Quantitative proteomics studies require an absolute quantification step to accurately measure changes in protein concentration. Absolute quantification using liquid chromatography-mass spectrometry (LC-MS) traditionally combines triple quadrupole instrumentation with stableisotope-labeled standards to measure protein concentrations via their enzymatically produced peptides. Chemical modification of peptides using labels like mass differential tags for relative and absolute quantification (mTRAQ) provides another route to determine protein quantities. This chapter describes a cost-effective and high-throughput chemical labeling method that utilizes five amine-reactive, isotopic N,N-dimethyl leucine (iDiLeu) reagents. These tags enable generation of four-point calibration curves in one LC-MS run to determine protein concentrations from labeledpeptides. In particular, we provide a detailed workflow for protein quantification using the iDiLeu reagent that includes important considerations like labeling conditions and isotopic interference correction.
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