Abstract: The characterisation of individual nanoparticles by single particle ICP-MS (SP-ICP-MS) was carried out at the Forel Institute focussing on gold, various metal oxide and clay colloids. SP ICP-MS has paved the way for the analysis of smallest biological systems. The current presentation summarise how to adapting this method for single viruses (SV) identification and counting. With multi-channel high resolution ICP-MS records in SV detection mode, the counting of master and key ions can allow analysis and identification of single viruses. The counting of 2-500 virial units can be performed in 20 s. Analyses are proposed to be carried out in Ar torch for master ions: 12C+, 13C+, 14N+, 15N+, and key ions: 31P+, 32S+, 33S+ and 34S+. All interferences are discussed in detail. Applications for two virus types: SARS-COV2 (RNA) and T5 sipho bacteriophage (DNA) are investigated using time scan and fixed mass analysis. Recommendations for interference free assays are emphasised together with optimisation of VOC vs DOC (and CO2) signals. For the selected virus characterisation the use of N/C, P/C and S/C molar isotopic ratio’s is carried out together with the quantification of their number concentration. Biography: After a Master 71-72 in analytical and radio- chemistry at the University of Liège, Claude Degueldre got his Doctorate degree in 1978 in the same Department. During his oversea Professorship (80-85) for the Belgian Foreign Affair and Cooperation Ministry at the University of Algiers and at the CEN, he familiarised with the ICP techniques for the analysis for traces of uranium in phosphate ores. During his 30 year activities at the Paul Scherrer Institute (85-05), he became the father of the Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) installation. He performed the first ICP-MS tests during the Grimsel colloid exercise (published as a CEC Report in 2000), and contributed to the analysis of actinides in solution. Visiting professor at The University of Geneva since 1990 he contributed to the installation of the hybrid ICP -MS –OES at the Forel Institute, University of Geneva in the late 90’s. He performed the first Single Particle Inductively Coupled Plasma Mass Spectroscopy (SP ICP-MS) tests at the Forel Institute in 2001 and presented the results in 2002 at the EMRS Spring Meeting and in a proceedings paper, see Degueldre & Favarger (2003). This method was later tested on gold nano- particles that are used as substrate in nano- pharmacy, and published subsequently i.e. Degueldre et al (2006). SP ICP-MS was also tested on dioxides e.g. ZrO2, ThO2, UO2, see Degueldre et al (2004 & 2006). Professor at the Engineering Department of The Lancaster University, he works on the extraction of uranium from seawater, analysing by ICP-MS uranium in solution down to the ppt level. This work is carried out at the LEC for a PhD project supervised by C. Degueldre in collaboration with Professor Dr Hao Zhang. He derived the SV ICP-MS methodology based on the SP ICP-MS in 2020 which concept was published in Feb. 2021.