Sometimes, particularly for chromatographic peaks with small signal-to-noise
ratios, the background ion signals are significant in the mass spectrum.
The can render interpretation of the spectrum difficult. In this case
it may be helpful to subtract a background spectrum from the spectrum.
Exercise: With the spectrum of interest selected and displayed (call this spectrum A), begin by printing the tabular data for the spectrum. This is simply the numeric mass-intensity data. Select a scan (or range of scans to average) in the baseline; usually, the scans immediately before the peak are chosen. The background spectrum will be displayed (call this spectrum B), but spectrum A remains in memory on the 'stack.' Print a tabular listing of the background spectrum. Select SUBTRACT from the Spectrum Menu, the Spectrum A minus Spectrum B will be displayed. Print a tabular listing and verify how the numeric data were obtained.