Electron and Chemical Ionization

Question 1

Three Methods for Making Ions

Answer 1

1. remove a charged species such as an electron or proton
2. add a charged species which as an electron, cation (H⁺) or anion
3. separate an ion pair (AB⇒A⁺ +B^- )

• removing an electron is the fastest and most simple approach

Question 2

Electron Ionization (EI)

Answer 2

• electrons are emitted from a metal of low work function such as tungsten or rhenium
• electrons are accelerated accross an electric potential (typically 70V)
• accelerated electrons remove an electron from the analyte which goes on to form a positive ion
• positive ions are pushed into the mass analyzer via a repeller which generates a positive electric field

M + e^-_accelerated→M⁺ + e^-_ejected + e^-_{scattered/accelerated}

Question 3

Electron-Molecule Interactions

Answer 3

• accelerated electrons have a velocity which can be calculated and typically interact with the molecule on the order of 100 attoseconds

v_e^- = sqrt((2*q(Coulombs)*P(V))/(mass))

Question 4

Characteristics of Electron Ionization

Answer 4

• primarily yields singly-charged ions
• may form ions with high internal energy which may lead to extensive fragmentation of the molecular ion (M⁺)
• can ionize any vaporized molecule

Question 5

Electron Ionization - the Good

Answer 5

• reproducable
• frgmentation
• nonselective
• can be interfaced with GC

Question 6

Electron Ionization - the Bad

Answer 6

• nonselective
• fragmentation
• sample has to be volatile

Question 7

Photoionization

Answer 7

• energy from a photon may be sufficient to eject an electron
• amount of energy absorbed can be controlled by controlling the laser wavelength
• low residual energy thus minimal fragmentation

Question 8

Chemical Ionization (CI)

Answer 8

• soft ionization method
• electrons from a filament are accelerated into an ion chamber filled with a reagent gas
• electrons radicalize reagent gas molecules which scavenge a proton from neighboring reagent gas molecules
• eventually the cationic reagent gas molecule transfers the proton to the analyte
• the key for selecting a reagent gas is proton affinity when compared to the analyte

Question 9

Mechanism of Chemical Ionization

Answer 9

Question 10

Proton Affinity

Answer 10

• the tendency of a molecule to accept a proton
• PA(Analyte) < PA(Reagent Gas): No analyte ionization
• PA(Analyte) > PA(Reagent Gas): Ionization of analyte
• PA(Analyte) >>> PA(Reagent Gas): Fragmentation

Question 11

Chemical Ionization - the Good

Answer 11

• molecular mass determinatino
• selective ionization
• fewer interferences from other substances

Question 12

Chemical Ionization - the Bad

Answer 12

• molecular mass may not be sufficient for identification
• efficiency of the ionization depends on reagent gas pressure, composition, and temperature and is therefore less reproducible
• not suitable for large molecules