Agriculture Reference
In-Depth Information
6.3.2.2 QqTOF-MS Analyzer
The development of the hybrid QqTOF
-
MS analyzer was closely related to the
progress in TOF
MS. For this analyzer, Q refers to a mass-resolving quadrupole; q
refers to a radio frequency-only quadrupole or hexapole collision cell, and TOF refers
to a TOF
-
-
MS analyzer [75]. Hybrid QqTOF
-
MS instruments combine the advan-
tages of quadrupole and TOF
-
MS analyzers. In this way, this con
guration shows the
bene
ts of high sensitivity, mass resolution, and mass accuracy. Therefore, QqTOF
offers a great potential for screening analysis and con
rmation of positive samples
on the basis of acquisition of product ion full scan spectra at high resolution.
Moreover, it is possible to perform MS/MS experiments with accurate mass mea-
surements, facilitating the structural elucidation of nontarget/unknown compounds by
valuable fragmentation information. Thus, Q permits precursor ion selection, but at
low resolution, which will be fragmented in q and their fragments will be monitored in
the TOF. The measurement of accurate mass in TOF allows the assignment of the
elemental composition of a compound, whereas QqTOF also allows the establishment
of the elemental composition of all product ions obtained [79].
QqTOF
MS instruments allow the possibility of performing MS/MS acquis-
itions in several ways. As aforementioned, the simplest method is the selection of
the precursor ion in Q and its subsequent fragmentation in q; then, all product ions
can be scanned in the TOF. For this mode, it is necessary to have a prior knowledge
of the ions, usually by previous full scan MS. However, it cannot be used for
nontarget and unknown compounds and only one precursor ion can be monitored in
each acquisition function. Therefore, alternative acquisition modes have been
developed to improve this. One of them was the data-dependent acquisition
(DDA) mode, in which the MS instrument switches from full scan MS mode to
MS/MS mode when an eluting peak rises above a prede
-
ned threshold [80]. Another
alternative is the acquisition mode named MS
E
, which involves the simultaneous
acquisition of exact mass data at high and low collision energy [81]. These two types
of monitoring modes permit the acquisition of full scan and MS/MS spectra during
the analysis for all the ions generated in the ion source, increasing the qualitative
data obtained.
6.3.2.3 Orbitrap-MS Analyzer
Since its introduction in 2005, the Orbitrap
MS analyzer has proven to be a valuable
analytical tool with a range of applications in different
-
fields of chemistry such as
proteomics [82], metabolomics [83], environmental analysis [84], and food
safety [85,86]. This analyzer uses the principle of orbital trapping in electrostatic
fields [87] and it is the latest MS analyzer developed so far; in other words, it is not a
hybrid system but a totally new mass analyzer.
Orbitrap-MS consists of an inner and an outer electrode, which are shaped to
create a quadro-logarithmic electrostatic potential. Ions oscillate harmonically along
its axis (
z
-direction), with a frequency dependent on their
m
/
z
values, generating an
image current transient that is converted to a frequency spectrum using a Fourier
