Label-free quantitative proteomics: Why has it taken so long to become a mainstream approach?

In recent years, mass spectrometry (MS) based proteomics has moved from being a qualitative tool (used to mainly identify proteins) to a more reliable analysis tool, allowing relative quantitation as well as absolute quantitation of a large number of proteins. However, the developed quantitative methods are either specific for certain types of samples or certain types of mass spectrometers. In some cases, developing expertise on how to use a given method may take a long time and the use of these methods is therefore limited to few laboratories. Other quantitative methods are suitable for simple standard protein mixes which are far from the complexity of real samples. As a consequence, the number of available quantitative methods is high and choosing the right one is challenging.