prescan

MS:
In QIT mass spectrometry when automatic gain control is active a prescan preceeds every analytical scan to determine the proper analytical ion injection time. It consists of a short (<10 ms) ion injection to determine the ion current.

AES:
In atomic emission spectrometry with CCD/CID detection systems with limited linear working range a prescan preceeds every analytical measurement to determine the proper exposure time for every channel in order to stay within the upper electron capacity of the detector element.