DAL
TON
PERSPECTIVE
J. Chem. Soc., Dalton Trans., 1998, 3893–3899
3893
Francis Aston and the mass spectrograph
Gordon Squires
Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge, UK CB3 0HE
Received 18th June 1998, Accepted 28th July 1998
The chemical determination of atomic weights gives the
average weight for an aggregate of a large number of
atoms. Although this is useful in many applications, the
determination of the masses of individual atoms gives
further important information, in particular the stability
of the atoms or more precisely of their nuclei. The first
accurate determination of the masses of individual atoms
was made by Aston in 1919. His measurements demon-
strated the existence of isotopes in non-radioactive
elements and paved the way for our present picture of the
nuclear atom.
Early life
Francis William Aston was born on 1 September 1877 at
Harborne, Birmingham. He was the third child of a family of
seven. His father and paternal grandfather were metal mer-
chants and farmers, and Francis was brought up on a small
farm. From an early age he showed a keen interest in mechan-
ical toys and scienti
fic apparatus. He had a ‘laboratory’ over a
stable and amused his sisters with home-made
fireworks and
large tissue-paper hot-air balloons. These were dispatched with
stamped addressed postcards, which were sometimes returned
from great distances.
1
Aston entered Malvern College in September 1891 and two
years later went to Mason College (which subsequently became
the University of Birmingham), where he studied chemistry
and physics. The professor in physics was John Poynting (of
Poynting’s vector). While at Birmingham Aston acquired skill
with tools and glass-blowing which proved invaluable in his
later work. Faced with the need to earn a living after graduating
he took a course in fermentation chemistry and in 1900 started
work in a brewery in Wolverhampton. In his spare time he
experimented at home, designing and building new forms of
Sprengel and Tœpler vacuum pumps. This experience was again
to stand him in good stead later on.
In 1903 Aston returned to Birmingham University and
physics. He worked on the properties of electrical discharges in
Gordon Squires is a retired
lecturer in the Department of
Physics at the University of
Cambridge, and his current
interest is the history of the
Cavendish Laboratory. His
field of research is the scatter-
ing of thermal neutrons. He is
the author of textbooks on
practical physics, quantum
mechanics, and the theory of
thermal neutron scattering.
Gordon Squires
gases and measured the variation of the length of the Crookes
dark space with current and pressure. In 1908 his father died,
and he used a legacy to travel round the world. On his return he
was appointed a lecturer in Birmingham University, but after
one term he received an invitation from Joseph (J. J.) Thomson
to come to the Cavendish Laboratory at Cambridge as his
assistant. Poynting, a close friend of Thomson, had recognised
Aston’s great gifts as an experimenter and recommended him
for the post. Aston accepted the invitation and thus began a
career that had momentous consequences for chemistry and
nuclear physics.
Historical background to Aston’s work
To appreciate the signi
ficance of the work Thomson was doing
and Aston’s subsequent role we need to go back in the history
of chemistry. In 1803 John Dalton put forward an atomic
theory, which laid the foundations of modern chemistry. One of
the postulates was that atoms of the same element are similar to
one another and equal in weight. About ten years later William
Prout suggested that the atoms of the elements were made up
of aggregates of hydrogen atoms. If this were true the weights
of atoms would be expressed as whole numbers, i.e. integers,
and, on the basis of Dalton’s postulate that all the atoms of an
element had the same weight, atomic weights would also be
whole numbers. However, experiment showed that although the
atomic weights of many of the elements were whole numbers,
far more than could be attributed to chance, there were a few,
for example, magnesium, atomic weight 24.3, and chlorine,
atomic weight 35.5, which were not. Therefore, Dalton and
Prout could not both be correct, and around 1900 it was
Dalton’s rather than Prout’s hypothesis that was accepted.
In 1896 Henri Becquerel discovered radioactivity, and from
then until the outbreak of the
first World War many radioactive
substances were found. An interesting feature was that two and
sometimes three of the substances with quite di
fferent modes of
decay appeared to be chemically similar. For example, in 1906
Bertram Boltwood found that once salts of thorium and
ionium were mixed they could not be separated by any chemical
means.
2
Another example was radium B and lead; not only were
their chemical properties the same, but Ernest Rutherford and
Edward Andrade found that they had identical X-ray spectra.
3
In 1913 Frederick Soddy
4
proposed the word isotopes to
describe these chemically similar materials, because they
occupy the same place in the Periodic Table of the elements. He
observed ‘They are chemically identical, and, save only as
regards the relatively few physical properties which depend
upon atomic mass directly, physically identical too’.
