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

Mass Spectrometry 155

Once the ions are formed through a process of electron can be accommodated in other mass analyzers. This very
ionization, they are trapped within the QIT by application low pressure can be reached, but engenders additional dif-
of appropriate voltages (DC) and rf signals on the elec- ficulty in creating and transporting ions from the outside
trodes. Again, in a simple case, the DC voltages can be world into the cell. The interfacing of an ESI source to a
at ground potential. A rf drive potential of about 1 MHz FTMS has been accomplished, but the practical difficulty
frequency is applied to the ring. Ions within a broad mass of transporting ions efficiently through a pressure differ-
range trace stable orbits near the center of the QIT. In most ential of 10–12 orders of magnitude is considerable. The
instruments, helium gas at a pressure of approximately advantages of high-resolution mass spectral data are suf-
10 −3 torr aids in keeping the ions in orbits near the center ficient impetus for development of this instrument and its
of the trap. As the amplitude of the rf drive potential is expanding applications. The presence of multiple charges
increased, the ion motions progressively change. Eventu- onanESI-generatedionincreasesthemassrangeofFTMS
ally, ions develop an unstable trajectory along the z-axis of by, in effect, making the ion easier to manipulate in the
symmetry (see figure), and pass through the ion exit holes cell and easier to detect. In that the ion is not destroyed in
in the end-cap electrode to be detected by the electron its detection, the sensitivity of FTMS is high. The limit-
multiplier. Appropriate potentials are used so that ions are ing steps are maintaining charge on the ions (larger ions
ejected from their orbits in mass order, and thus the device have a tendency to relinquish their charge unimolecularly)
scans across the mass range of the mass spectrum. and maintaining those ions within a stable and coherent
orbit. With enough time, the signal frequency can be de-
termined for a group of only 100–1000 ions of the same
5. Fourier Transform Mass Spectrometers
mass, which may correspond to only a very small amount
The basis of high resolution Fourier Transform mass spec- of sample, or a very low abundance ion in a mass spec-
trometry (FTMS) is the measurement of the frequency of trum. In fact, the presence of too many ions complicates
an ion orbiting in a static magnetic field. The ions are not the measurement through introduction of secondary space
destroyed by this measurement of frequency. Instead, the charge effects that distort the measured frequency. Capa-
ions orbit continuously within the confining cell of the bilities of FTMS are discussed again in the final section
FTMS instrument (held there by a combination of poten- of this overview on launch points for the next fifty years.
tials applied to a cubic cell and a static magnetic field) until
removed by application of an external electrical pulse, or D. Detectors
scattered out of a stable orbit by collisions with neutral
With the exception of the Fourier transform mass spec-
residual gas molecules. The key to an accurate measure-
trometer described above, ion detection in other mass
ment of frequency, and therefore an accurate determina-
spectrometers is the destructive event in the sequence.
tion of mass, is maintenance of a coherent ion orbit. With
Each individual ion carries such a small charge and is
extended measurement time, the impact of random errors
of such low absolute mass that direct detection is difficult
in the determined frequency value are reduced (the usual
(but not impossible). Considerable amplification of the ion
measurement statistic in which random noise decreases
charge or the ion mass is necessary to make mass spec-
with the square of the number of measurements). The keys
trometry practicable. Electron multiplier detectors, known
to the maintenance of ion orbit are the homogeneity of the
since the 1950s, provide the requisite amplification of the
static magnetic field, the symmetry of the trapping poten-
charge into an easily manipulated current.
tials generated within the ion confinement cell, and the
base pressure within the FTMS instrument. At low base
1. Electron Multipliers
pressures within the FTMS instrument, the ion orbits are
stable for periods of seconds and longer. Extraordinary The electron multiplier detector is a transducer that con-
resolutions have been achieved not only for simple atomic verts the impact of ions on its front surface into an am-
ions but also recently for complex ions derived from com- plified electron current then sampled by modern digital
plex organic and biomolecules, as well as for ions from electronics. The sensitivity of mass spectrometric analysis
polymers and organometallic compounds. The maximum is supported by the high gain (as high as a millionfold)
resolution of 100,000 reached by sector instruments has provided by modern electron multiplier. This impressive
been surpassed easily by a resolution of 1,000,000 readily gain is achieved by a combination of the electron emissive
achievable in FTMS. properties of the active surface of the multiplier and the po-
Perhaps the most significant practical impediment to tential difference maintained along its length. The active
the use of FTMS has been the need to maintain a low surface of the modern Channeltron version of the electron
pressure in the cell where the ions orbit. A pressure of multiplier consists of emissive layers of silicon dioxide
10 −9 torr is desirable, as compared to the 10 −6 torr that overlying a conductive layer of lead oxide supported by