In a secondary ion mass spectroscopic (SIMS) method, and a mass spectrometer for implementing the method, for depth-profiling analysis of alkali metals in a sample which comprises an insulating material or is an insulator. The sample is irradiated by an ion beam as an analysis beam for desorption of secondary ions from the uppermost layers, such that the surface of the sample is removed with the same or a further ion beam. The ion beam used for removal of the sample surface comprises essentially gas clusters or consists of gas clusters.