Here, a workflow for sample preparation and quantification of brain and pancreatic tissue proteins with use of heavy labelled protein standards (QPrESTs) is described. The standards are added before the digestion ensuring high reproducibility and elimination of nuances apearing during proteolytical cleavage of proteins and further steps such as solid phase extraction. QPrESTs are heavy labelled recombinant protein fragments covering regions of proteins with low homology and span over 50-150 aminoacids. They were produced as a part of Human Protein Atlas project in auxotrofic E.coli and individually purified, serving as an excelent resource for MS-based protein quantification.