Why should we prepare protein samples from dicing SDS-polyacrylamide gels for mass spectrometry? Does this step the so-called "reduction" in the LC-MS/MS sample preparation workflow? Is this step unnecessary in Label-free quantitation? Does the necessity of this step depend on the quantitation methods? Confused. Consider that the protein mixtures from a specific treatment sample would be digested into smaller pieces of peptides for further detection. This molecular weight of peptides should be all under 10kDa. So, what's the difference between dicing a band with a specific range of molecular weight for subsequent analyses and by directly uploading a total mixture of trypsin digested peptides? And, if it should be uploaded with a particular band within a specific range of molecular weight, what's the optimal range?

In addition, what's the foundamental principle to estimate the protein abundance by integrating information of its derivative peptides?