Reduce, Reuse, Recycle
11 March 2012
Well, hello. I admit I have been lazy lately. To top off the laziness, I will open this page with an essay I wrote in 2009.
There have been vast armies of forgotten scientists for every one remembered by the general populace. The stories of those men and women, those lost in the corners and crevices of history, continue to exist through those they had influenced and the knowledge they had strove so hard to obtain and share. The following is the story of a man who quietly changed the world from his bench at a small laboratory in Cambridge, England. A man whose beloved hobby just so happened to lead him to some of the most incredible conclusions ever realized in the field of atomic physics . . .
MASS
REVOLUTIONARY AT CAVENDISH LABORATORY
Nick
Mulder
Great minds are often built
on foundations of childhood imagination and mischief. Francis
William Aston was a natural-born explorer and tinker, and thus fit
the mold well. Helen Aston, sister of Francis, recalled that he was
enamored with fireworks and mechanical objects from a young age.l
His first memorable experiment involved the study of soap bubbles,
soon followed by his study of the reaction of sulfuric acid and zinc.
The latter was carried out in secrecy in his family’s pigsty. The
real experimentation began after building himself a laboratory over
the stable on his family farm. His adoration of fireworks drove him
to design small picric acid bombs and giant tissue paper fire
balloons, which he used to put on shows for the community. He had no
fear of delving into complex technical issues, and dedicated his time
to things such as glassblowing and “winding miles of wire for his
x-ray coil.”1 The laboratory over the stable was an
excellent place for him to hone his scientific methods, but formal
studies allowed him to explore in ways that the farm could not.
Francis
William Aston was born August 31, 1877, in Camomile Green, England.2
His father was a metal merchant, and his mother the daughter of a
prominent gunsmith. Aston spent most of his childhood playing on his
family’s small farm, as their financial security allowed him ample
time to build and explore. His schooling began under a woman by the
name of Misses Tonks in Harbome and at age 12 he was enrolled in
Harborne Vicarage School. Aston’s introduction to formal
scientific instruction began two years later at Malvern College where
he quickly rose to the top of his class in both mathematics and
science. He moved his studies to Mason College two years later. The
stay at Mason allowed him to feed his scientific appetite. Much of
his free time was spent reading in the Grand Library, honing his
glassblowing skills, and mastering the art of carpentry. Aston loved
woodworking so much that he happily participated in the construction
of the first chemistry laboratory at Mason. His formal studies there
included chemistry instruction under Franklin and Tilden, and physics
instruction under Poynting.3
After
passing London Matriculation, Aston moved back into his father’s
house where he built a laboratory in the attic. His initial studies
focused on organic chemistry and “... in 1898 he was awarded the
Forster Scholarship to work with Franklin on the optical rotational
powers of a complex tartaric derivative.”l They
finished their research and published a paper on the subject in 1901.
It then became clear to Aston that he needed to find a source of
income. With the intention of becoming a brewer, he enrolled at the
School of Brewing where he studied fermentation chemistry under
Adrian Brown. He was swiftly hired by Messrs. Butler & Co. in
Wolverhampton and held the position for three years. Aston’s
scientific studies, however, never took a back seat to his
occupation.
Throughout
his time as a brewer he had remained fascinated with vacuums and
x-rays, which had been discovered in 1875. Much time was spent
improving variations on the Sprengel pump, an instrument he had been
making since childhood. Aston had become, by this time, an excellent
glass blower. As his skill improved, so did his ability to make more
precise pieces. He was able to make an “induction coil capable of
giving a 3-inch spark”l and built his own version of a
Toeler pump. His research led him to study electrical discharges in
gasses. Of particular interest was what is called the Crookes dark
space. Crookes dark space is a dark area located between the cathode
glow and the vacuum glow in a low-pressure vacuum tube. Aston found
while using the Toeler pump that he could make “a form of
irreversible discharge tube with which the discharge of an induction
could be rectified.”1 This meant he had found a way of
measuring the Crookes dark space accurately. The great realization
led him to return to Mason College, which had since become Birmingham
University.
