11.2.4 Tandem Mass Spectrometry
Tandem mass spectrometry employs two stages of mass analysis, one to
preselect an ion and the second to analyze the product ions produced most
commonly by collisional activation with an inert gas like argon, helium, or
nitrogen. This dual analysis can be tandem-in-space or tandem-in-time (de
Hoffmann, 1996). Tandem-in-space instruments are constructed with two
physically separated mass analyzers. Representative tandem-in-space instru-
ments include two-sector instruments comprised of a magnetic sector and an
electric sector, triple quadrupole instruments, or hybrid tandem instruments
(Q-TOF, Qq-LIT, LIT–FTICRMS, LIT–FT-OrbitrapMS, etc). Tandem-in-
time MS/MS can be achieved on trapping devices such as FTICRMS and ion
traps. These devices are capable of storing ions within the device. Selection
of an ion of interest is achieved by ejecting all the other ions. The selected ion
is then excited and fragmented during a selected time frame and the product
ions can then be mass analyzed. This process can be repeated for product
ions; therefore, this type of instrument is capable of performing MSn
experiments.
Four scan functions can be performed in a tandem mass spectrometry
experiment (Schwartz et al., 1990). These are product ion scan, precursor ion scan
(PIS), constant neutral loss scan (CNLS), and selected/multiple reaction
monitoring (SRM/MRM). In product ion scan, an ion of a givenm/zvalue is
selected, activated, and fragmented. The product ions are then analyzed to
provide structural information. This scan function is available for both types of
tandem mass spectrometers. In precursor ion scan mode, the second mass
analyzer is set to only pass the ions with a selectedm/zvalue, while the first mass
analyzer is scanned over a defined m/z range. This type of experiment is
particularly useful for monitoring groups of compounds that fragment to
produce common fragment ions. In CNLS experiments, both mass analyzers are
scanned while them/zdifference between the mass analyzers is kept constant.
Since precursor ion scan and neutral loss scan require two distinct mass analyzers,
these two scan functions are only possible with tandem-in-space type instruments.
In selected/multiple reaction monitoring, both of the analyzers are static as ions
of interest are selected and transmitted through the first mass analyzer, while the
fragment ions of interest are selected by the second mass analyzer. Liquid
chromatography/tandem mass spectrometry (LC/MS/MS) is by far the most
powerful technique for metabolite profiling and identification (Clarke et al., 2001;
Kostiainen et al., 2003; Ma et al., 2006; Nassar and Talaat, 2004).
11.3 Metabolite Identification––Role of LC/MS
11.3.1 Metabolite Characterization in Drug Discovery
The utilization of LC/MS for metabolite characterization in drug discovery is
vital and extensive. Different laboratories use the technology differently
METABOLITE IDENTIFICATION––ROLE OF LC/MS 329