Nitrogen is a Group 15 element. The element seemed so inert that Lavoisier named it azote, meaning "without life". Nitrogen gas is colourless, odourless, and generally inert. As a liquid it is also colourless and odourless. Prof. James Marshall U of North Texas
Nitrogen is the recommended gas for leak testing, purging, and brazing In the HVAC & R industry.
Air, freon, water, propane, oil, and so on and so on will react to changes in temp. My point is that nitrogen will not and makes a better gas to check for leaks.