Nielen obtained his PhD at the VU university (Amsterdam, NL) in 1986. After a career at TNO and in chemical industry at AkzoNobel, he became program manager veterinary drugs at RIKILT Wageningen UR. In 2007 he also became extraordinary professor at Wageningen University, focusing on research and development of bioactivity-related multimethods for the detection of chemical contaminants in the food chain. Since 2012, he is principal scientist at WFSR (formerly known as RIKILT) and holds a special chair on analytical chemistry at Wageningen University. He is former president of the Dutch Society for Mass Spectrometry. From 2011 till 2016 he also acted as scientific director of TI-COAST, the Dutch public-private partnership on analytical science and technology; currently he is executive board member of that organisation. Since January 2017 he is coordinator of the EU Marie Curie Innovative Training Network FoodSmartphone which aims for the development of smartphone analyzers for on-site testing of food quality and safety. Nielen (co-)authored more than 195 peer-reviewed papers (>8800 citations, H-index 52) covering a wide range of analytical technologies, including mass spectrometry, chromatography, capillary electrophoresis and biosensors.