Abstract: The invention describes compounds useful for labelling molecules of interest (i.e. analytes), particularly biomolecules such as peptides, proteins, oligonucleotides and nucleic acids, and also methods for analysing, detecting and/or isolating these labelled molecules using mass spectrometry. The compound in one aspect is a mass marker for labelling of an analyte detectable by mass spectrometry such as neutral loss mass spectroscopy, in which the mass marker comprises a neutral loss mass modifier linked via a first collision cleavable linker to a reactive group having reactive functionality for attachment to the analyte. The neutral loss mass modifier upon cleavage from the analyte during mass spectroscopy is uncharged.