Aston
was given a scholarship at Birmingham University to study the
electrical discharges. The research mainly involved measuring the
Crookes dark space of gases at varying currents and pressures. He
published his first paper on the subject, “The length of the
Crookes dark space” in Proceedings of the Royal Society in 1905.1
Further research into the subject earned Aston his first true
discovery in 1907, which was detailed in the paper “The discovery
of a new primary dark space,” also published in Proceedings of the
Royal Society. Dubbed ‘Aston dark space,’ his discovery was that
of an intensely dark region within the Crookes dark space.
Quantification of the properties of this region “[pointed] to a
fresh way for finding the respective contributions of positive ions
and electrons to the current.”4 The Aston dark space
existed in a thin region directly next to the cathode in a Geissler
discharge. In 1908, upon the death of his father, Aston took leave
from his work to travel the world. From 1908-1909 he traveled
throughout the South Pacific, Canada and the United States. A warm
welcome met him upon his return to Birmingham University where he was
granted the position of physics lecturer. During his first term as
lecturer a man named Sir J.J. Thompson contacted Aston. At the
request of Thompson, a prominent British physicist, Aston promptly
left Birmingham University and moved his studies to Cavendish
Laboratory at Cambridge.
Thompson
was motivated to contact Aston by J.H. Poynting, a good friend of
Thompson and former professor to Aston.1 Poynting had
realized Aston’s potential very early in his career. The research
Aston and Thompson performed was very similar in nature, thus
Poynting decided it necessary for the two great minds to collaborate.
Thompson
had made a name for himself by this time and was very well respected.
His research on cathode rays caused him to ponder the nature of
electricity, and in 1906 he made known his theory of the electron,
the basic unit of electricity. In 1907 Thompson became interested in
a discovery made in 1886 by Eugen Goldstein. While studying the same
types of gas discharge in which Thompson was interested, Goldstein
discovered that if a slit were to be cut in the cathode, “a beam of
light appeared remote of the anode” which was characteristic of the
specific gases involved.5 In 1898, Wilhelm Wien found
that these beams of light could be deflected by a strong magnetic
field, and that the deflection paths indicated the beams consisted of
positively charged particles of atomic dimensions.5 The
subject matter was intriguing to Thompson, who began to study the
positive rays in 1907. Just as negatively charged particles formed
at the cathode, species at the anode were positively ionized.
Positive rays were attracted to the cathode, whose high field
strength allowed the ions to pass directly through the slit. The
positive rays were qualitatively different from the negative rays, as
they changed color along the path and were not very responsive to
magnetic fields as shown by Wien.
J.J.
Thompson performed the first successful deflection of positive rays
some time in 1909. Using both magnetic and electrostatic fields in
parallel, he found the rays could be separated into beams of their
constituents based on their response to the fields. The
crossed-deflection paths, recorded on photographic plates, allowed
Thompson to determine quantitatively how many different species were
present and their respective charge to mass ratio. The paths formed
by these particles were parabolic with their “axes parallel to the
direction of the electrostatic field.”1
Recently
hired by Thompson, Aston spent a great deal of time working on the
already operational instrument. His extensive background in
high-vacuum research and mastery of glassblowing gave Aston a very
solid foundation. The instrument operated best at very low
pressures, so precise parts were of utmost importance. Aston, the
perfectionist that he was, believed that the separation and precision
achieved by Thompson could be greatly improved upon. Barring a brief
intermission during WWI, the next few decades of his life were
dedicated to evolving Thompson’s primitive apparatus to one of the
most technically complex, yet incredibly versatile analytical
instruments used today.
The
instrument as designed by Thompson had very poor resolution.
Improvements made with the help of Aston allowed “parabolas
corresponding to mass differences of 10%”5 to be
resolved by 1912. Early research became puzzling to the men during a
study of the parabolic paths of neon gas. As purification was still
an emerging art at the time, it was not uncommon for impurities to
appear in samples. The mystery, however, involved two parabolas on
the plate corresponding to the atomic masses of 20 and 22. The
parabola at 20 was attributed to 20Ne, but the nature of
the much lighter parabola corresponding to a mass of 22 was unknown.