Thomson’s work on positive rays
In 1886 Eugen Goldstein was investigating the properties of the
electric discharge obtained when a large voltage is applied
across a pair of electrodes in a vessel containing a gas at low
pressure. He found that if a channel or canal was cut through
the cathode a beam of light appeared on the side remote from
the anode. He called the beam Kanalstrahlen, canal rays.
5
In
3894
J. Chem. Soc., Dalton Trans., 1998, 3893–3899
1898 Willy Wien
6
managed to de
flect the beam with a strong
magnetic
field, in a direction which showed it was due to a
stream of positively charged particles. They are in fact the posi-
tive ions resulting from the atoms in the gas that have lost one
or more electrons. In 1907 Thomson
7
started to investigate
the positive rays. He measured the mass of the particles by
de
flecting the rays with electric and magnetic fields. His appar-
atus was the forerunner of Aston’s mass spectrograph and it is
instructive to consider it
first.
The essentials of the apparatus are shown in Fig. 1. The
discharge occurs in the spherical tube T. The anode A is located
in a side arm, and the positive rays pass through the cathode C,
which is a
fine tube. The rays then pass between the poles N and
S of an electromagnet, the pole pieces P
1
, P
2
of which are insu-
lated from the magnet by thin sheets of mica. By this means
a potential di
fference may be applied across the pole pieces,
giving an electric
field E in the same direction as the magnetic
field B, this direction being at right angles to the path of the
particles. The particles
finally strike the screen H, where they
produce a
fluorescent spot. In the absence of the two fields the
particles travel in a straight line, and the spot is in the centre of
the screen in line with the
fine tube in the cathode.
Take a set of right-handed axes x, y, z, with the initial direc-
tion of the particles as the z axis, and the common direction of
E and B as the x axis. We consider the electric and magnetic
de
flections separately. The deflection produced by the electric
field is shown by the diagram in Fig. 2. The particles coming
from the left with velocity v enter the region between the pole
pieces P
1
and P
2
, across which a potential di
fference V is
applied. If the pole pieces are a distance d apart this gives an
electric
field E = V/d, which causes an acceleration eE/m, where
e is the charge and m the mass of the particles. If l
E
is the length
of the plates, the particles spend an approximate time l
E
/
υ
between the plates, and when they emerge from the plates they
have acquired a component of velocity in the x direction given by
υ
x
= eEl
E
/m
υ.
(1)
Since
υ
x
Ӷ υ, the angle through which the particles are deflected
by the
field is approximately
θ =
υ
x
υ
=
eEl
E
m
υ
2
.
(2)
A magnetic
field B whose direction is at right angles to
the path of the particles de
flects them into a circular path of
radius R as shown in Fig. 3. The force due to B is Be
υ, and its
direction is at right angles to the directions of both B and v. The
acceleration in the circular path is
υ
2
/R. Thus
m
υ
2
/R
= Beυ, i.e. mυ = BeR.
(3)
Fig. 1
Diagram of Thomson’s positive ray apparatus.
Fig. 2
The de
flection of positively charged particles by an electric
field.
The particles are de
flected in the y direction through an angle
φ =
l
B
R
=
eBl
B
m
υ
,
(4)
where l
B
is the length of the path in the magnetic
field.
Now let E and B act together. The screen H, which contains
the x and y axes, is shown in Fig. 4, with the position O of the
spot for the unde
flected beam as the origin. The field E deflects
the particles in the x direction by an amount proportional to
the angle
θ, while B deflects them in the y direction by an
amount proportional to the angle
φ, both the angles being
small. The coordinates of the spot are therefore
x
= c
1
eE
m
υ
2
,
y
= c
2
eB
m
υ
,
(5)
where c
1
and c
2
are constants depending on the geometry of the
apparatus. Eliminating the velocity
υ between these two expres-
sions gives
y
2
x
= c
3
e
m
B
2
E
,
(6)
where the constant c
3
depends on the geometry of the appar-
atus. Thus, for a beam of ions with the same value of e/m and
varying velocities, the pattern on the screen is a parabola, Fig.
4. Particles with di
fferent velocities arrive at different points on
the parabola.
If ions with di
fferent masses are present there will be several
parabolas corresponding to the di
fferent e/m values. The value
of e is the electronic charge, 1.60 × 10
Ϫ19
C, or a simple multiple
of it. For singly charged ions the y value of the parabola at a
constant value of x is proportional to 1/
√m. So the ratio of two
masses is given by the square of the inverse ratio of the two y
values at the same value of x. This is independent of the form
of the apparatus and of the values of E and B. If an atom of
mass m loses two electrons in the discharge tube, the doubly
charged ion appears on a parabola corresponding to a singly
charged ion of mass m/2. A photographic record is made of the
Fig. 3
The de
flection of positively charged particles by a magnetic
field. The direction of the field is down, perpendicular to the plane of
the diagram.
