Precursor ion quantification (for example, SILAC™) is an isotopically labeled quantification method. In precursor ion quantification, protein abundance is determined from the relative MS signal intensities of isotopically labeled samples and an unlabeled control sample.
However, this type of quantification does not necessarily have to use labeled and unlabeled samples. Some laboratories use heavy and medium labels to eliminate the bias of unlabeled peptides.
Stable-isotope labeling by amino acids in cell culture (SILAC) is a proteomics identification and quantification technique that uses in vivo metabolic labeling to detect differences in the abundance of proteins in multiple samples. SILAC is a type of isotopically labeled quantification that uses stable (non-radioactive) heavy isotopes as labels.
You can also introduce the stable isotopes by chemical labeling, for example, dimethyl labeling, at the protein or peptide level with isotopomeric tags.