Biology Reference
In-Depth Information
instrument is another hybrid instrument, which combines a linear
ion trap with a Fourier Transform Ion Cyclotron Resonance
(FT-ICR) MS detector [
62
]. This configuration provides rapid MS
and MS/MS analyses, similar to the “data-dependent scanning”
found on standard 3-D Paul traps, but with substantially improved
internal scan dynamic range, mass measurement accuracy, mass
resolution, and detection limits [
62
].
The new millennium also witnessed the introduction of the
Orbitrap™ mass analyzer [
63
]. The roots of the Orbitrap stem
from the principle of orbital trapping in an electrostatic field
defined by Kingdon in 1923 [
64
], and modified in 1981 by Knight,
who introduced a modified outer electrode that included an axial
quadrupole term that confines the ions on the trap axis [
65
]. The
Orbitrap developed by Alexander Makarov in 1996-1997 is an ion
trap that only uses an electrostatic field to trap ions radially around
a central spindle electrode [
63
]. An outer electrode confines ions
axially. Mass-to-charge values are measured from the frequency of
harmonic ion oscillations along the
z
-axis of the electric field. The
back and forth movement of ions around the central electrode is
described by a simple harmonic oscillator with angular frequency
of axial oscillations (ω, in rad/s) = [(
q
/
m
)
k
]
1/2
, being
k
the field
constant [
63
]. The generated image current is amplified and pro-
cessed by fast Fourier transforms to obtain the mass spectra, result-
ing in comparable sensitivity and resolution than FT-ICR MS [
66
,
67
].
In the Orbitrap, the mass resolving power,
m
/Δ
m
= (1/2Δω)
(
kq
/
m
)
1/2
, increases linearly by increasing the magnetic field.
However, since the resolving power also depends on the time-
domain data acquisition period, in LC-MS experiments the rele-
vant parameter is the resolution achievable over the limited time of
the chromatographic separation (
m
/Δ
m
of around 120,000 (at
m
/
z
400) has been demonstrated at 1 scan/s) [
67
].
A shortcoming of the Orbitrap analyzer is that when an ion
decays under the dynamic trapping, its fragments will have the same
velocity as the precursor ion. As their energy is proportional to their
individual mass-to-charge ratios, the trajectories of daughter ions
with
m
/
q
typically below 30-50 % of that of the precursor ion
become highly elliptical and eventually these ions are lost. In addi-
tion, mass analyzers associated with ion-trapping technology, such
as quadrupolar (3D) ion traps, FT-ICR mass analyzer, and Orbitrap,
are unable to perform precursor ion or neutral loss scans; for these
screenings beam-type instruments have a definite advantage. The
challenge of performing MS/MS and data-dependent scans were
main reasons behind the concept of using the Orbitrap as an accu-
rate-mass detector for another mass analyzer. The first hybrid sys-
tem, the LTQ-Orbitrap™ [
68
], was commercialized in 2005. The
ion storage device linking the linear ion trap to the Orbitrap ana-
lyzer is called the C-trap [
69
], which has a high space charge capac-
ity and can also be used for additional fragmentation methods such