Fig. 4
Parabolas obtained with Thomson’s positive ray apparatus.
The ratio of the masses of the particles for the two parabolas is given by
m
1
/m
2
= (Y
2
Y
2
Ј/Y
1
Y
1
Ј)
2
. The parabolas with negative y are obtained by
reversing the magnetic
field.
J. Chem. Soc., Dalton Trans., 1998, 3893–3899
3895
traces. If the mass is known for one of the parabolas, measure-
ments of the y values at constant x give the values of all the
other masses. The axis Ox is not marked on the photograph.
The magnetic
field is therefore reversed for the second half of
the exposure, which puts the pattern in the
Ϫy region and
allows the y values to be measured.
When Aston arrived at the Cavendish Laboratory in 1909
Thomson’s positive ray apparatus was already working. With
his assistance the apparatus was greatly improved, and by 1912
parabolas corresponding to mass di
fferences of 10% could be
resolved. In November of that year some gas containing neon
was analysed. The photograph, Fig. 5, showed a strong
parabola corresponding to a mass of 20 (on the scale oxygen
=
16) and a much weaker one at a mass of 22.
8
Various possi-
bilities for the 22 parabola were considered. One was that it was
due to doubly charged CO
2
. However, when the gas was passed
through liquid air, the parabola at 44, due to singly charged
CO
2
, disappeared, while the one at 22 was not a
ffected. Another
speculation was that the 22 trace was due to a compound
NeH
2
.
From density measurements the atomic weight of neon was
known to be 20.2. So the novel, and at the time revolutionary,
suggestion was made that neon could exist in two forms, which
were isotopes, just like the isotopes suggested by Soddy in
radioactive elements. If the isotope of mass 20 was 9 times
more abundant than the one of mass 22, that would give the
measured atomic weight of 20.2. In other words, neon did not
consist of identical atoms of mass 20.2, but of two di
fferent
atoms of mass 20 and 22, in line with Prout’s hypothesis.
Aston set to work to see if he could separate the two con-
stituents of neon. He
first tried fractional distillation, but with-
out success. He then tried di
ffusion through fine pores, using
clay tobacco pipes, and after much labour obtained a small
e
ffect.
9
Then, in his own words ‘the whole of the lightest
fraction was lost by a most unfortunate accident’. (It is said that
he dropped the
flask containing the specimen!) However,
undeterred, he carried on with the heaviest fraction and ultim-
ately obtained two samples with densities 20.15 and 20.28 on
Fig. 5
Positive ray parabolas of neon obtained by Thomson in 1912.
the scale O
2
= 32. These results were just on the borderline of
the experimental uncertainty.
The work was interrupted by the
first World War. Aston
was sent to the Royal Aircraft Factory, later the Royal Aircraft
Establishment, at Farnborough. Frederick Lindemann, later
Lord Cherwell, and George Thomson (J. J. Thomson’s son)
were also there. In after years Thomson recollected that Linde-
mann was sceptical of Aston’s isotope hypothesis, preferring
the idea of CO
2
or NeH
2
for the 22 parabola.
10
He said that
Lindemann was a much better theoretician than Aston and
always won the argument, but Aston ‘had faith and next morn-
ing was still of the same opinion’. In 1914 Aston crashed in an
experimental aeroplane, but escaped unhurt. He worked at
Farnborough as a chemist, studying among other things the
properties of the doped canvas with which aeroplanes were then
covered.
Aston’s
first mass spectrograph
After the war Aston returned to the Cavendish Laboratory.
While at Farnborough he had meditated on an improved form
of the apparatus to measure the masses of the positive ions, and
in 1919 he built his
first mass spectrograph.
11
Like Thomson’s
parabola apparatus it employed electric and magnetic
fields to
de
flect the particles, but the two fields were in different regions
along the path of the particles. Unlike Thomson’s apparatus in
which particles with the same e/m value, but di
fferent velocities,
were distributed along the parabola, in Aston’s spectrograph
these particles were focused to the same point on the screen.
This was a big advantage. The focused beam was much more
intense, thus permitting
finer slits to be used, which improved
the resolution and accuracy of the instrument.
The principle of the instrument is illustrated in Fig. 6. The
path of the positive particles emerging from the discharge tube
is de
fined by a pair of narrow slits S
1
and S
2
. The particles then
pass between a pair of plates P
1
and P
2
across which a potential
di
fference is applied. The particles are deflected downwards by
the electric
field towards the negative plate P
2
. They are
de
flected continuously in the region between the plates, but as a
first approximation we may assume that the paths come from a
point Z in the middle of the plates on the line de
fined by S
1
and
S
2
. A group of the rays is allowed to pass through a narrow
diaphragm D, which selects those de
flected through angles
between
θ and θ ϩ δθ. They then pass between the poles of an
electromagnet which has its north pole above the plane of the
diagram. This de
flects the particles in the opposite direction to
that of the electric
field.
