This invention relates to a mass spectrometer including a reaction cell and to a method of using such a mass spectrometer. In particular, although not exclusively, this invention relates to a tandem mass spectrometer and to tandem mass spectrometry. The invention provides a method of mass spectrometry using a mass spectrometer having a longitudinal axis, comprising guiding ions to travel along the longitudinal axis of the mass spectrometer in a forwards direction to pass through an intermediate ion store and then to enter a reaction cell, to process the ions within the reaction cell, to eject the processed ions to travel back along the longitudinal axis to enter the intermediate ion store once more, and to eject one or more pulses of the processed ions in an off-axis direction to a mass analyser.