Janssen is senior scientist Analytical Chemistry at Unilever R&D Wageningen and part-time professor Recognition-based Analytical Chemistry at WUR. Janssen obtained his PhD at the Eindhoven University of Technology in 1991. After a career at that university as assistant- and associated professor he joined Unilever in 1999 to become senior scientist compositional analysis. In 2004 he also became extraordinary professor at Amsterdam University, focusing on research and development of systems for the analysis of (bio)macro-molecules. In 2019 he left Amsterdam University and joined the Laboratory for Organic Chemistry of the WUR as a part-time professor next to his position at Unilever. Janssen has been secretary of the working group Separation Methods of the Royal Dutch Chemical Society and board member of TI-COAST, the Dutch public-private partnership on analytical science and technology. Janssen (co-)authored more than 200 peer-reviewed papers covering a wide range of analytical technologies, including mass spectrometry, chromatography and hyphenated analyzers. His specific focus area is food quality analysis.