The same notation is used as in the discussion of Thomson’s
apparatus. Eqns. (2) and (4) still apply. For particles of velocity
v, charge e and mass m, the electric
field E gives a deflection θ,
and the magnetic
field B gives a deflection φ. The position of
the diaphragm D
fixes the angle θ, and hence the velocity of the
particles passing through. The spread
δθ in θ gives rise to a
spread
δυ in υ, which in turn gives a spread δφ in the deflection
produced by the magnetic
field. The relations between δθ, δυ
and
δφ are obtained from eqns. (2) and (4). For a constant value
Fig. 6
The paths of the particles in Aston’s mass spectrograph. The
particles have the same mass value but varying velocities. The path of
the fastest particles is shown in blue and that of the slowest in red. For
clarity the electric and magnetic de
flections are shown as abrupt
changes in direction, rather than the actual continuous changes shown
in Figs. 2 and 3.
3896
J. Chem. Soc., Dalton Trans., 1998, 3893–3899
of e/m,
θ is proportional to 1/υ
2
, and
φ is proportional to 1/υ.
Therefore
δθ
θ
= Ϫ2
δυ
υ
,
δφ
φ
= Ϫ
δυ
υ
,
(7)
whence
δφ/δθ = φ/2θ.
(8)
The minus signs in eqns. (7) indicate that the faster particles,
indicated by the blue path in Fig. 6, are de
flected less in both the
electric and the magnetic
fields than the slower particles indi-
cated in red. Since the electric and magnetic de
flections are in
opposite directions the rays passing through D are brought
together at a point F.
It is readily shown that the angle between the line ZF and the
initial direction ZC of the particles is equal to
θ. Fig. 7 shows
the mean paths of the particles. The angle between FZ and OZ
is denoted by
ρ, where O is the centre of the magnetic field. The
angle GOF
= φ, and ZFO = φ Ϫ ρ. Now ρ and φ Ϫ ρ are small;
in Aston’s apparatus they were of the order of 1/10 rad. The
line LM in Fig. 6 is therefore almost perpendicular to the lines
ZL, ZM, FL and FM, and, to good approximation, its length
is given by
LM
= aδθ = b(δφ Ϫ δθ),
(9)
where a and b are the lengths OZ and OF. Similarly, in Fig. 7, if
ON is the perpendicular from O to the line ZF, its length is
ON
= aρ = b(φ Ϫ ρ).
(10)
Therefore, from eqns. (10) and (9),
a
b
=
φ Ϫ ρ
ρ
=
δφ Ϫ δθ
δθ
,
(11)
whence
φ
ρ
=
δφ
δθ
=
φ
2
θ
.
(12)
The last step follows from eqn. (8). Eqn. (12) shows that
ρ = 2θ, i.e. the angle FZC = θ. The position of the point F on
the line ZB depends on the value of
φ, and hence from eqn. (4)
on the value of e/m. So particles with di
fferent e/m values come
Fig. 7
Mean paths of the particles in Aston’s mass spectrograph: Z is
the centre of the electric
field, and O the centre of the magnetic field;
OZ
= a, OF = b.
to a focus at di
fferent points along the line ZB. A photographic
plate is placed along this line to record the traces.
The position of the focus point on the line ZB for a given e/m
value may be calculated from the values of E, B, and the geom-
etry of the apparatus. However, the quantities required are the
ratios of masses, and these are obtained most accurately by
empirical methods. Aston
first calibrated the instrument using a
set of lines given by atoms and compounds with masses spread
over a suitable range, and whose relative masses were known to
the accuracy required. An example of such a set was: 6, C
2
ϩ
; 8,
O
2
ϩ
; 12, C; 16, O; 28, CO; 32, O
2
; 44, CO
2
. (The integer before
each atom or compound is the e
ffective mass number, i.e. the
actual mass number divided by the number of charges on the
ion.) This provided a set of points on a calibration curve. He
filled in the gaps between the calibration points by taking the
spectrum with the same set of ions, which were made to give
lines at a di
fferent place by changing the value of the magnetic
field.
Aston gave a preliminary account of the spectrograph in
August 1919.
11
The instrument was an immediate success. The
two isotopes of neon, mass 20 and 22, were easily resolved.
12,13
Similarly, chlorine was found to be a mixture of isotopes of
mass 35 and 37.
14
By the time his
first book Isotopes appeared
in 1922 he had studied 27 elements.
