Biology Reference
In-Depth Information
Linear Ion Trap (LIT), Orbitrap, and Fourier Transform-Ion
Cyclotron Resonance (FTICR). Tandem configurations can be
classified into two groups: tandem-in-space instruments and
tandem-in-time instruments. Tandem-in-space instruments require
separate mass analyzers to be utilized for each MS stage and are
associated with beam-type technology such as quadrupole mass fil-
ters, and TOF mass analyzers. Tandem-in-time instruments per-
form separate the different MS stages by time, with the various
stages of MS/MS being performed in one mass analyzer. Hybrid
instruments that combine both in-space with in-time mass analysis
have been also built (Fig.
3
).
The Q-TOF, which results from replacing the third quadru-
pole in a triple stage quadrupole by a TOF analyzer [
52
] is one of
the most popular configurations. Targeted absolute quantitation of
peptides is a fast developing field with triple quadrupole instru-
ments being traditionally used for this type of work. Recently, AB
Sciex has launched its next generation triple quadrupole-TOF
(TripleTOF
®
) family of spectrometers. The new TripleTOF
®
5600
+
System is a high sensitivity and high-resolution mass spectrometer
for simultaneous high-performance qualitative and quantitative
analysis. It delivers new innovation for biomolecule research with
MS/MS
ALL
with SWATH™ acquisition [
53
]. The TOF spectrom-
eter of hybrid systems is usually equipped with a reflectron. The
function of the reflectron, an electrostatic mirror, is to compensate
for small differences in the velocities of ions with the same
m
/
z
,
increasing thereby the resolution of TOF MS [
54
]. The TOF-TOF
tandem mass spectrometer was introduced in 1993 [
55
]. It con-
tained two dual-stage reflectron analyzers and a collision region for
producing product ions by collision-induced dissociation. Current
TOF-TOF instruments incorporate delayed extraction of ions
from the ion source towards the flight tube by ns-μs in order to
allow ion equilibration in the plasma produced during the desorp-
tion/ionization, thereby improving mass resolution [
56
]. A crucial
aspect of the MS/MS experiment is the unimolecular ion dissocia-
tion that occurs between the two MS stages. High-energy CID of
singly charged (M + H
+
) peptide ions occurring in TOF-TOF
instruments generates incomplete series of sequence-specific
daughter ions, challenging
de novo
structure elucidation. However,
combined peptide mass fingerprinting (in TOF1) and partial MS/
MS spectra of selected peptide ions detected (in TOF2) represents
a high confident approach for identifying protein sequences pres-
ent in the searched databases [
57
]. Automated high-throughput
TOF-TOF MS analysis and database search enables a time- and
cost-effective protein identification strategy where genome
sequence of the particular organism under study is available.
Tandem and multiple-stage MS (MS/MS and MS
n
, respec-
tively) can also be achieved by the use of additional sequence of
operations in the scan function of ion traps. However, a limitation
