Describe the 3 main differences between MALDI and ESI and with what mass analyser they couple best
ESI (liquid phase & continuous ion stream & multiple charged ions) –> quadrupole or ion trap
MALDI (solid phase & pulsed ion packets & singly charged ions) –> TOF
Give the operational definition of ESI
Electrospray Ionisation is a soft ionisation method that converts molecules from a liquid solution into gas phase ions by generating a fine spray of charged droplets under a strong electric field
Describe the Electrospray Process until the formation of the charged droplets
- Sample introduction: liquid through fine capillary
- High voltage: electric field (2-5kV) between needle and counter-electrode
- Taylor cone formation: electrostatic force overcome surface tension, producing jet of microdroplets
- Charged droplet spray: Droplets carry excess charge and move towards MS inlet
Describe Desolvation & Coulombic explosion
Evaporation: solvent evaporates and radius decreases –> Charge buildup –> Rayleigh limit: electrostatic repulsion exceeds surface tension –> Coulombic explosion –> repeated
Describe the two main ,models of how ions emerge from the final charged droplets
- Charged Residue Model (large molecules): Solvent evaporates until only analyte is left
- Ion Evaporation Model (small molecules): Ions are ejected from droplet surface when local electric field exceeds solvation energy
Explain the 4 key parameters you can change in ESI and what they cause
- Voltage: increase = finer spray & higher charge states
- Flow: lower flow –> smaller droplets –> improved desolvation & sensitivity
- Solvent surface tension: decrease = more stable spray. Solvent chemistry governs droplet behaviour & charge formation
- Distance tip/radius: affect stability & sensitivity
Explain what nanospray is and what advantage it has
Nanospray is when a smaller needle is used. This creates smaller initial droplets –> more efficient droplet sampling + fewer droplet fission events –> more efficient & softer
Describe why we need MALDI
Some (large) biomolecules are non-volatile and thermally unstable –> unsuitable for ESI
Describe the three main steps of MALDI
- Sample preparation: Sample mixed with UV-absorbing matrix compound & co-crystallised on target plate
- Laser desorption: Nanosecond UV laser pilse excited matrix –> desorption of matrix + analyte to gas phase
- Ionisation: gas-phase proton transfer/ cationisation from matrix to analyte–> singly charged ions!
Describe the 2 roles of the matrix in MALDI
- Energy Absorber
- Ionisation Mediator
Describe the 2 main types of MALDI lasers
- Gas laser (nitrogen) –> cheap, variability
- Solid state laser –> more expensive, stable
Describe 5 Limitations of MALDI
- Matrix background at low m/z
- Less compatible with LC
- Dependent on good crystallisation
- Singly charged ions only
- Matrix ions can interfere with spectrum
Describe 5 advantages of MADLI
- Soft ionisation : minimal fragmentation
- Simple sample prep
- Broad mass range > 150kDa
- Fast, pulsed analysis
- Ideal for imaging
Explain what MALDI-MS Imaging (MSI) is and why spatial molecular information matters
MSI is a form of MALDI which is used to record one spectrum per ‘pixel’, to create molecular maps of for example a piece of tissue. It is important in tissue proteomics, clinical diagnostics and drug distribution studies. –> Spatially correlated protein analysis.
Outline the MALDI-MSI workflow
tissue –> matrix coating (at specific spots (profiling) or everywhere (imaging) –> laser raster –> spectra recorded –> image reconstruction
Describe the differences between MSI Imaging and Proflining
Imaging is the full spatial acquisition, and profiling is of specific locations/spots
Describe how MALDI-Imaging can help with biomarker discovery
MSI offers molecular specificity with spatial context to be able to identify compounds related to certain diseases (cancer etc).
