We describe the design and performance of a spectrometer that simultaneously measures the size-dependent concentration and chemical composition of particles ranging from 5 to 40 nm. The spectrometer consists of a differential mobility analyzer for size analysis and an inductively coupled plasma mass spectrometer for composition analysis. For a particle size of 30 nm, the lower detection limit of the spectrometer for particle concentration is about 1 x 10(5) particles cm(-3). (C) 2002 Elsevier Science Ltd. All rights reserved.