15
Among them were the
following (masses, where oxygen is 16, in parentheses): lithium
(7, 6), boron (11, 10), magnesium (24, 25, 26), argon (40, 36),
krypton (84, 86, 82, 83, 80, 78) and xenon (129, 132, 131, 134,
136, 128, 130). The isotopes are given in the order of the inten-
sities of the lines. Reproductions of the spectra for neon and
chlorine are given in Fig. 8.
Although the discovery of many isotopes in light non-
radioactive elements was of great importance, even more sig-
ni
ficant was Aston’s result that the masses of all the particles
are whole numbers. (The only exception was that of hydrogen
whose mass was 1.008, see below.) This whole number rule as it
was called gave a simple model for the atomic nucleus. The only
particles known at the time were the proton and the electron,
with relative masses of 1837. It was therefore proposed that the
nucleus of an isotope of mass M and charge Z, both being
integers, consisted of M protons and M
Ϫ Z electrons. Thus,
for example, the nucleus of
7
Li consisted of 7 protons and 4
electrons, while that of
6
Li consisted of 6 protons and 3 elec-
trons. Although this model gave the correct mass and charge of
the nucleus, and satis
fied the whole number rule, it had two
grave defects. First, from the uncertainty principle, if an elec-
tron were con
fined to a region as small as an atomic nucleus, its
momentum and hence energy would be much larger than the
binding energy of the nucleus. Secondly, the spins of some of
the nuclei were anomalous on this model. For example, the
nucleus of
14
N would consist of 14 protons and 7 electrons,
giving a total of 21 particles. Since the spin of both the proton
and the electron is ¹¯², the spin of the nucleus with an odd number
of particles would be half-integral; in fact the spin of
14
N is 1.
The discovery of the neutron by James Chadwick
16
in 1932
removed these di
fficulties. The present model is that a nucleus
of charge Z and mass number M contains Z protons and
M
Ϫ Z neutrons. Isotopes are thus nuclei with the same number
of protons and a di
fferent number of neutrons. They have the
same chemical properties, but di
fferent nuclear properties.
Fig. 8
Mass spectra obtained by Aston, 1919–1920, of (a) neon showing the 20 and 22 isotopes, and (b) chlorine showing the 35 and 37 isotopes.
14
A number of other ions are present in both spectra. For example, the line at 28, prominent in both spectra, is due to CO. The lines at 36 and 38,
present in the chlorine spectrum, are due to H
35
Cl and H
37
Cl.
J. Chem. Soc., Dalton Trans., 1998, 3893–3899
3897
There are no electrons in the nucleus, and the nucleus
14
N
contains 14, an even number, of particles of spin ¹¯².
Aston’s
first mass spectrograph could separate particles with
a mass di
fference of 1 in 130, which may be compared with a
value of about 1 in 10 for Thomson’s parabola apparatus. The
values of the masses were obtained with an accuracy of about 1
part in 1000.
The second and third mass spectrographs
Aston and other scientists soon grasped the reason for the
departure of the mass value of hydrogen from an integral
value, namely that it is the only atom with a non-composite
nucleus. The masses of all the other atoms are reduced owing
to the binding energy of their constituents, which results in
hydrogen having a slightly higher mass relative to its mass
number. The next step in mass spectrometry was therefore to
improve the accuracy of the instrument to measure
divergences from the whole number rule for all the atoms,
which would give basic information on the binding forces
within nuclei. For nuclei with mass numbers greater than
about 20, the binding energy per nucleon is roughly constant,
with a value between 8 and 9 MeV, which is about 1% of the
energy equivalent of the mass of a nucleon. So to determine
the binding energy to 1% the mass of the nucleus must be
measured to an accuracy of about 1 part in 10
4
. Aston started
designing an improved version of his spectrograph in 1921,
though he continued to use the original instrument until it was
dismantled in 1925.
In the second mass spectrograph
finer slits were used and
they were placed farther apart, thus more accurately de
fining
the paths of the particles.
17
The electric de
flecting plates were
curved, so that the particles remained midway between them
as they were de
flected. The electric deflection θ was doubled
to 1/6 rad. The potential for the de
flection came from a set of
500 accumulators, each one built by Aston himself. They were
charged twice a year and gave a voltage constant to better
than 1 part in 10
5
during a single experiment. To achieve a
constant magnetic
field with minimum heating, the core of the
magnet was wound with over 6000 turns of wire weighing
over 100 kg. A current of 1 A through the coils produced a
magnetic
field of 1.6 T, which was sufficient to deflect the
heaviest and most energetic particles through an angle
φ of
2/3 rad. (A detailed calculation shows that, when
φ = 4θ, the
position of the line varies linearly with mass, which is con-
venient for interpolation.) The pole pieces of the magnet were
greatly reduced by changing their cross-section from the circu-
lar shape of the
first instrument to a sickle shape, thereby
producing a magnetic
field only in the region close to the
particle beam. Aston demonstrated the improved resolving
power of the second instrument by separating six isotopes of
mercury with mass numbers ranging from 198 to 204.
