A substantially planar porous substrate has an end which tapers to a point; the substrate may be triangular. A skin-print and a fluid (perhaps a methanol:water solvent) are applied to the substrate. A voltage (maybe 4.5 kV) is applied to the substrate to generate an electric field. This ionises droplets of the fluid, which are emitted from the point as a Taylor cone and analysed using a mass spectrometer. A developing agent, e.g. Ninhydrin or silver nitrate, may be applied to the substrate and developed using electromagnetic radiation or ultraviolet light, and colourmetric analysis performed. In an aspect, a cartridge comprises a housing and a substantially planar porous substrate having an end which tapers to a point. The cartridge may comprise a shutter which may be moved to enable capture of a skin-print on the substrate, then closed to secure the sample for transport. A tamper-evident feature may be provided.
