Agriculture Reference
In-Depth Information
There are various types of mass analyzers available for the detection of ions in GC.
For fast GC, however, elution of narrow chromatographic peaks dictates the
requirements for mass spectrometers. In particular, it is necessary to collect mass
spectra at high acquisition rates. Also, for the detection of analytes in (ultra)trace
analysis, high sensitivity is required. To achieve these requirements, a quadrupole MS
is typically operated in selected ion monitoring (SIM) mode, and triple quadrupole
instruments are operated in multiple reaction monitoring (MRM) mode to achieve
high sensitivity (SIM and MRM) as well as selectivity (MRM). The increase in
selectivity in MRM mode is achieved through monitoring one or more characteristic
product ions formed within collision-induced dissociation of the parent ion. While
there is some probability that several analytes/matrix components will form ions with
the same nominal mass, it is much less probable that two compounds will produce
identical fragmentation pattern. In general, the use of these mass spectrometers limits
not only the number of ions/transitions that can be monitored for each analyte but also
the total number of analytes that can be analyzed to obtain acceptable detectability in
(ultra)trace analysis. In contrast to these scanning instruments, a time-of-
ight MS
(TOFMS) allows acquisition of full mass spectra without the loss of sensitivity [11].
Currently, three types of TOFMS instruments differing in their basic characteristics
are available: (i) high-resolution/accurate mass analyzers (7000 full width at half
maximum (FWHM) providing only moderate acquisition speed (up to 20 spectra/s),
(ii) unit-resolution instruments that feature high acquisition speeds (up to 500 spectra/s),
and (iii) high-speed high-resolution/accurate mass analyzers permitting high
acquisition speeds (up to 200 spectra/s) as well as high mass resolving power
(50,000 FWHM).
Regarding ionization,
MS, electron ionization (EI) and chemical
ionization (CI) represent the fundamental ionization techniques. On the basis of
the scienti
in GC
-
c literature abstracted in SciFinder Scholar, EI was used in
95% of all food GC
-
MS applications, while the remaining applications (5%)
employed CI.
EI is preferred not only for con
rmation of target component identity through
consistent
cation of unknowns and
determination of molecular structure [12]. Unfortunately, EI fragmentation can
be too extensive, leaving little or no trace of a molecular ion, which makes the
determination of the molecular weight dif
ion abundance ratios but also for identi
cult or impossible. Use of low energy or
ionization techniques such as positive chemical ionization (PCI) can enhance
the detection of molecular ion-based species. Because little or no fragmentation
occurs during PCI, this ionization technique is less suitable for con
soft
rmation.
However, this is useful in some analyses because the ion corresponding to the
molecular species (e.g., protonated molecule, adduct with reagent gas)
is
more intense and speci
c than lower mass fragment ions [13]. For a limited
number of compounds, such as analytes containing a halogen atom, a nitro group,
or an extended aromatic ring system, negative chemical ionization (NCI) can
provide signi
cant improvement in sensitivity (2 orders of magnitude or even
greater) and selectivity compared to EI and PCI because only a limited number of
analytes are prone to ef
cient electron capture during NCI [11].
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