The Stand-By mode should be used to 'store' the GC/MS overnight when it is being used regularly. It is better to leave the system running than it is to pump it down and vent it every day or couple of days (Remember that 3 hour recommended equilibration time, and overnight is better). If the instrument is not to be used for several weeks or longer, turning it off may be the preferred mode.
Do NOT turn the GC OFF with the MS Rough or Turbo Pumps running. If the instrument is to be in 'Stand-By' with any part ON, the whole instrument should be ON.
The main feature of Stand-By mode is to reduce the carrier flows and especially the split flow. The carrier flow can be set to about 1 mL/min (for capillary columns) and the total flow reduced to just above the carrier flow. This will save gas, since the HP 5890 does not have a 'Gas Saver' mode to reduce split flow when the instrument is not in use.
Remember to reset the Total Flow before using the instrument, or there will be no Split/Septum Purge.