Mass Spectrometry Overcomes PD-L1 IHC Limitations Associated with Aged Tissue. PD-L1 IHC was evaluated in stored tissue with mass spectrometry (MS). Increased humidity and temperature resulted in considerable immunoreactivity loss, particularly among antibodies recognizing extracellular and discontinuous epitopes, which could be partially mitigated with the use of desiccant. However, even in significantly affected tissues MS was able to quantitate PD-L1 expression and assess peptide oxidation. These results suggest MS is suited for biomarker assessments using stored sections—a sample type commonly encountered in oncology research.