The Background page of the PeakErazor enables you to perform two related but different functions:

Subtract background: If you are running a series of samples you may want to know if they share a common contaminant. You copy a number of mass lists onto the clipboard one at a time and then paste them into the Background page using the ‘+ Add’ button. Up to 10 mass lists can be pasted.
Then set the Combine and Precision fields (combine mass values when at least n values fall inside of the precision set). When satisfied, you press the ‘<blank>’ button and the mass values will be copied into the current Erazor list with the name <blank>. In this process they will replace any current mass values with the name <blank>.

Calibrate against a known protein digest: Copy a mass list onto the clipboard that contains the known MH+ values of the peptides of interest. This can most conveniently be done from GPMAW: From a peptide list (e.g. automatic digest) you right-click and from the pop-up menu you select ‘Copy/Export’ and then ‘Copy mass list MH+’.
You then paste it into the background list twice (press the ‘+ Add’ button twice), set the Combine field to 2 and the Precision to 10 ppm. Press the ‘Calculate’ button, this will put the whole mass list into the white list box. Pressing the ‘<blank> button puts the list into the Erazor list with the name of <blank> for each mass value.
You will now be able to calibrate any mass list against the known protein digest.

Example.