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 was principal scientist at WFSR (formerly known as RIKILT) and holding a special chair on analytical chemistry at Wageningen University. He is a 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. From January 2017 until March 2021 he was the coordinator of the EU Marie Curie Innovative Training Network FoodSmartphone which aimed for the development of smartphone analyzers for on-site testing of food quality and safety. At the end of 2021 he retired from WFSR and on June 1st, 2022, he retired from Wageningen University, changing his status to emeritus special professor. Nielen (co-)authored more than 220 peer-reviewed papers (10k citations, H-index 56 in Scholar) covering a wide range of analytical technologies, including mass spectrometry, chromatography, capillary electrophoresis and biosensors.