 The study presents a high-resolution diagnostic method using an orbit-rap mass spectrometer to investigate OLED degradation, revealing that oxygen loss at the EMLE-TL interface is a dominant chemical degradation mechanism in devices with short lifetimes. Additionally, modifying host materials can increase device lifetime by minimizing interfacial degradation. This article was authored by Gustavo F. Trindade, Sir Juan Sol, June Hayak Kim, and others.