Isolation air gap

You would have to create a custom PCB with the slot in it. To do that you create an SVG of the board shape you want.

http://fritzing.org/forum/thread/1369/

You can have a read through my tips and tricks on using Inkscape as it has a few tips about creating a board with holes. It is important that the slot be created using a difference operation and it has to be the last operation done to the shape of the SVG before export.