The principle of TIMS is based on the formation of atom ions on a heated filament and the separation of these ions according to their mass/charge ratio in a mass spectrometer. In general, TIMS is applied for highprecision measurements of isotope ratios. In the basic design of a modern thermal ionisation source, a surface material is used in the shape of a flat ribbon filament. Usually, a solution (separated from the matrix and containing the element of interest in high concentration) is deposited in the centre of the filament as a liquid. Applying a current to the filament will evaporate and ionise the analyte deposited on the filament. In the case of a single-filament geometry, evaporation and ionisation of the analyte is accomplished with one filament, whereas in a double filament design, evaporation and ionisation is performed on two filaments placed parallel to each other. The analyte is evaporated on one filament, with ionisation occurring when the evaporated atoms strike the ionisation filament. A magnetic sector field is used as mass separator.