Ion source Flashcards
What two things are important in MS?
Mass and charge
What was MS first conducted in?
A cathode ray tube
What is a mass spectrometer?
An instrument that defines the covalent structures of substances by ionising, separating and detecting molecular and fragment ions according to their mass-to-charge ratios
What are the 3 different ionisation methods?
Electron impact, eletrospray, matrix-assisted laser desorption
What is electron impact ionisation?
Hard ionisation. Small molecules: 1- 1000 Da
What is electrospray ionisation?
Soft ionisation. Proteins, oligosaccharises, peptides: >500,000 Da
What is MALDI?
Soft ionisation. Proteins, DNA, peptides
What are molecular ions?
Radicals
What are the lasers used for MALDI?
Nitrogen gas UV laser and solid state UV laser
Common MALDI matrices
alpha-cyano-4-hydroxycinnamin (alphaCHCA) for peptides + proteins, and 2,5-dihydroxybenzoic acid (2,5-DHB) for carbohydrates
What is delayed extraction?
Pulse of ions kept in source for short time after laser pulse to allow time for ions deep in matrix to emerge and catch up with those formed near the surface
What is the point of delayed extraction?
In ensures ions have the same velocity when entering the analyser
What happens when delayed extraction is not used?
Ions of the same mass coming from the target have diff speeds due to uneven energy distribution when ions formed by laser pulse (depends on where in the crystal the ion is formed)
What aids ES?
Drying gas (nitrogen) that flows around the outside of the capillary
What happens when the solvent evaporates in ES
The droplet becomes unstable due to high surface charge and explodes into smaller droplets - eventually naked ions
