A multireflection time of flight mass spectrometer comprising: a primary ion mirror arrangement having a longitudinal axis, the primary ion mirror arrangement comprising a plurality of primary ion mirrors (10a, 10b, 10c, 10d) in stacked arrangement, each primary ion mirror (10a, 10b, 10c, 10d) having a major axis, a minor axis and a longitudinal axis, wherein each primary ion mirror extends a greater distance in its major axis than in its minor axis and wherein the longitudinal axis of each primary ion mirror (10a, 10b, 10c, 10d) is parallel to the longitudinal axis of the primary ion mirror arrangement, the stacked arrangement being such that the longitudinal axis of each primary ion mirror (10a, 10b, 10c, 10d) is offset in the direction of the minor axis from each adjacent primary ion mirror (10a, 10b, 10c, 10d), the major axis of each primary ion mirror (10a, 10b, 10c, 10d) is parallel to the major axis of every other primary ion mirror (10a, 10b, 10c, 10d) and the minor axis of each primary ion mirror (10a, 10b, 10c, 10d) is coincident with the minor axis of every other primary ion mirror (10a, 10b, 10c, 10d); wherein each primary ion mirror (10a, 10b, 10c, 10d) comprises: a top electrode parallel to a plane containing the longitudinal axis and the major axis; and a bottom electrode parallel to the top electrode such that a bottom electrode of at least one primary ion mirror (10a, 10b, 10c, 10d) is shared with a top electrode of an immediately adjacent primary ion mirror (10a, 10b, 10c, 10d) in the stacked arrangement; a secondary ion mirror generally opposed to the primary ion mirror arrangement, the secondary ion mirror having a major axis, a minor axis and a longitudinal axis, wherein the secondary ion mirror extends a greater distance in its major axis than in its minor axis and wherein the longitudinal axis of the secondary ion mirror is parallel to the longitudinal axis of the primary ion mirror arrangement, the secondary ion mirror comprising: a first side electrode parallel to a plane containing the longitudinal axis and the major axis of the secondary ion mirror; and a second side electrode parallel to the first side electrode; and a field-free region between the electrodes of the primary ion mirrors (10a, 10b, 10c, 10d) and the electrodes of the secondary ion mirror; characterised in that the major axis of each of the primary ion mirrors is perpendicular to the major axis of the secondary ion mirror and in that the minor axis of each of the primary ion mirrors is perpendicular to the minor axis of the secondary ion mirror. a primary ion mirror arrangement having a longitudinal axis, the primary ion mirror arrangement comprising a plurality of primary ion mirrors (10a, 10b, 10c, 10d) in stacked arrangement, each primary ion mirror (10a, 10b, 10c, 10d) having a major axis, a minor axis and a longitudinal axis, wherein each primary ion mirror extends a greater distance in its major axis than in its minor axis and wherein the longitudinal axis of each primary ion mirror (10a, 10b, 10c, 10d) is parallel to the longitudinal axis of the primary ion mirror arrangement, the stacked arrangement being such that the longitudinal axis of each primary ion mirror (10a, 10b, 10c, 10d) is offset in the direction of the minor axis from each adjacent primary ion mirror (10a, 10b, 10c, 10d), the major axis of each primary ion mirror (10a, 10b, 10c, 10d) is parallel to the major axis of every other primary ion mirror (10a, 10b, 10c, 10d) and the minor axis of each primary ion mirror (10a, 10b, 10c, 10d) is coincident with the minor axis of every other primary ion mirror (10a, 10b, 10c, 10d); wherein each primary ion mirror (10a, 10b, 10c, 10d) comprises: a top electrode parallel to a plane containing the longitudinal axis and the major axis; and a bottom electrode parallel to the top electrode such that a bottom electrode of at least one primary ion mirror (10a, 10b, 10c, 10d) is shared with a top electrode of an immediately adjacent primary ion mirror (10a, 10b, 10c, 10d) in the stacked arrangement; a secondary ion mirror generally opposed to the primary ion mirror arrangement, the secondary ion mirror having a major axis, a minor axis and a longitudinal axis, wherein the secondary ion mirror extends a greater distance in its major axis than in its minor axis and wherein the longitudinal axis of the secondary ion mirror is parallel to the longitudinal axis of the primary ion mirror arrangement, the secondary ion mirror comprising: a first side electrode parallel to a plane containing the longitudinal axis and the major axis of the secondary ion mirror; and a second side electrode parallel to the first side electrode; and a field-free region between the electrodes of the primary ion mirrors (10a, 10b, 10c, 10d) and the electrodes of the secondary ion mirror;