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The solid phase isobaric mass tagging (SP1MT) approach is presented for simultaneous protein quantitation and identification. The novelty of the SP1MT strategy relies on a CID-based differentiation of regioisomeric species for quantitalion of tagged proteolytic peptides. SPIMFs are unlabeled mass-tagging reagents, which consist of a reporter group, a mass balance group, and a spacer with a amine-specific reactive group, able to be linked to any N-terminal peptide. Therefore SPIIWF-llnked peptides from a two-plex set appear as a single unresolved precursor ion in MS, whereas the reporter groups lead to quantitation signals of m/z 168.2 and 182.2 Da upon tandem mass specUvimetiy (MS/MS) analysis with matrixassisted laser desorption time-of-flight/time-of-flight (MAh Dl TOF/TOF). This strategy allows ease protein identification by direct submission of MS and MS/MS data to the MASCOT database. SPIMf approach showed an excellent quantitation linearity, detecting any relative concentration differences of peptides in two solutions over a 5-fold concentration range without losing sequencing informadon. Therefore, SPIMT's are an attractive, simple, and low cost alternative for two-plex quantitation of proteins and offer possibilities of tuning the two-plex signal mass window by replacing the spacer. [ABSTRACT FROM AUTHOR]