Inner source assembly and associated components

An inner source assembly for a mass spectrometer, the inner source assembly comprising:

• a base;
• and
• a volume housing removably connectable to the base for retaining a repeller assembly therebetween,
• wherein the volume housing comprises a first protrusion and a second protrusion and the base comprises a first slot and a second slot to receive and retain the first protrusion and the second protrusion, respectively,
• wherein the first protrusion is longer than the second protrusion, and a guard is associated with a mouth of the second slot, presenting an inner radial surface, and the distance between the inner radial surface and a longitudinal axis of the base is greater than a length of the second protrusion and smaller than a length of the first protrusion.