His chief scientific legacy comes from the Fifties and Sixties, when he established the surprising fact that in chemical reactions in gases in which one component of a molecule is eliminated there can be an intermediate stage in which the separation of electrical charges is important. He enlarged our understanding of chemical processes in gases into a comprehensive scheme that is largely unchallenged after 40 years. His work adds to the fundamental basis of knowledge from which technological advances arise.
Maccoll was born in Edinburgh in 1914. His family migrated to Australia when he was eight. They lived in the Sydney seaside suburb of Manly with its famous beaches and outdoor opportunities. He became a weekend surf lifesaver and represented Sydney University in water polo. Throughout his life he remained unmistakably Australian; he was gregarious, robust, genial and a bon viveur. He graduated from Sydney with first class degrees in Mathematics and Chemistry, and was appointed lecturer there as a very young man.
After the Second World War he was one of the first winners of the ICI and Turner and Newall Research Fellowships awarded to overseas students for postgraduate research. At this time there were no PhD programmes in Australia, and graduate students almost without exception were attracted to Britain. He arrived in 1945 with his wife Margaret, an economist, and joined Sir Christopher Ingold, one of the greatest chemists of the century, in his laboratories in University College London.
Allan Maccoll liked to tell of his reception in London. The illustrious and fully occupied Professor Ingold took immediate personal charge of the new boy, showing him the worst and the best of what there was left of the college after the 1941 bombings - the space where Physics had been, the near-destroyed library, the dreary and ill-maintained Department of Chemistry, and finally, with pride, the genius loci, the auto-icon of Jeremy Bentham, seen as the founding father of the university. The tour concluded with a glass of exceedingly weak post-war ale.
Maccoll's quality was quickly recognised; soon after completing his PhD he was appointed to the teaching staff and became the leader of the group of young chemists whom Ingold selected to restore the departmental research effort after a disruptive move to Aberystwyth and Bangor during the war. He rose through the academic grades, becoming professor in 1963. He spent his entire career in London at UCL. He was active in college and university affairs, serving on the Senate and Academic Council of the university in the years 1965-74.
He had taken to University College a research project stemming from his first work in Sydney with Thomas Iredale. It was to study the speed of a class of chemical reactions taking place in gases. Visitors to the laboratory he now set up saw a formidable apparatus spread across an entire laboratory wall. An enthusiastic Maccoll explained that it was centred on a reaction vessel containing the target gas. This vessel was joined to a complex array of glass tubing that allowed gases to be fed in and drawn out and the pressure measured. The construction was itself a technical feat of glass-blowing that few could have managed. Conclusions were built on large numbers of precise measurements of reaction rates made over months.
Following indications that were at first tenuous, then more and more inescapable, Maccoll showed that, against all received wisdom, some well- known reactions involved pulling apart electrical charges, and he developed a unified theory that brought together reactions in the gas and in solution in a single explanatory framework. It has proved a durable accomplishment.
Maccoll was one of the first in Britain to exploit the mass spectrometer in organic chemistry. In the mass spectrometer molecules are hit by fast electrons. Typically they become positively charged and break into fragments. The fragments are spread into a spectrum according to their masses and electric charges and can be identified.
His central purpose was to find the steps the original molecule took in its passage to the final fragments. He worked with instrument manufacturers in solving the technical problems of getting low- energy electron beams of precisely defined energy, doing less damage to the molecules and making easier the detective work of relating fragments to original targets. He thus illuminated fragmentation processes in broad classes of molecules. The achievement was recognised by the award in 1989 of the first Aston medal of the British Mass Spectrometry Society.
Maccoll's influence on mass spectrometry was great. He was the founding editor of the journal Organic Mass Spectrometry, which was of high importance in turning mass spectrometry from a poorly understood analytical art into a soundly based discipline. In the Sixties and Seventies he rarely missed a conference; his mastery of the subject and his gently phrased but incisive comments were looked for and respected.
He had many research students from developing countries and was sensitive and sympathetic to their problems of science education. He took the opportunity given by London University's scheme of Special Relationship to lecture in Ibadan, Ghana, Makerere, Nairobi, Dar es Salaam and Rhodesia, and for the British Council in countries of Eastern Europe, Chile and Venezuela.
Allan Maccoll made his contribution to chemistry through the originality of his research, but those who were lucky enough to have been his colleagues and his friends will remember him for his generous spirit, bonhomie and loyalty, and for his pride in and devotion to the institution he served.
Allan Maccoll, physical chemist: born Edinburgh 25 July 1914; Lecturer in Chemistry, University College London 1947-53, Reader 1953-63, Professor of Chemistry 1963-81 (Emeritus); Editor-in-Chief, Organic Mass Spectrometry 1969-81; married 1940 Margaret Riley (two daughters); died Claygate, Surrey 16 February 1999.