Some aspects of this invention provide reagents and methods for the sensitive, quantitative and simultaneous detection of target analytes in complex biological samples by liquid chromatography tandem mass spectrometry (LC MS/MS). Some aspects of this invention provide affinity reagents encoded with mass reporters for the sensitive and quantitative translation of an analyte of interest into a mass tag. The reagents and methods provided herein have general utility in analyte detection and encoding, for example, in biomolecular profiling, molecular diagnostics, and biochemical encoding.