaging: Spatially resolved lipid, peptide and drug distribution mapping across tissue sections.
- Time-of-flight calibration: Mass calibration using known standards across linear and reflectron flight paths.
- Ion optics development: Characterisation of ion source and reflectron geometries for resolution optimisation.
- Delayed extraction studies: Reduction of initial velocity spread effects on peak width and mass accuracy.
- Detector response analysis: Evaluation of microchannel plate detector linearity and dynamic range across mass ranges.
- Matrix optimisation: Selection and co-crystallisation conditions for different analyte classes to maximise ionisation efficiency.
- Synthetic polymer characterisation: Molecular weight, dispersity and end-group determination for polymer chemistry.
- Small-molecule analysis: Reaction monitoring and compound identification where soft ionisation preserves molecular ion integrity.
- Surface and materials chemistry: Direct analysis of surface-bound species and self-assembled monolayers.
- Microbial identification: Species-level bacterial and fungal ID from whole-cell spectra matched against reference libraries.
- Protein characterisation: Intact mass determination, peptide mapping and post-translational modification screening.
- Tissue imaging: Spatial mapping of peptides, lipids and metabolites directly onto tissue morphology.
- Biomarker discovery: Comparative profiling of clinical samples for disease-associated molecular signatures.
Application areas
Deep reading
- Fundamentals of MALDI-ToF-MS Analysis —foundational reading
- MALDI-TOF Mass Spectrometry for Microbial Infection Control in Healthcare Settings
- Use of MALDI-TOF in Clinical Microbiology
- Applications of MALDI-TOF-MS in structural characterization of synthetic polymers
- MALDI-TOF Mass Spectrometry for Microbial Identification and Diagnosis
- MALDI-TOF MS in conjunction with machine learning