Download Ionization methods - 2-CI - Florida International University

EI: 10-6 Torr
CI: 0.1-2.0 Torr
EI vs. CI
R (excess)
(Secondary
ions)
Reactant gas
Primary ions
Short mean free paths
(~ 2 x 10-4 mm)
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 + NH4+  [M + NH4+]+
M + C2H5+  [M + C2H5+]+
M + C3H7+  [M + C3H7+]+
M+·
[M + H]+
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.
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.
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,
NH3 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
NH4+ < C4H9+ < C2H5+ < CH3+ < H3+
• The degree of fragmentation caused by these common reactant gas ions increase in the
same order.
CH3+ and H3+: For structure elucidation
NH4+ and C4H9+: For confirmation of relative molecular mass of a
more fragile molecule.
“Self-CI”
•
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
Chemical ionization
a) Proton Transfer, b) Nucleophillic Addition, c) Hydrogen Abstraction
Chemical ionization: Common Reagent Gases
Methane:
good for most organic compounds
usually produces [M+H]+, [M+CH3]+ adducts
adducts are not always abundant
extensive fragmentation
Isobutane:
usually produces [M+H]+, [M+C4H9]+ 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+NH4]+ adducts
basic compounds produce [M+H]+ adducts
non-polar and non-basic compounds are not ionized
Methane Chemical Ionization
CH4+· will also fragment (although not a major pathway):
CH4+·  CH3+ + H ·
CH4+·  CH2+· + H2
CH3+ + CH4  C2H5+ + H2
CH2+· + CH4  C2H3+ + H2 + H·
C2H3+ CH4  C3H5+ + H2
• If sample M is a saturated hydrocarbon RH
RH + CH5+  R+ + CH4 + H2 (hydrogen abstraction)
Isobutane Chemical Ionization
Isobutene Chemical Ionization
Ammonia Chemical Ionization
• NH3+· + NH3  NH4+ + NH2·
• NH4+ + NH3  (NH4 + NH3)+
• RNH2 + NH4+  RNH3+ + NH3 (for basic compounds)
• R + NH4+  RNH4+ (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+·)*
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Ar does not have vibrational degree of freedom
Excess of energy after ionization is 15.755 eV
Excess of energy will transfer to M
Excess of after M is ionized: (15.775 – 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+. + 2eM + e-  M -.
Photo–ionization
Ionization of any species by photon (hv):
M + hv  M+ + eElectrons and photons do not ‘impact’ on molecules or atoms. They interact with
them in ways that result in various electronic excitations including ionization.