The invention relates to the use of mass labelled probes to characterize nucleic acids by mass spectrometry. Thus the invention provides methods of detecting the presence of a target nucleic acid in a sample, using a circularizing probe in which a mass tag is present in the probe. Further methods of detecting the presence of a target nucleic acid are provided, which in contrast use a probe detection sequence in the circularizing probe, wherein the probe detection sequence is detected with a probe attached to a mass tag. Methods for determining a genetic profile from the genome of an organism also form part of the invention.