EI: Torr CI: Torr

EI vs. CI Primary ions Short mean free paths (~ 2 x mm) R (excess) Reactant gas (Secondary ions) Generally ions with an even number of electrons (cations, RH + ) are more stable than ions with an odd number of electrons (radical cations) like A+· primary ions.

The secondary ions are strong Lewis acid, therefore donate a proton. Less usually, a hydrogen ion is abstracted from M.

Adduct ions Depending on the acidity of the reactant gas and the basicity of the sample molecules, hydrogen transfer may not occur and instead the secondary ions react with molecules (M) to give adduct ions, rather than protonated molecules, M + NH 4 +  [M + NH 4 + ] + M + C 2 H 5 +  [M + C 2 H 5 + ] + M + C 3 H 7 +  [M + C 3 H 7 + ] +

In general, quasi-molecule ions obtained by chemical ionization have greater relative Abundances than molecular ions obtained by EI. [M + H] + is more stable than M+· because the former is produced with a little excess of internal energy and because it is Even-electron species, unlike the radical cations (M+·) formed by EI. [M + H] + M+·M+· After chemical ionization, C-C bonds tend to cleave only if the product of the dissociation are particularly stable. Often, the carbon skeleton remains intact and cleavage is restricted the bonds of functional groups, such as C-O, C-S, and C-N bonds.

Chemical Ionization and Reagent gases Degree of fragmentation can be controlled by changing reagent gases, because –The amount of excess of energy imparted on an [M + H]+ ion on its formation depends on the relative affinities of the conjugate base of the reactant ion (CH 4, NH 3 and so on) and the compound M. Decrease in PA (proton affinity) of the conjugate base (or increase in acidity of the reactant gas ion) causes increase in fragmentation because more energy is transferred to them during their formation. Acidity increase in the order NH 4 + < C 4 H 9 + < C 2 H 5 + < CH 3 + < H 3 + The degree of fragmentation caused by these common reactant gas ions increase in the same order. CH 3 + and H 3 + : For structure elucidation NH 4 + and C 4 H 9 + : For confirmation of relative molecular mass of a more fragile molecule.

In EI, [M + H] + ion can be formed if the sample pressure becomes too high in the source M +· + M  [M + H] + + [M –H] · This process is frequently observed when 1)the normal abundance of molecular ions M +·, in EI spectra is low. 2)the molecule of interest contains a site of high PA (like amino acid). 3)Self-CI can be reduced or eliminated by analyzing a smaller amount of sample “Self-CI”

a) Proton Transfer, b) Nucleophillic Addition, c) Hydrogen Abstraction Chemical ionization

Methane: good for most organic compounds usually produces [M+H] +, [M+CH 3 ] + adducts adducts are not always abundant extensive fragmentation Isobutane: usually produces [M+H] +, [M+C 4 H 9 ]+ adducts and some fragmentation adducts are relatively more abundant than for methane CI not as universal as methane Ammonia: fragmentation virtually absent polar compounds produce [M+NH 4 ] + adducts basic compounds produce [M+H] + adducts non-polar and non-basic compounds are not ionized Chemical ionization: Common Reagent Gases

Methane Chemical Ionization CH 4 + · will also fragment (although not a major pathway): CH 4 + ·  CH H · CH 4 + ·  CH 2 + · + H 2

CH CH 4  C 2 H H 2 CH 2 + · + CH 4  C 2 H H 2 + H· C 2 H 3 + CH 4  C 3 H H 2

If sample M is a saturated hydrocarbon RH RH + CH 5 +  R + + CH 4 + H 2 (hydrogen abstraction)

Isobutane Chemical Ionization

Isobutene Chemical Ionization

NH 3 + · + NH 3  NH NH 2 · NH NH 3  (NH 4 + NH 3 ) + RNH 2 + NH 4 +  RNH NH 3 (for basic compounds) R + NH 4 +  RNH 4 + (no or little basic character) No efficient ionization for saturated hydrocarbons. Ammonia Chemical Ionization

Charge Transfer (exchange) Chemical Ionization Reactant gas ions are formed from monoatomic species like argon Ar + e  Ar + · Ar + · + M  Ar + (M + ·)* Ar does not have vibrational degree of freedom Excess of energy after ionization is eV Excess of energy will transfer to M Excess of after M is ionized: ( – I) eV I is ionization energy of M M has vibrational degree of freedom and will fragment Knowledge of the precise amount of excess of energy given to M Sometimes can selectively ionize a mixture of compounds.

Chemical ionization: Negative Ions

Ionization Nomenclature Electron attachment A resonance process whereby an electron is incorporated into an atomic or molecular orbital of an atom or molecule. A + e -  A - Charge–exchange (charge transfer) ionization Occurs when an ion/atom or ion/molecule reaction takes place, in which the charge on the ion is transferred to the neutral species without dissociation of either. A + + B  A + B + Dissociative charge transfer Occurs when an ion/molecule reaction takes place, in which the charge on the ion is transferred to the neutral species. The new ion then dissociates to one or more fragment ions. A + + B  A + B +  A + (F1 +, F2 +,....Fn + ) Ion–pair formation An ionization process in which a positive fragment ion and a negative fragment ion are the only products. A B  A + + B -

Ionization Nomenclature Electron ionization (Electron Impact) Ionization of any species by electrons. The process may be written: M + e -  M e - M + e -  M -. Photo–ionization Ionization of any species by photon (hv): M + hv  M + + e - Electrons and photons do not ‘impact’ on molecules or atoms. They interact with them in ways that result in various electronic excitations including ionization.