A method of determining the structure of a macromolecular assembly (MMA) comprises the steps of (a) generating precursor ions of an MMA species to be investigated; (b) transporting the MMA precursor ions to a fragmentation zone; (c) carrying out pulsed fragmentation of the MMA precursor ions in the fragmentation zone; (d) for a first plurality of MMA precursor ions, detecting both a spatial distribution of the resultant MMA fragment ions, and an m/z distribution of the MMA fragment ions; (e) analyzing the spatial and m/z distributions of fragment ions formed from the said first plurality of precursor ions of the MMA species to be investigated, to determine the relative positions of those fragment ions within the structure of the precursor MMA; and (f) reconstructing the three dimensional (3D) structure of the MMA from the analysis of the spatial and m/z distributions of fragment ions.