GC 8890 - Injected volume affects retention time

Seems like you evaporate your methanol into a volume that is far bigger than your liner volume (Consider CS2 volume plus methanol volume). This is called overloading, you see that the end of your peak stays the same, that's also typical for overloading. You still have not so much, that the typical fronting can be seen. You can find some general information about overloading here: Help! My Peaks Look Strange - Fronting and Tailing in GC - Wiki - Consumables - Agilent Community

Other peaks can be influenced because your methanol vapour plug changes the mass transfer between phase and carrier gas vs. phase and carrier gas+methanol plug.

As a second comment: CS2 is highly apolar and you use a polar phase, this also affects the mass transfer.

But still Paul is right you only see a retention time shift of 0.1 min max (6 secs), which is not critical.

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In split, one reason the RTs move forward as more is injected is because of the higher instantaneous pressure in the inlet caused by the larger volume of solvent vapor created.  It looks interesting here, but the difference from 0.3uL to 1.5uL is only 0.03 minutes at 4.45 minutes --- 1.8 seconds.

RT shift with different injection volumes is also related to a solvent effect – with larger injection volumes you’re introducing a lot more solvent, which can start to have an effect on analyte solubility in the stationary phase and thus affect retention. Most customers don’t see this because they stick with stable split and injection volumes (as they should), but retention behavior is a function of the whole method which includes how much solvent is introduced to the system. This is really only observable for peaks eluting in the neighborhood (relatively) of the solvent peak; real high boilers eluting at the end of the run will not show much impact due to their ability to cold trap and chromatograph much more slowly through the system, well after the solvent has been sent on its way.