A composition formed of a mixture of a first compound and a second compound wherein the first compound has a different chemical structure than the second compound. The first compound and the second compound are both organic compounds. At least one of the first compound and the second compound contains at least one less abundant stable isotope atom. The first compound has an evaporation temperature T1 and the second compound has an evaporation temperature T2 where both T1 and T2 are between 100 to 400 ° C. and the absolute value of T1-T2 is less than 20 ° C. The first compound has a concentration C1 in said mixture and a concentration C2 in a film formed by evaporating said mixture in a high vacuum deposition tool with a chamber base pressure between 1×10−6 Torr to 1×10−9 Torr, at a 2Å/sec deposition rate on a surface positioned at a predefined distance away from the mixture being evaporated; and wherein absolute value of (C1-C2)/C1 is less than 5%.