KB001672: What are the Steps to Quantitate a Peak Which Elutes before the Solvent? - MS ChemStation 

This can be done with Event time programming. For example, consider the case when the solvent peak elutes at 5.5 to 5.7 minutes, the target peak elutes before the solvent at 4.5 minutes, and another target peak occurs after the solvent at 7.5 minutes. In the [Device Control] screen, open the [Device] menu and click the "MS SIM/Scan" command" to open the "MS SIM/Scan Parameter" dialog box, then set the solvent wait time to 4 minutes (as the first target peak occurs at 4.5 minutes). Click [Time Event] at the bottom of the screen to open the [MS Time Event Table] dialog box. Set the [Time] field to 5.0 (minutes), the "Event Type" to detector, and [Parameter 1] to off, and then click [Add]. Similarly, set the [Time] field to 7.0 (minutes), the "Event Type" to detector, and [Parameter 1] to on, and then click the "Add" button again. When you have finished, click [OK]. Performing the measurement with these settings turns off the detector for the solvent elution time only, allowing that measurement to be performed without damaging the filament.