Thompson believed it to be NeH2 or doubly charged CO2.
Aston had a quite different hypothesis influenced by recent research
into radioactivity.
Radiation
was discovered by Henri Becquerel in 1896. Frederick Soddy coined
the term isotope to describe radioactive elements with similar
properties in 1913.5 Soddy realized that “[isotopes]
are chemically identical, and, save only as regards the few physical
properties which depend upon atomic masses directly, physically
identical too.”5 Aston hypothesized that isotopes were
not exclusive to radioactive elements. He thought the lighter path
corresponding to a mass of 22 was due to 22Ne. If the
isotopes were abundant in a ratio of 9:1 20Ne:22Ne,
the average weight would be equal to the accepted weight of Ne gas,
20.2 amu. After several failed attempts to separate the components
by fractional distillation, he halted his research and left The
Cavendish Laboratory to join the war effort.
Aston returned to Cavendish after the war with a head full of ideas. Initial research had shown the limitations of the parabolic method they had been using. Although a good and easily quantifiable method, he realized precision could be improved. As stated by Aston, “many rays are lost by collision in the narrow canal-ray tube, the mean pressure in which must be at least half that in the discharge bulb; very fine tubes silt up by disintegration under bombardment; the total energy available for photography falls off as the fourth power of the diameter of the canal-ray tube.”7 He overcame the first obstacle by evacuating both chambers and the space between them as much as possible. Silt was done away with by replacing the canal-ray tube with one made of aluminum, a much more corrosion-resistant material. The real trouble with the method involved controlling the intensity of the parabolas without jeopardizing accuracy and precision. Aston sought to create the finest parabolas possible without sacrificing intensity or separation. He found that magnetic field strength was proportional to the ray thickness, but that any change in the magnetic field strength had to be offset by a corresponding change in the strength of the electrostatic field. Changing the electrostatic field strength directly affected the intensity by drawing out the parabolas. He eventually realized that using two parallel slits in the cathode could increase the beam intensity. With this method there was no change in beam dimensions. The positive beams were focused using a method described by Dempster in 1918.8 Aston stated that “beams of charged particles which are homogeneous electrically (constant mv2/e) or magnetically (constant mv/e) can be focused like rays of light,” but that those generated by a discharge bulb are heterogeneous both electrically and magnetically.9 His remedy for this problem was to design a device that focused rays of constant mass regardless of their velocity, which varied throughout the path. The focused beams were more intense and allowed the use of finer slits, further increasing the instrument’s accuracy.
The
first incarnation of Aston’s mass spectrometer was a much more
complex instrument than Thompson’s early design. The positive beam
traveled toward the cathode through a low-pressure discharge tube.
The beam would then pass through two narrow parallel slits in the
cathode. The “resulting thin ribbon,” as Aston called it, “[was]
spread out into an electric spectrum by means of the parallel
plates.”9 A diaphragm was used to select which rays
would pass through the magnetic field. This field deflected the rays
toward a photographic plate, also designed by Aston. Of specific
interest is the fact that Aston separated the two fields, whereas
Thompson directly crossed them. Accurate quantification of relative
masses was possible with this instrument, allowing Aston to begin his
research on elemental masses.
He
resumed research on neon gas after the completion of his first mass
spectrometer in 1919. The first spectra had incredible resolution
and accuracy compared to those created by the previous instrument.
It once again showed the existence of parabolas corresponding to
masses of both 20 and 22, although much more clearly. Still very
confident in his theory of isotopes and isotopic abundance, Aston
moved to other gases. He soon discovered chlorine existed as 35Cl
and 37Cl, another surprising realization. Some elements,
such a krypton, were more complex. Krypton itself had isotopes of
78, 80, 82, 83, 84 and 86. The method proved successful for a number
of species, amounting to 27 elements by 1922.5 Aston’s
work during this period was immensely important to chemistry and
physics. He had proven the existence of isotopes of light,
non-radioactive elements and had stumbled upon the realization that
the masses of elements are whole numbers. The whole number rule, as
it was called, had serious implications for atomic theory.