18
With
the original instrument the lines appeared as an unresolved
blur.
Aston’s third mass spectrograph in 1937 incorporated further
improvements.
19
The widths of the collimating slits could be
adjusted externally, obviating the laborious opening of the
apparatus which was necessary for such adjustments in the
first
two instruments. The stability of the magnetic
field was
improved by monitoring its strength with a
fluxmeter; in the
previous instruments only the exciting current had been kept
constant. Any variation in the magnetic
field was compensated
for by manual adjustment of a spiral mercury resistor. Another
advance was in the greatly improved sensitivity of the photo-
graphic plates used to record the lines, which resulted after
extensive trials carried out with the collaboration of the Ilford
photographic company.
The biggest advance came in the use of the doublet method
for comparing two masses. This consists of measuring the small
di
fference in the masses of two ions with the same mass number
M. The mass of an atom X with mass number M is denoted by
m(
M
X). Since the value of m is close to the integer M, we may
express it as
m(
M
X)
= M(1 ϩ δ),
(13)
where
δ, known as the packing fraction, is small compared to 1.
As an example we show how Aston measured the mass of the
hydrogen atom in terms of the mass of the carbon atom. He
measured the di
fference in mass ∆
1
between the deuterium atom
and the hydrogen molecule (doublet with M
= 2), and the dif-
ference
∆
2
between the masses of the triatomic deuterium
molecule and the doubly charged carbon atom (doublet with
M
= 6). Then
∆
1
= 2m(
1
H)
Ϫ m(
2
H)
=
2(1
ϩ δ
H
)
Ϫ 2(1 ϩ δ
D
)
= 2δ
H
Ϫ 2δ
D
,
(14)
∆
2
= 3m(
2
H)
Ϫ m(
12
C
2
ϩ
)
=
6(1
ϩ δ
D
)
Ϫ 6(1 ϩ δ
C
)
= 6δ
D
Ϫ 6δ
C
,
(15)
δ
H
Ϫ δ
C
= (3∆
1
ϩ ∆
2
)/6.
(16)
Aston’s values for
∆
1
and
∆
2
were (15.2 ± 0.4) × 10
Ϫ4
and
(423.6 ± 1.8) × 10
Ϫ4
respectively, giving
δ
H
Ϫ δ
C
= (78.2 ± 0.4) ×
10
Ϫ4
.
At the time of Aston’s measurements the atomic mass unit,
denoted by u, was de
fined by taking the mass of the atom
16
O to
be exactly 16, but in 1962 the de
finition was changed
20
so that
the mass of the atom
12
C is taken as exactly 12, i.e.
δ
C
= 0. Thus
on the present scale Aston’s value for the packing fraction of
hydrogen was
δ
H
= (78.2 ± 0.4) × 10
Ϫ4
, giving m(
1
H)
= 1.00782 ±
0.00004 u. The example shows the intrinsic advantage of the
method of measuring the di
fference in mass of doublets with
the same mass number. The mass di
fferences ∆
1
and
∆
2
are
measured to accuracies of the order of 1%, but the mass of the
hydrogen atom obtained is accurate to 4 parts in 10
5
. It may be
noted that Aston’s value is in complete agreement with the
present value,
21
m(
1
H)
= 1.00782504 ± 0.00000001 u.
The particles whose masses are measured in the spectrograph
are ions that have lost one or more electrons, but the mass
values quoted relate to the neutral atoms, i.e. the mass of one
or more electrons is added to the measured masses. The mass
of the electron, 5.486 × 10
Ϫ4
u, is small but not negligible at
the accuracy of Aston’s measurements. On the other hand,
the binding energies of the atoms in a molecule, being of
the order of electronvolts, correspond to mass values of the
order of 10
Ϫ9
u. So the mass of a diatomic molecule such as
hydrogen may be taken to be twice the mass of the hydrogen
atom.
Aston improved the resolving power of his spectrographs
from 130 for the
first instrument to 600 for the second and 2000
for the third. He claimed an accuracy of 1 in 10
4
for the second
instrument and approaching 1 in 10
5
for the third. If we com-
pare his mass values (changing them to the
12
C scale) with the
present-day values, which are accurate to about 1 in 10
6
or
better, the di
fferences for the values obtained from the 1927
instrument are on average about 1.5 in 10
4
, and for the 1937
values about 2.5 in 10
5
. So his claimed accuracy is well substan-
tiated. The third mass spectrograph, without the magnetic
field
components, is in the Museum of the Cavendish Laboratory;
it is shown in Fig. 9. Aston’s
first mass spectrograph is in the
Science Museum in South Kensington.
