This brief report presents mass spectral resolution of H3O+ and F+ at mass 19 in a high vacuum system. Typically, this mass spectral peak observed in residual gas analyzers has always been attributed to fluorine. Using Fourier transform mass spectrometry, the presence of the hydronium ion was fully resolved from fluorine and it is shown that there is ample time,for the formation of the hydronium ion through ion-molecule interactions. (C) 2003 American Vacuum Society.