faster acquisition rates or scanning speed. In general, the mass
resolutions of FTICR and Orbitrap mass spectrometers are signifi-
cantly higher than that of Q-TOF mass spectrometer at long scan
time. It is reported that mass resolutions of FTICR and Orbitrap
mass spectrometers are seriously decreased if faster scan speed is
adopted. Therefore, it is important to coordinate the chro-
matographic gradient of U(H)PLC and the scan speed of Orbitrap
or FTICR mass spectrometers. Electrospray ionization is the popu-
lar ionization mode in LC-MS, which include the positive and
negative modes. It is recommended that both modes should be
performed to ensure a wide panel of metabolites in a sample. For
targeted metabolomics, LC usually is hyphenated to tandem mass
spectrometers, such as triple quadrupole (QqQ) or hybrid triple
quadrupole/linear ion trap (QTrap), which has high sensitivity and
specificity of metabolite qualification. The multiple reaction moni-
toring (MRM) mode is mostly applied, where the precursor ion
filtered in the first quadrupole is fragmented in the collision cell of
the second quadrupole, and subsequently the characteristic frag-
ments (product ions) are further selected in the third quadrupole.
Compared to UPLC-Q-TOF-MS and UPLC-Q-Orbitrap-MS,
LC-MS/MS can more effectively detect the low abundance
metabolites.
LC-MS-based platform can detect more metabolites than
GC-MS platform. However, the inherent matrix effects in ESI
mode are the major limitation of LC-MS for the quantification of
co-eluted metabolites [39, 64, 69]. Recently, submetabolome is
introduced to improve quantitative metabolomics, where QC sam-
ple derivatized by^13 C-labeled chemical reagents is mixed with the
samples derivatized by^12 C-labeled chemical reagents to eliminate
the disturbances of matrix effects [44, 47, 48].4.3 NMR-Based
Detection Platforms
NMR technique is based on the energy absorption and reemission
of the atom nuclei due to variations in an external magnetic field
[70]. High-resolution NMR spectroscopy is a quantitative and
nondestructive technique. The increase in field strength will tre-
mendously improve the resolution and sensitivity, and more meta-
bolites are observed. Generally, 500 or 600 MHz NMR
instruments are used in metabolomics studies. Additionally, the
introduction of cryo-cooled NMR probes reduces the level of
thermal noise, and a four- or fivefold increase of the signal-to-
noise (S/N) ratio is achieved as compared to a room temperature
probe, which can detect metabolites with micromolar (μM)
concentration [71].
High-resolution^1 H NMR spectroscopy is a powerful technol-
ogy of metabolite detection, which can present comprehensive
metabolic profile within a relatively short time scale. However, for
the^1 H NMR spectroscopy, the dominant signal of the water pro-
tons could cover signals of low abundant metabolites and hinderMetabolomics: A High-Throughput Platform for Metabolite Profile Exploration 277