Other workers and modern developments
Although Aston is recognised as the pioneer in mass spec-
troscopy there were other major workers in the
field from 1918
onwards. Arthur Dempster, at the University of Chicago, con-
3898
J. Chem. Soc., Dalton Trans., 1998, 3893–3899
structed a mass spectrograph in 1918
22
and in the next few
years found isotopes in magnesium, lithium, potassium, cal-
cium and zinc. His instrument involved bending monoenergetic
ions into a semicircular path by a uniform magnetic
field, which
gives direction focusing, i.e. ions diverging in direction at the
entrance to the magnetic
field are brought to a focus after
completing a semicircle. Kenneth Bainbridge, at the Franklin
Institute, Swarthmore, improved on Dempster’s instrument by
using a velocity
filter before the magnetic analyser, thereby
removing the need for a monoenergetic source.
23
With his
apparatus he made the
first measurement of the mass of the
deuterium atom
24
and also provided one of the
first experi-
mental demonstrations of Einstein’s mass-energy relation.
25
The detailed motions of ions in electric and magnetic
fields
were calculated by Richard Herzog and Josef Mattauch in
Vienna
26
and others in the early thirties. The results led to the
design of high-resolution double-focusing instruments in which
ions with both a spread in velocities and a spread in initial
directions were brought to a focus. Instruments making use of
double focusing were built by Alfred Nier at the University
Fig. 9
Aston’s third mass spectrograph, now in the the Museum of
the Cavendish Laboratory. The discharge tube is on the right. The white
discs are for adjusting the widths of the slits S
1
and S
2
. The magnet, not
shown, acts over the region of the tube in the left-hand wooden sup-
port. The photographic plate is placed in the focus plane just before the
left end of the tube to record the spectra. The overall length of the
instrument, excluding the discharge tube, is 105 cm.
Fig. 10
Aston working with his apparatus for the separation of the
isotopes of neon by fractional distillation, 1914.
of Minnesota
27
and several other workers.
28
Further improve-
ments came from replacing photographic plates by electrical
detectors and from advances in vacuum technology.
29
The value
of 2000 for the resolving power of Aston’s third mass spectro-
graph has been extended to values exceeding 10
5
, and accur-
acies of the order of 1 part in 10
8
or 10
9
have been obtained.
30
Mass spectroscopy is now applied in several branches of
chemistry, biology, geology, and physics. In many of these
applications the classical method of electrostatic and magnetic
de
flection described in this paper has been replaced by timing
methods. The highest precisions are obtained by cyclotron
resonance in which the frequencies of ions rotating in a uniform
magnetic
field are measured and analysed by Fourier transform
techniques. Sophisticated ionisation methods have been
developed for the analysis of complex biomolecules and
macromolecules.
31
Aston’s later life
Aston’s
first mass spectrograph brought him immediate
acclaim. He was appointed to a Fellowship in Trinity College,
Cambridge in 1920 and was made a Fellow of the Royal Society
in 1921. He was awarded the Nobel Prize in Chemistry in 1922
for, in the words of the citation, ‘his discovery, by means of
his mass spectrograph, of isotopes in a large number of non-
radioactive elements, and for his enunciation of the whole
Fig. 11
The research students and sta
ff of the Cavendish Laboratory
in 1922, the year that Aston was awarded the Nobel Prize. Aston is
fourth from the left in the front row, Thomson is
fifth, and Rutherford,
the Cavendish Professor, is sixth. Edward Appleton is second from the
right in the front row, Patrick Blackett is second from the right in the
second row, and Peter Kapitza is at the right-hand end of the back row.
Fig. 12
Aston working with his third mass spectrograph, ca. 1937.
J. Chem. Soc., Dalton Trans., 1998, 3893–3899
3899
number rule’. In proposing the toast of the laureates at a dinner
in December of that year, Svante Arrhenius, the Director of
the Nobel Institute, commented that never before had the
Nobel Prize been handed over to a group of such distinguished
laureates, which, besides Aston, included Niels Bohr, Albert
Einstein, and Frederick Soddy. The last two were the 1921 prize
winners in Physics and Chemistry respectively, but the awards
were made in 1922.