Aston
initially saw the whole number rule simply as proof of his theories
of isotopes and abundance. The combined research of Aston and
Rutherford, however, had also led to the formulation of a feasible
physical model of the atom. Rutherford determined that atoms
consisted of a nucleus surrounded by mostly empty space during his
experiments with gold foil. This basic description was improved by
the whole number rule, which helped to describe the composition of
the nucleus. The discovery of one deviation from the whole number
rule, 1.008H, also had more weight than initially realized
by Aston.
The
1920’s were an era of great advancement in chemistry and physics.
Quantum theory was still in its infancy, unproven and extremely
complicated. Physicists attempting to explain the structure of atoms
and their relation to physical properties were baffled at first by
Aston’s discovery, but soon devised an incredible theory. It was
determined that the mass of hydrogen deviated from the whole number
rule due to its physical nature. Using Einstein’s theory of
mass-energy equivalence, it was decided that H deviated slightly from
the rule because it is the only non-composite element. All other
elements, composed of more than one H atom, have reduced masses due
to the binding energy in the nucleus. Aston’s research
reverberated throughout the scientific community. The success of his
instrument was monumental, and in 1925 it was dismantled to provide
parts for the second incarnation.
The
second mass spectrometer had greatly improved resolution and
accuracy. The care Aston took in designing the instrument was
unprecedented. He focused the parabolic paths by using finer slits
and orienting them a bit further apart. He altered the electric
field by curving the potential plates and increasing the deflection
to l/6 rad.5 The magnetic field was increased to 1.6T,
and the design of the poles changed from a circular cross-section to
a sickle shape. Most intriguing of all, Aston powered the instrument
with 500 lead accumulators, designed and built by himself, which
provided a constant voltage to within 1:105.5
The sole purpose of this instrument was to quantify deviations from
the whole number rule. Accurate to a degree of l:10,000, the spectra
obtained from the second mass spectrometer were of much higher
resolution. For example, the spectra of Hg on his first instrument
appeared to be a blur, but with the second Aston was able to identify
six separate paths corresponding to the six isotopes of krypton. The
third incarnation, built in 1937, trumped the former two with its
accuracy of l:l00,000.l This development was of specific
importance to nuclear scientists due to their need for extremely
accurate mass values.
The
significance of Aston’s research cannot be understated. His work on
mass spectrometry directly influenced knowledge of atomic structure,
elemental abundance, and the dependence of atomic properties on mass.
Rightfully recognized for his contributions early in his career,
Aston was presented the Nobel Prize in Chemistry in 1922. His later
endeavors, however, saw no slump. Between 1910 and 1940 the work of
Francis Aston fueled innovation in physics, chemistry and biology.
The author of Aston’s Nature obituary, G.P. Thomson, remarked that
“[the] use of isotopes as tracer elements both in chemistry and in
biology is only in its early stage, but even now the results are
highly important and it is difficult to put a limit on the
possibilities of this field.”3 Incorporation of
isotopes into modem chemical and biological techniques has increased
exponentially in recent decades and, unbeknownst to Aston, who passed
away in 1945, the mass spectrometer has become one of the most widely
used analytical tools in the world.
- Fin -
References
1. Hevesy, G. Obituary Notices of
the Fellows of the Royal Society 1948, 5,
1468-1475.
2. Downard, Kevin. Mass Spectrometry
Reviews 2007, 26, 713-723.
3. Thomson, G. P. Nature 1946,
157, 290-292.
4. Carmichael, M.; Emeleus, K. Nature
1928, 121, 14.
5. Squires, Gordon. J. Chem. Soc.,
Dalton Transactions 1998, 3893-3899.
6. Aston, F. W. Mass Spectra and
Isotopes; Longman’s, Green & Co.: New York, 1933.
7. Ref. 6, p. 38.
8. Ref. 6, p. 28.
9. Ref. 6, p. 39.
Written by Nick Mulder, 2009.
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