Gas
Chromatography- Mass Spectrometry (GC-MS)
Thermo
Finnigan Polaris Q GC/MS. This
is an ion trap type instrument with MSn
capability, and can be used for electron impact ionisation
(EI), and positive and negative chemical ionisation (PCI
and NCI) and electron capture ionisation (ECI) with either
methane or ammonia as the reagent gas.
The
Thermo Polaris Q GC-MS permits separation of complex mixtures
into single components before ionization and mass analysis.
A variety of capillary columns with different stationary
phases are available to optimise the separation of complex
mixtures. This is particularly useful when analysing relatively
low levels of target compounds derived from complex biological
matrices. The target analyte must be relatively volatile
or must be susceptible to conversion to a volatile derivative
to permit GC separation. In general, the derivatized analyte
should have a MW of less than 1000 Da.
The
instrument may be used for quantification, typically using
an internal standard, and for identification using the MS
library or user supplied standards.
Electron
Ionisation
Electron ionization (EI) is widely used in mass spectrometry
for relatively volatile samples that are insensitive to
heat and have relatively low molecular weight. The gas phase
molecule is hit by an energetic electron (typically 70eV)
and some of the energy is transferred to the neutral molecule.
If the energy transferred is sufficient to displace an electron,
a positive radical ion is created (M+.)
and any
excess energy is then dissipated by fragmentation of covalent
bonds to lose either a neutral or a radical.
Ionisation:
M + e-
g M+.
+ 2e-
Fragmentation:
M+.
g m1+
+ n.
radical loss
M+.
g
m1+.
+ n
neutral loss
The
spectra, usually containing many fragment-ion peaks, are
useful for structural characterization and identification.
Libraries of spectra are available for searching.
Chemical
Ionisation (Positive and Negative)
Chemical Ionization (CI)
is applied to similar samples and is used to enhance the
abundance of the molecular ion (MH+
). Thermal electrons in a gas filled source react with the
ionising gas (commonly methane, isobutane or ammonia) to
generate reactive species which can ionise the neutral analyte
via one of four general pathways.
M
+ [BH]+
g
[M+H]+
+ B
proton transfer
M + B
g
[M-H]-
+ [BH]+
proton transfer
M + X+
g
[M+X]+
electrophilic
addition
M + X+
g
[M-A]+
+ AX anion
abstraction
M + X+.
g
M+.
+ X
charge
exchange
In
any CI plasma, both positive and negative ions are formed
simultaneously by either deprotonation or by anion attachment
and one can choose, by selecting the polarity of the accelerating
voltage, which ions are extracted from the ion source for
MS analysis.
CI
is known as a “soft” ionisation method as it generally results
in minimal fragmentation of the analyte molecule and little
structural information is contained in the spectrum.
Electron-Capture
Ionisation
Electron-Capture Ionisation (ECI) is used for molecules
containing highly electrophilic moieties (e.g. F, Cl, NO2,
CN, etc). These electron-capturing groups can also be added
into the target analyte by derivatisation, usually after
isolation and before mass spectrometric analysis.
M
+ e- (thermal)
g
M-.
resonance electron capture
M
+ e- (thermal)
g
[M-A]-
+ A.
dissociative
electron capture
M
+ e- (thermal)
g
[M-B]-
+ B+
+ e-
ion-pair formation
The
sensitivity of ECI analyses is generally two to three orders
of magnitude greater than that of CI or EI analyses. Little
fragmentation occurs during ECI, and this mode of ionization
is generally employed for quantification of trace amounts
of known compounds.
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