Bubble plasma ionisation probe for analysing liquids by mass spectrometry. A means of a detecting analytes dissolved in a liquid by mass spectrometry is described. Gas flows from a source through a first conduit 105 and thereafter through a coaxial second conduit 103 that also serves as the inlet to the mass spectrometer 102. The coaxial arrangement of conduits is submerged in the liquid to be analysed 301. Using a feedback loop, the gas pressure is adjusted and controlled such that an attached bubble 302 forms at the open end of the first conduit 105. A plasma 305 is provided in the bubble. The plasma is preferably generated by a dielectric barrier discharge between a collar electrode 107 and mass spectrometer inlet 103. Analytes dissolved in the liquid are both desorbed form the gas-liquid interface and ionised by the action of the plasma. Ions formed in this way become entrained in the gas flow and are consequently transferred to the mass spectrometer, where they are analysed.