GC/MS data is a three dimensional matrix of information: scan number
(related to the time since injection), mass and signal intensity.
As a run progresses, the spectrometer is typically scanned (for scan mode operation) over the mass range specified in the method. During each scan, the entire mass spectrum is collected and stored. In essence, the mass spectrum is collected as a function of time. Data can also be collected in the Selected Ion Monitoring (SIM) mode; in this mode, only some mass data are collected rather than the full spectrum.