Abstract: The invention provides a method for labeling an analyte comprising a primary amino group, the method comprising: a. a labeling process comprising reacting the analyte with a dialdehyde in the presence of a label, wherein the label bears a charge, and b. an analysis process comprising subjecting the labeled analyte to MS, preferably LC-MS-MS. Herein, preferably, the labeling process comprises reacting an analyte with a dialdehyde, wherein the dialdehyde carries a label bearing a charge, to provide a labeled analyte carrying the charge. The present invention also provides a labeling method to provide a labeled analyte carrying a charge, wherein the labeling method comprises a labeling process comprising reacting an analyte with a dialdehyde, wherein the analyte comprises a primary amino group and wherein the dialdehyde carries a label bearing the charge. The dialdehyde is preferably an aromatic dialdehyde, most preferably an aromatic 1,2- or 1,3-dicarboxaldehyde.