Dr. Karl Lark-Horovitz, professor of physics at Purdue, had a keen interest in the development of the cyclotron and the application of physical techniques to solve biological problems, and sought to develop methods that utilized radioactive tracers produced from the cyclotron. With the assistance of Leng and Donald Tendam, radioactive tracers were employed following an intense regimen to develop these methods. Key studies concerned sodium and potassium in the human body and their uptake, distribution and excretion; sodium and potassium distribution in human blood cells; and the analysis of enteric coatings for medications. Leng was awarded an American Association of University Women fellowship for work at Purdue. The fellowship permitted her the freedom to pursue the pioneer research on radioactive tracer materials.
In 1943, Leng moved to New York City to accept a faculty appointment in physics at Rensselaer Polytechnic Institute (RPI) and in 1966 was promoted to become RPI's first female full professor.