Why is the first vaporized fraction removed during molybdic oxide purification?

During the purification of technical molybdic oxide into chemically pure molybdenum trioxide (MoO${}_3$) via sublimation, the first vaporized fraction—which accounts for the initial 2–3 percent of the charge—is removed because it contains the majority of the volatile impurities. By discarding this initial fraction, the process ensures that the subsequent, final fraction achieves the high level of purity (99.95 percent) required for the production of molybdenum chemicals and metallic molybdenum.1