https://phys.org/news/2018-06-proteomics-method.html

The basic tool for counting proteins is a machine called a mass spectrometer. Cell samples can be run through this type of instrument one at a time, but this is laborious and it can be difficult to detect any changes between different samples. An alternative approach is to label all of the proteins in a particular sample with a unique "isobaric" tag. Multiple samples—up to 11—can then be mixed together and run through the mass spectrometer at the same time, with the isobaric tag functioning as an identifying barcode that tells the researcher which sample the protein originally came from. This speeds things up and makes it easier to quantify any changes in the protein composition of different samples.

"However, with the simplest version of isobaric tagging, known as TMT-MS2, there are major difficulties in distinguishing real signals from background noise," Wühr explains. "That makes the readouts unreliable and only semi-quantitative."