This will depend on the type of device and coil that you’re using. If your coil has a resistance of under 1.0Ω, it classes as a sub-ohm, which will mean you’ll need a freebase nicotine liquid that is thicker and has a mix of around 70VG/30PG. For MTL (Mouth To Lung) devices that have a coil with a resistance of 1.0Ω or above, you’ll need to use a thinner freebase liquid, with a PG of 50% or less such as 50VG/50PG mix or even high PG liquids.