mass spec - ionisation

Question 1

ionisation

Answer 1

process of producing an ion from your analyte (M)

either a molecular ion M+’ o an adduct ion [M+X]+ (where X = cation)

Question 2

Electron ionisation (EI)

Answer 2

gaseous molecules interact with an energised electron beam leading to ionisation
stream of electrons are formed through thermionic emission by heating a tungsten fire filament
electrons attracted across source to positive electron trap (anode)
off-axis magnet makes electron beam spiral to increase the interactions between the electron and vaporised analyte and thus improve ionisation efficiency
electron beam interacts with gaseous analyte molecules leading to electronic disruption and ultimately electron ejection
at 70 eV the de Broglie wavelength of energised electrons matches the typical covalent bond length - maximises ionisation efficiency

Question 3

EI mechanisms

Answer 3

Several competing processes
Fragment ion formation, molecular ion formation and secondary fragmentation
Molecules lose an electron to form the molecular ion (M+’)
Spontaneous (primary) fragmentation occurs for labile ions
ions possess considerable energy after ionisation (HARD IONISATION TECHNIQUE) leading to secondary fragmentation
If thermally unstable will thermal fragment before reaching gas stage

Question 4

Chemical ionisation (CI) - schematic

Answer 4

gaseous analyte molecules interact with an ionised reagent gas leading to ionisation

Question 5

CI - mechanisms

Answer 5

complex series of reactions leading to analyte ionisation
e.g. methane
mechanism produced by EI of methane followed by proton transfer
Protonated analyte [M+H]+ produced by proton transfer with mechanism
side reaction can lead to adduct formation - [M+29] in spectrum
less residual energy (SOFT IONISATION) and less fragmentation

Question 6

Electronspray ionisation (ESI)

Answer 6

ionising thermally unstable and involatile analytes. analyte solution sprayed through a capillary
potential difference creates charged droplet
droplets are dried by a heated counter-current flow of N2 gas
charge passed to analyse as solvent evaporates
a spray of charged droplets is formed
solvent evaporation increases charge density on surface of droplets
coulombic explosion occurs when repulsion of like charges overcomes surface tension of the solvent droplets
results in ion formation
low energy ionisation with little residual energy and fragmentation (SOFT)
ions can be multiply charged
spectra dominated by cat ionised species
negative ions possible