What does “component” or “SCX classification” in iTRAQ/TMT experiment represent?

The iTRAQ/TMT experiment performed 2D-LC-MS on the peptides. The first-dimensional chromatographic analysis mainly used SCX or high-pH HPLC to perform preliminary separation of the labeled peptide mixed samples. The sample is separated into 10 components to reduce the complexity of the peptide sample in each component, and then the ten components are separately subjected to LC-MS to achieve better mass spectrometry results.