Aston never married and for the last 35 years of his life lived
in Trinity College. Outside his work his main interests were
sport, travel and music. He was a keen cross-country skier, and
played tennis up to tournament class. He played golf in a
famous foursome with Ernest Rutherford, Ralph Fowler, and
Geo
ffrey (G. I.) Taylor. He was an enthusiastic cyclist, once
cycling 200 miles in 22 h. He was also an excellent photographer
and combined this hobby with his love of travel to help at
several solar eclipse expeditions. He was an omnivorous reader,
Sherlock Holmes being his favourite. An acquaintance once
described him as the highest lowbrow that he had ever met.
Aston died on 20 November 1945. In an obituary in Nature
32
G. P. Thomson wrote ‘Aston was a man in whom a great zest
for life was combined with a simplicity of character almost
approaching naivety. Though a good occasional lecturer, he
had no gift for teaching, and a few early attempts were not
persisted in. His attitude to physics was essentially that of the
experimenter and visualizer. He preferred the model to the
equation, the concrete to the abstract. He was a Conservative
in politics as in life, and though he would admit that a change
might be good, he preferred it to happen as gradually as
possible’.
In summary, Aston was not only a one-experiment man, he
was e
ffectively a one-instrument man. However, what he meas-
ured was of the highest importance and he did it extremely well.
To quote G. P. Thomson once more,
33
‘Aston was a superb
experimenter. His
first mass spectrograph was a triumph; few
but he could have got it to work at all’.
Acknowledgements
Fig. 8 is reproduced from the Philosophical Magazine, 1920, by
permission of Taylor & Francis. Figs. 5 and 9 to 12 are from the
Photographic Archives of the Cavendish Laboratory.
References
1 G. Hevesy, Obituary Notices Fellows R. Soc., 1948, 5, 635.
2 B. B. Boltwood, Am. J. Sci., 1907, 24, 307. The nomenclature is that
used at the time. The thorium referred to here is
232
Th; ionium is
230
Th; radium B, so called because it is one of the decay products
following from radium, is
214
Pb.
3 E. Rutherford and E. N. Da C. Andrade, Philos. Mag., 1914, 27,
854.
4 F. Soddy, Nature ( London), 1913, 92, 399. The word isotope was
suggested to Soddy by Dr. Margaret Todd, a medical doctor with a
practical knowledge of Greek (A. Fleck, Biogr. Mem. Fellows R.
Soc., 1957, 3, 203).
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6 W. Wien, Verh. Phys. Gesell. Berlin, 1898, 17, 10.
7 J. J. Thomson, Philos. Mag., 1907, 13, 561.
8 J. J. Thomson, Royal Institution, Weekly Meeting, January 17, 1913.
9 The same method of gaseous di
ffusion was employed at Oak Ridge
during the second World War to separate the uranium isotopes 235
and 238 in the gas UF
6
.
10 G. P. Thomson, J. J. Thomson and the Cavendish Laboratory in his
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11 F. W. Aston, Philos. Mag., 1919, 38, 707.
12 F. W. Aston, Nature (London), 1919, 104, 334.
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14 F. W. Aston, Philos. Mag., 1920, 39, 611.
15 F. W. Aston, Isotopes, Edward Arnold, London, 1922. Aston wrote
a second book in 1933, entitled Mass-Spectra and Isotopes, that
dealt more with the experimental results and less with the general
theory. This was followed by a second edition in 1942.
16 J. Chadwick, Proc. R. Soc. London, Ser. A, 1932, 136, 692.
17 F. W. Aston, Proc. R. Soc. London, Ser. A, 1927, 115, 487.
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20 B. W. Petley, The Fundamental Physical Constants and the Frontier
of Measurement, Adam Hilger, Bristol and Boston, 1985, p. 93.
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Fundamental Physical Constants, Codata Bulletin, 63, Pergamon,
Oxford, 1986.
22 A. J. Dempster, Phys. Rev., 1918, 11, 316.
23 K. T. Bainbridge, Phys. Rev., 1932, 40, 130.
24 K. T. Bainbridge, Phys. Rev., 1932, 42, 1.
25 K. T. Bainbridge, Phys. Rev., 1933, 44, 123.
26 R. Herzog and J. H. E. Mattauch, Ann. Phys., 1934, 19, 345.
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28 H. E. Duckworth, R. C. Barber and V. S. Venkatasubramanian,
Mass Spectroscopy, 2nd edn., Cambridge University Press, 1986,
ch. 5. This book gives an excellent account of the subject.
29 The term mass spectrograph is reserved for an instrument that
records the spectra on a photographic plate, an instrument that
employs electrical detection being termed a mass spectrometer. The
last reported use of a mass spectrograph was in 1972.
30 Ref. 28, p. 159.
31 P. G. Gates, World Wide Web, http://www-methods.ch.cam.ac.uk
32 G. P. Thomson, Nature (London), 1946, 157, 290.
33 Ref. 10, p. 139.
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