Biology Reference
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sometimes called full-scan mode, and this method is useful when scanning matrices for
a broad range of possible unknown compounds. Its disadvantage is that the sensitivity
of the method will be reduced. In other cases, however, one might know exactly what
one is looking for, such as pyrethroid and pyrethrin metabolites (
Martínez Vidal and
Garrido Frenich, 2006
). In this case one would use selected-ion monitoring (SIM).
Using SIM, the analyst programs the instrument such that, at a given retention time,
the mass analyzer will scan for only a few specific ions. In this way the amount of
dwell time for each ion can be greatly increased and therefore the sensitivity of the
method is increased as well. Again there are several different types of mass analyzers to
consider.
The real workhorse in this area is the quadrupole, as it is compact, robust, and rela-
tively inexpensive. As the name implies, the basic configuration consists of four rods
that are arranged in parallel around a central channel. A fluctuating electric field is
formed in this channel by applying DC and RF voltages across opposing pairs of rods.
By modulating the currents, only ions with a particular
m/z
ratio can traverse the
channel to reach the detector at the opposite end. More recently, these quadrupole
instruments have been set up in tandem to derive much more structural information
from the samples. These instruments, generally referred to as a triple-quad or MS/MS
instruments, use a technique called collision-induced dissociation. In this case a par-
ticular ion passes through the first quadrupole, then it enters a mixing chamber where
more ions are formed through a chemical ionization technique, and then these resul-
tant ions enter a second quadrupole mass analyzer. These instruments are particularly
useful in biological applications and are very commonly paired with HPLC.
A newer design that is rapidly becoming more prevalent is the ion trap (
Ballesteros-
Gómez, 2011
). The principle of ion separation for the ion-trap instrument is simi-
lar to that for the quadrupole, but rather than taking place along a linear channel, it
occurs in an inner-tube-shaped ring with caps on the end. This circular magnet “traps”
the ions and they are then swept out by RF voltage. The advantage of the ion trap is
that it can hold the ions in place and perform the MS/MS analysis in situ, without
having a second mass analyzer. Therefore, the analyst is not constrained by MS/MS,
but rather could repeat the process to achieve MS/MS/MS/
…
/MS
x
.
A new mass
analyzer that is similar in principle to the ion trap is the Orbitrap. This mass analyzer
traps the ions around a central spindle electrode and the
m/z
values are measured from
the frequency of harmonic ion oscillations, along the axis of the electric field (
Hu
et al., 2005
).
The final method to address here is the time-of-flight (TOF) analyzer. Essentially
the TOF works on the principle that, if ions are accelerated through a fixed distance by
a high voltage, larger ions will move more slowly than smaller ions. Thus, the ions are
resolved based on the amount of time it takes them to traverse the tube. This analyzer
is often paired with the MALDI technique for the determination of large molecules.
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