[citation needed] Ruben and colleague Martin Kamen, a University of Chicago Ph.D. and researcher in chemistry and nuclear physics working under Ernest O. Lawrence at the Berkeley Radiation Laboratory, set out to elucidate the path of carbon in photosynthesis by incorporating the short-lived radioactive isotope carbon-11 (11CO2) in their many experiments between 1938 and 1942.
Aided by the concepts and collaboration of C. B. van Niel, at Stanford University's Hopkins Marine Station, it became clear to them that reduction of CO2 can occur in the dark and may involve processes similar to bacterial systems.
This interpretation challenged the century-old Adolf von Baeyer theory of photochemical reduction of CO2 adsorbed on chlorophyll which had guided decades of effort by Richard Willstätter, Arthur Stoll, and many others in vain searches for formaldehyde.
Their results, confused by absorption of the products on proteinaceous residues, initially failed to reveal the path of carbon in photosynthesis but succeeded in exciting the interest of scientists worldwide in the search and revelation of metabolic processes, beginning a revolution in biochemistry and medicine.
[citation needed] Use of carbon-14 in tracer experiments was hindered by the difficulty measuring the weak beta emission of the radioactive decay and by the onset of World War II that shut off production of the isotope.