[62]. GCGC-TOFMS or recently tested GCGC-qMS has
been applied to metabolomics studies for different types of samples
(cell, biofluid, tissue, bacteria, yeast, plant) [62, 63].
GC-MS is a highly attractive metabolomics tool due to the
inherent advantages: (1) the technology is easy to use and low
cost, (2) has excellent chromatographic resolution power, and
(3) has readily available mass spectral databases independent of
instrument manufacturers [64]. However, GC-MS is limited to
the analysis of small volatile molecules, and molecules can be
volatilized [65].4.2 LC-MS Platform LC coupled with MS is a powerful platform for metabolomics
study, which can be tailored for targeted or untargeted strategies.
This platform is attributed to the remarkable development of
ultrahigh-performance liquid chromatography (UPLC or
UHPLC) based on sub-2μm size of packing particles and high-
resolution mass spectrometer. The UPLC or UHPLC significantly
increase chromatographic resolution and lead to narrow peak width
[39], which can detect more metabolites at less time compared with
conventional HPLC [66]. Mostly published LC-MS-based untar-
geted metabolomics studies used reverse phase (RP, e.g., C18)
silica-based columns with less than 2μm particle sizes so as to
obtain high resolution, sensitivity, and throughout. In general,
RP stationary phase is biased in favor of the separation of nonpolar
and polar metabolites rather than highly polar metabolites (e.g.,
glucose 6-phosphate and ATP). These highly polar metabolites are
usually eluted without chromatographic retention, and the repro-
ducibility of quantification is poor due to the matrix effects. Nor-
mal phase (NP) and hydrophilic interaction chromatography
(HILIC) are the complementary separation technologies to
RP. Recently, HILIC has been preferable technology in metabolo-
mics studies for the separation of highly polar metabolites because
it uses the same mobile phase and salt modifiers as RP chromatog-
raphy [67, 68]. In general, the application of RP and HILIC
technology together improves the chromatographic separation of
different metabolites, which is vital to the qualification and quanti-
fication of metabolomics.
High-resolution mass spectrometry (HRMS, typically> 5000
full width at half maximum) is a prerequisite in untargeted meta-
bolomics for unknown metabolite identification. The application of
HRMS will significantly increase the accuracy of mass-to-charge
(m/z) as well as reduce the number of unknown molecule candi-
dates. Time-of-flight (TOF), Fourier transform ion cyclotron
(FTICR), and Orbitrap are the most commonly used HRMS ana-
lyzers. The application of hybrid mass spectrometers such as
Q-TOF, FTICR, and Q-Orbitrap further improves the accuracy
of identification of metabolites through integrating the profile of
mass fragments. The narrow peak width by U(H)PLC requires
276 Jing Cheng et al.