An inductively coupled plasma-time-of-flight mass spectrometer has been combined with a capillary gas chromatograph for the speciation analysis of inorganic and methylmercury. The derivatisation and volatilisation of the ionic mercury species from the sample was achieved by in-situ ethylation with sodium tetraethyl borate (NaBEt₄) in a purge-and-trap injection (PTI) system. The potential of the ICP-TOFMS for detection of fast transient signals with peak widths significantly shorter than 1 s with high temporal fidelity and ultra-trace sensitivity was demonstrated. A thorough optimisation of the instrumental parameters led to absolute detection limits of 20 fg and 1.5 pg and relative detection limits of 2 and 150 pg l⁻¹for methylmercury and inorganic mercury, respectively. The accuracy of the method was demonstrated by the analysis of two certified reference materials, a freeze dried tuna fish (CRM 464) and a dogfish muscle (DORM-2), and by recovery measurements on spiked samples. Application to ultra-trace speciation analysis of organomercury compounds in ice from the French Alps has been addressed.