based on mass spectrometry (MS) and proton nuclear magnetic
resonance spectroscopy (NMR) platforms. However, none of these
techniques alone can detect the entire metabolome of an organism.
NMR is a global and nondestructive technique characterized by
high-throughout, reproducible, and minimal sample pretreatment.
The main limitation of NMR is low sensitivity compared to MS
technology. Although MS can be successfully operated in flow
infusion analysis mode, it is usually linked with an additional device,
such as GC, LC, and capillary electrophoresis (CE), to increase the
resolution of analytes and lower the matrix effects (ion suppression
or enhancement when ionization in ion source). In general, the
combination of MS- and NMR-based approaches can detect more
metabolites.4.1 GC-MS Platform GC-MS is a widely used analytical platform for the identification
and quantification of volatile and semivolatile organic compounds
in complex matrix due to its high sensitivity, peak resolution, and
excellent reproducibility [57]. For the GC-MS method, the struc-
tural information of compounds can be obtained through matching
detecting data to the well-established metabolite spectrometry
database, which facilitates the identification of metabolites and
the elucidation of physiological or pathological mechanisms. The
available databases include the commercial NIST, Wiley, Fiehn
Metabolomics library, and the free Golm Metabolome Database.
For GC-MS platform, the compound identification is quite
straightforward and robust because of the extensively reproducible
fragmentation pattern across mass spectrometers in electron impact
(EI) ionization with full-scan mode. Moreover, EI ionization in
GC-MS is less prone to matrix effect due to its use of the gaseous
phase and the nature of EI ionization; therefore GC-MS in EI
ionization has been the standard reference method for quantifica-
tion of many compounds [58].
Time-of-flight (TOF) and quadrupole mass analyzers are most
frequently applied in GC-MS metabolomics. TOF mass analyzer
provides faster spectral acquisition (up to 500 Hz) and more scan
points recorded for a peak; thus TOF mass analyzer can more
accurate deconvolution of mass spectra for co-eluted analytes than
other mass analyzer. The total run time of each GC-TOFMS analy-
sis can be remarkably shorten since both chromatographic separa-
tion and spectral acquisition are faster than conventional GC-MS
analysis. Therefore, the predominant advantage of GC-TOFMS is
the higher throughout as compare to the conventional GC-MS
based on quadrupole mass analyzer [59]. Currently, comprehensive
two-dimensional gas chromatography (GCGC) is applied for
metabolomics study, which provides superior separation capacity,
chemical selectivity, and sensitivity than conventional GC for com-
plex sample analysis [60, 61]. All analytes in a complex sample can
be separated by nonpolar column and subsequently a polar column
Metabolomics: A High-Throughput Platform for Metabolite Profile Exploration 275