Biology Reference
In-Depth Information
as ETD (electron transfer dissociation) [
70
,
71
]. This hybrid system
retains the high sensitivity, high charge capacity, accurate control of
ions, and short cycle time of the linear trap, and the high-resolution
measurements of the Orbitrap. The LTQ-Orbitrap system has also
the capability of performing data-dependent scanning for precursor
fragmentation studies. The Orbitrap analyzer is used to scan the
precursor species and the LTQ mass analyzer is set to pick them,
isolate them within the linear C-trap and fragment them. Product
ions can then either be detected using the C-trap or the Orbitrap.
A variant of the ESI-LTQ-Orbitrap is the MALDI-LTQ-Orbitrap,
which couples a vacuum MALDI source to a hybrid LTQ Orbitrap
and is particularly suited for Peptide Mass Fingerprint and Tissue
Imaging applications [
72
]. Other Orbitrap configurations include
the Q Exactive (hyperbolic quadrupole mass filter-Orbitrap) mass
spectrometer (140,000 resolution at
m
/
z
200 and <1 ppm mass
accuracy) [
73
]. This system enables new quantitative methods
based on high resolution and accurate mass measurements, includ-
ing targeted analysis in MS mode (single ion monitoring, SIM) and
in MS/MS mode (parallel reaction monitoring, PRM) [
74
]. The
LTQ Velos™ family [
75
] (Velos™, Velos Pro™, Orbitrap Elite™)
was designed to improve sensitivity and scan speed: the first ion trap
efficiently captures and fragments ions at relatively high pressure
whereas the second ion trap realizes extremely fast scan speeds at
reduced pressure.
Drift-time ion mobility mass spectrometry (DT-IMMS)
developed in the 1950s and 1960s by Earl W. McDaniel at the
Georgia Institute of Technology. With the help of mechanical
engineering student, Dan Albritton, McDaniel constructed in
1964 a “drift tube” that revolutionized the field of ion transport.
The paper reporting this achievement [
76
] was chosen as one of
the top 100 papers ever published in the reputed journal Physical
Review. Drift-time IMMS measures the time that an ion forced to
drift in a gas cell takes to migrate through the buffer gas (at a
pressure of up to about 0.7 Torr) in the presence of a low electric
field [
77
-
79
]. Under low-field conditions ion mobility can be
thought of as “directed diffusion”, and the velocity of the ion is
directly proportional to the electric field. The ion mobility con-
stant (
K
) is related to the ion's collision cross-section by:
(
)
12
/
12
/
mM
mM
+
=
3
16
q
N T
2
π
1
K
,
Ω
where
q
is the charge of the ion,
N
is the number density of the
buffer gas,
k
is the Boltzmann's constant,
T
is the absolute
temperature,
m
is the mass of the buffer gas,
M
is the mass of the
ion, and Ω is the collision cross-section of the ion [
80
]. Drift-
time IMMS can be used to separate isobaric ions differing in
