The dynamic range
In mass spectrometry, the dynamic range can be defined as the ratio between the minimal and the maximal concentration of compounds that can be detected simultaneously in a given sample.
This dynamic range depends on the sample complexity and on the type of mass spectrometer that is used. Thus, proteins present at very low concentration levels will be very difficultly or not detected in the presence of highly abundant proteins.
Depending on the sample, prefractionation methods can be very usefull to decrease the sample complexity and thus increase the number of detected proteins.
The protein expression dynamics combined with the dynamic range explain why it is impossible to detect all proteins present in a complex biological sample.