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Mass Spectrometry 157

ions are collected. Once these positive ions are accelerated tributes such as exact mass and measuring their intensity
into the body of the electron multiplier, the electron release as formed from a dissociating molecule. In interpreting a
and amplification process occurs as described above. For mass spectrum, we explore issues of chemical reactivity,
the efficient detection of high-mass ions, the conversion often based on kinetics, but expanding more recently into
dynode may be held at a very high potential to acceler- thermochemical arenas. This chemical focus represents
ate larger ions to a velocity sufficient to release on impact the fifty years of modern mass spectrometry in contrast to
a large number of lower-mass positive ions that are then Aston’s world of isotopic discovery that dates from 1919.
detected by the electron multiplier. Prognosticators look to the next fifty years, lest the con-
tent of an overview such as this become archaic even as it
2. Array Detectors appears.
The electron multiplier is a single channel device. The
mass analyzer selects the ions by mass, and the ion cur- A. Exact Mass Measurement
rentisgeneratedthroughamplificationasdescribedabove,
Sincetheexactmassesofindividualatomicionsareknown
measured, and recorded as the value for the ions at that
with high accuracy, the exact mass of an ion of a given
mass. Since the mass analyzer has to scan through the
empirical formula (a known combination of atoms) is also
mass range, at any given instant, all ions except those
known with high accuracy by simple summation. In mass
of the one mass passing through to the detector are lost.
spectrometry, measurement of the exact mass of an ion is
The Mattauch–Herzog geometry of double focusing mass
used to deduce information about the empirical formula
spectrometer used a focal plane detector in which ions of
(not the molecular formula) of the ion. If the total number
different masses were brought simultaneously into focus
of possible combinations is small, and the measurement of
at a planar detector that most often was photographic film.
the exact mass sufficiently accurate, the measurement can
The sensitivity of this integrating detector was high. With
be used to derive an ion empirical formula. The traditional
newer developments in multichannel or multipoint col-
exact mass measurement is usually limited to the more
lectors and their associated electronics, photographic film
abundant ions observed in the mass spectrum, since higher
has been replaced with array detectors, which are assem-
mass resolution in sector instruments was achieved at the
blages of small devices that act individually much like the
cost of lower ion signal.
electron multiplier described above.
High-performance sector instruments could provide
The term array is used to describe an assemblage of
mass resolutions of up to 100,000, but the typical daily op-
small single point ion detectors (sometimes called ele-
eration was a resolution of a few thousand. Excursions to
ments) arrayed in a plane. Each of these elements acts as
higher resolutions required concerted efforts with a clean
an ion current amplifier. Each element acts as a detector
and stable instrument, and the availability of a relatively
for the ion mass that is directed onto that particular space,
large amount of sample. As described above, the basis of
and that particular element. All ions of all masses are de-
tected simultaneously, and the array detector acts as an highresolutionFTMSisthemeasurementofthefrequency
of an ion orbiting in a static magnetic field. The ions are
integrating detector (ignoring the electronics of readout
not destroyed by this measurement; the measurement can
and measurement, and transfer of values into the data sys-
be completed on only a few hundred ions kept within the
tem). Clearly the array detector will provide an increase
cell for an extended period.
in sensitivity over a single-channel detector. It is therefore
The ability to routinely make exact mass measure-
useful in trace analysis, or in analyses in which a fraction
ments as exemplified by FTMS is not a simple exten-
of the mass spectrum (perhaps across the molecular ion
sion of the use of such values to derive empirical formula.
region) should be recorded without scanning that cover
There are two related areas in which the mass measure-
a wide mass range. More recently, these detectors have
ment data is used in situations in which other information
become useful in mass spectrometers in which the pro-
provides additional restraints on possible empirical for-
duction of ions in the source is discontinuous, as in a laser
mulas, and a synergistic extension of abilities. Rodgers
desorption ionization source. Time variabilities in ion pro-
et al. (2000) show that stable isotope incorporation into
duction are muted by the integrating nature of the detector.
selected biomolecules increases the upper mass limit at
which accurate mass measurement can provide an empir-
II. LAUNCH POINTS FOR THE NEXT ical formula composition for the ion. Simply, the nom-
FIFTY YEARS inal mass difference between a “natural abundance ion”
13
and the corresponding C-enriched (99% enrichment) ion
The mass spectrometer represents our microscope into of the same form immediately yields the number of car-
the world of individual ions, recording their physical at- bon atoms in the molecule. Once that value is known, the