"This is
not the place to enter into the extremely
involved design of that instrument, which
is as ingenious as it is exact. Suffice to
say that the rays which have a constant
ratio between electrical charge and mass
are focussed ... (and) ... can be exactly
determined with the help of a photographic
plate. By this means there is attained
what is known as a mass spectrogram, that
is to say a series of lines in which each
line corresponds to a certain atomic
weight."
So said Dr. H.G. Soderhaum of the Royal
Swedish Academy of Sciences in presenting
Francis Aston with the Nobel Prize for
Chemistry 1922. What follows is a brief
overview of the history of mass
spectrometry from its earliest beginnings
to the present day.
(photo) J.J. Thomson
working with his cathode ray tube
- an early predecessor of the modern mass
spectrometer.
Mass spectrometry
has its roots in experiments performed at
the Cavendish
laboratory of The University of
Cambridge, England almost a hundred
years ago. Joseph John
(J.J.) Thomson discovered that electrical
discharges in gases produced ions and
these rays of ions would adopt different
parabolic trajectories according to their
mass when passed through electromagnetic
fields. This separation of ions according
to their mass (and charge) is a hallmark
of all modern mass spectrometry
experiments.
It was Thomson's student Francis William
Aston
who designed several subsequent mass
spectrographs in which ions were dispersed
by mass and focused by velocity. This led
to improvements in mass resolving power
and the subsequent discovery of isotopes
for many common naturally-occuring
elements. Aston's second mass
spectrograph is on display at the
National Science Museum in London.
Both Thomson and Aston were honored for
their achievements and received Nobel
Prizes in Physics and Chemistry in 1906
and 1922. By the early 1920s, Arthur
Dempster first in Cambridge then at the
University of Chicago developed a magnetic
analyzer that focused ions formed by
electron impact onto an electrical
collector. This design was adapted by
Josef Mattauch and his student Richard
Herzog, as well as Kenneth Bainbridge,
Alfred Nier and others, leading to major
discoveries in atomic and nuclear physics
throughout the 30s. Still the early
instruments were difficult to operate and
esoteric in nature and it wasn't until the
early 1940s that commercial mass
spectrometers first appeared. Throughout
the 1930s and 40s, Nier and many others
incorporated the latest vacuum
technologies and electronics of the time
to improve the performance of the early
designs. Double-focusing instruments,
which combine a magnetic and electrostatic
analyzer, were also introduced leading to
greater mass accuracies. These instruments
were originally developed for the purpose
of accurately determining the atomic
weights of the elements and their
isotopes.
By the mid 1940s, magnetic sector mass
spectrometers were being manufactured by a
number of companies in Europe and the
United States. This decade also saw the
parallel development of the time-of-flight
(TOF) mass spectrometer, a concept
proposed as a cheaper and simpler mass
analyzer in which ions are separated based
on differences in their velocities as they
are accelerated down a linear flight
tube.
Sector mass spectrometers of
Mattauch-Herzog and Nier-Johnson
geometries were still widely being used in
the 1950s to characterize organic
compounds. Another type of instrument
developed for the purpose of coupling mass
spectrometers to gas chromatographs also
emerged. The quadrupole mass filter uses a
quadrupolar electric fields comprising
both radiofrequency and direct-current
components to separate ions. This analyzer
was first reported by Wolfgang Paul of the
University of Bonn, who later shared the
1989 Nobel Prize in Physics for his work
on ion trapping.
By the 1960s, mass spectrometry had become
an established technique for
characterizing organic compounds. This was
aided by the introduction of the chemical
ionization source. Other so-called "soft
ionization" methods emerged including
field desorption, secondary ionization MS
(or SIMS), plasma desorption and laser
desorption MS. Several scientific journals
dedicated to the field appeared including
Organic Mass Spectrometry. Tandem mass
spectrometry (MS/MS) came to the fore with
the development of the collision-induced
dissociation procedure. Tandem mass
spectrometry enables structural
information to be obtained for components
of a mixture using two stages of mass
analysis.
In 1974 Fourier transform ICR mass
spectrometry (FT-ICR MS) was developed. A
major advantage of FT-ICR MS is that it
allows many ions to be detected
simultaneously and these mass
spectrometers also achieve very high mass
resolution. The 1980s saw the development
of ionization techniques capable of
efficiently ionizing biological molecules
that took advantage of these features.
Michael Barber in Manchester developed the
fast atom bombardment (FAB) technique
which uses a source of neutral heavy atoms
to ionize compounds from the surface of a
liquid matrix. John Fenn and colleagues at
Yale University refined an ion source
originally reported by Malcolm Dole of
Northwestern University almost two decades
earlier to develop the electrospray
ionization (ESI) technique.
Matrix-assisted laser desorption
ionization (MALDI) was also developed in
the late 1980s by Franz Hillenkamp and
Michael Karas. In this technique,
molecules are desorbed by a laser from a
solid or liquid surface containing an
organic matrix compound. The ESI and MALDI
techniques have enabled biological
molecules exceeding 1 million Daltons to
be introduced into mass spectrometers as
stable gas-phase ions.
Today mass spectrometers are used
throughout the world to study a wide range of
substances and materials with
ever-increasing precision and sensitivity.
Mass spectrometers are employed in
astronomical studies of the solar system,
in geophysics and geochemistry, for
chemical analysis and more fundamental
investigations of ion chemistry, in
toxicology, in the environmental sciences,
and increasingly in the biological and
biomedical sciences.
See additional history material within the
i-mass
guides.