What is the purpose of the desalting columns used in many of your kits (Zeba spin columns)?

These columns are size exclusion columns with a MWCO of 7 kDa. These are used to exchange buffers – to put the biomolecule to label in the correct buffer, such as a modification buffer. Small contaminants (e.g., various buffer components under 7 kDa will go into the bead matrix or resin, and retained, as larger molecules flow through the column. This both removes buffer components which might interfere with the reaction and puts the biomolecule of interest into a new buffer.