The atom probe field ion microscope is a powerful technique for the quantification of solute segregation at internal interfaces due to its ultrahigh spatial resolution. The segregation behavior for all elements is determined by collecting the atoms that originate in a cylinder centered on the boundary region. The method of analysis is dictated by the orientation of the boundary in the field ion specimen. The general case and the two special orientations are shown schematically in Fig. 1. The special case where the unit normal to the interface plane is at 90° to the unit vector parallel to the cylinder of analysis, ϕ = 90°, permits a large number of atoms to be collected but the spatial resolution is defined by the effective diameter of the probe aperture and is typically chosen to be between 1 to 5 nm. The special case where ϕ = 0° provides the highest spatial resolution but only a limited number of atoms are collected from the boundary in each cylinder of analysis.