Background is the mass spectrum of all substances present in the MSD in the absence of a sample. When a sample is injected, the mass spectrum becomes the spectrum of the background plus the spectrum of the sample. A new MSD, or one that has recently been vented, typically has many background peaks. This decreases with time. Give the MSD plenty of time to pump down. Make sure the ion source, mass filter (quad), and GC/MSD interface heaters are on and at normal operating setpoints. If high background is present after more than one day under vacuum, check the following: