Protein biopharmaceuticals have emerged as important therapeutics for the treatment of various diseases including cancer, cardiovascular diseases, diabetes, infection, inflammatory and autoimmune disorders. They come in many flavours and include monoclonal antibodies (mAbs), antibody-drug conjugates (ADCs), bispecific antibodies, antibody fragments, fusion proteins, hormones, growth factors, cytokines, therapeutic enzymes, blood factors, vaccines and anticoagulants. Given their obvious benefits in terms of safety and efficacy, these molecules have substantially reshaped the pharmaceutical market and today over 350 products have been approved for human use in the United States and the European Union. Protein biopharmaceuticals are commonly produced recombinantly in mammalian, yeast or bacterial expression systems. Next to the therapeutic protein, these cells produce endogenous host cell proteins (HCP) that can contaminate the biopharmaceutical product despite multiple purification steps in a process. Since these process-related impurities can affect product safety and efficacy, they need to be closely monitored. This application note reports on the use of micro pillar array columns (μPAC™) in combination with mass spectrometry (MS) for the characterization of HCPs and their monitoring during downstream processing.