Between approximately 1914-1918, she worked with Otto Hönigschmid at the Radium Institute of Vienna using analytical methods to demonstrate the first and second credible cases of isotopes in lead and thorium.

[6] At the recommendation of Lise Meitner,[1] Horovitz was recruited by Otto Hönigschmid at the Institute for Radium Research, Vienna in 1913 or 1914.

[2] Hönigschmid had studied under leading expert Theodore Williams Richards at Harvard and his work in determining precise atomic weights was well respected.

[1] Hönigschmid was asked by Fajans and Soddy to determine the atomic weight of lead from radioactive sources to demonstrate the existence of isotopes.

The purification process involved many rounds of washing, dissolving, filtering, and recrystallization to yield a lead chloride sample completely free of contamination.

[2] Horovitz and Hönigschmid later proved that ionium, a radioactive element discovered by Bertram Boltwood, was in fact an isotope of thorium.

[9] In a major career shift, she established a foster home in Vienna providing therapy for children with Alice Friedmann, an Adlerian psychologist.