Page 265 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN009N-406 July 18, 2001 23:32

Mass Spectrometry 153

components of the mass spectrometer that serve as its ion
optics are linked to a set value for V . Changing V to carry
out a mass scan will work, but results in a variation in
ion transmission known as the defocusing effect. Instead,
B is changed in the electromagnet as the current passing
through the magnet is changed. Typically, the magnetic
mass analyzer is scanned from a higher mass (higher cur-
rent) to a lower mass (lower current) to allow for maximal FIGURE 2 Schematic for a TOF mass analyzer.
heat dissipation. Hysteresis is reduced by allowing a settle
time at the beginning of each scan. The reluctance of the a quadrupole or an ion trap mass analyzer, or a MALDI
electromagnetisalimitingfactorinhow B canbechanged. TOF instrument are usually the first instrumental choice.
Additionally, the ions require a finite amount of time to
pass through the sector, and the magnetic field must not
2. TOF Mass Analyzers
change during ion transit such that the ion no longer passes
through to the ion detector. Scanning speed therefore is an The time-of-flight mass analyzer is a racetrack for ions
interconnected balance of magnetic field strength, magnet (Fig. 2). Ions are separated based on velocity differences,
size, accelerating potential, and associated ion optics. and therefore differences in the time required to tra-
As shown in the derivation above, a magnetic sector verse the length (L)ofthe flight tube from the ionization
alone is a dispersive device based on the momentum of source to the ion detector. Ions are accelerated from the
the ions. Proper mass analysis with the magnetic sector source through a potential difference that gives them all
requires that all ions leaving the source be provided with the same kinetic energy. But since the ions have differ-
precisely the same kinetic energy. The accelerating po- ent masses, they will have different velocities. Light ions
tential of the source can be regulated to a high precision. travel faster; heavier ions maintain a more leisurely pace.
However, the ions are formed from neutral molecules in Solution of the equations show that time through the flight
the gas phase, and these have a range of kinetic energies time (t) is related to ion mass: t = L(m/2zV ) 1/2 . A short
that is carried through their ionization, and then manifest pulse of ions enter the flight tube at the same time, and then
as a range of kinetic energies after acceleration through disperse along the path of ion movement over time. Mea-
the source potential. An electric sector (an older term is suring the time of arrival of the ions determines their mass.
electrostatic sector) is conjoined with the magnetic sector The TOF mass analyzer is based on an instrumental de-
in a double focusing mass spectrometer. The electric sec- velopment that first came to fruition in the 1950s, but then
tor consists of two parallel curved plates of specific radius. experienced a renaissance in the 1990s, bolstered both by
In the energy focusing achieved by the electric sector, an new technology (the ability to measure ion arrival in pro-
ion beam (for simplicity, consider all ions to be of the gressively narrower windows), the ability to compensate
same mass) with a spread of kinetic energies is dispersed for initial ion energy spread using an energy focusing ion
by transit through a radial electric field. This dispersion is mirror (Mamyrin, 1994), and the need for a mass analyzer
compensated for by the direction focusing character of the to complement the abilities of pulsed ionization exempli-
magnetic sector. The electric and magnetic sectors work fied by MALDI. Synergistically, the demand for higher
together as a unit, and therefore double focusing of the mass capabilities, and a simplified approach to mass mea-
ions is achieved. Under such conditions, the exact mass of surement, in biological analysis developed at the same
ions can be measured to a high degree of accuracy using time. The development of the TOF mass analyzer has lit-
a mass marker (an ion signal of known mass) and a pro- erally pushed mass spectrometry into totally new areas of
cedure known as peak matching. The voltages applied to application. Ion masses are routinely described in units
the plates of the electric sector do not scan, but rather are of kDa (kiloDaltons), and with higher resolution attained
set to the values appropriate for the range of ion energies in sophisticated instruments, these centroid masses are
that are to be passed. The mass spectrum is still scanned measured with accuracy and precision (about 0.1%
by varying B, the magnetic field strength. mass measurement accuracy). Even more amazing is the
The mass range of a sector-based mass spectrometer is demonstrated sensitivity; a few femtomoles of sample suf-
determined by the strength of the electromagnet, and the fices for the determination of mass. Issues for the future
value of V , the accelerating potential. Normal mass ranges relate to the ability to control and physically manipulate
of several thousand Daltons are typical at full accelerating such small amounts of sample outside of the mass spec-
potential. At reduced V , and therefore reduced ion trans- trometer. The analyst may engineer devices and processes
mission, higher mass ions can be passed through the to accomplish this goal. Nature precedes us; a single cell
mass analyzer. However, for higher mass ions, ESI with is a smartly packaged collection of sample molecules.