Source/Drain terminals in a MOSFET Sampling circuit
Theoretically the source and drain are interchangeable (although in practice they are not).  For an NMOS sampling switch, the source is at the lower potential.  Since your switch will have non-zero resistance, the source is the side connected to the capacitor and the drain is connected to your signal source.  You should follow this convention if you are using CMOS devices that are non-symmetric.