I wasn't going to comment, because my experience is decades old, but I would caution about using any sort of non-specific detector for nC4.
One problem is that low molecular HCs are almost ubiquitous at ppb concentrations around industrial areas. C1 is natural and from IC engine exhausts and evaporative emissions, C2-C5 can also be from liquified gases used for heating and fuels. As background concentrations vary, so sensitivity can change rapidly.
Note that HC flammability detectors are quite insensitive because of the narrow flammability limits ( 6 -14% ) in air.
Another problem is that, at such low leak rates, the sampling is critical. Having worked in a production environment with thermionic and electron capture detectors seeking similar leak rates of CFCs from refrigeration hermetic systems, I've seen the issues of directional leaks being missed by some handheld devices.
It is essential to have a contamination-free area, with some form of whole module sampling system, and yet be able to trap and remove the leaked emissions from devices that leaked badly, as well as those that leaked slightly.
My understanding is that in some situations, small MS systems have used, however I would suggest contacting some of the suppliers of industrial systems, eg
http://www.lacotech.com/Leak_Detection.html
A Google search should find many others...
Bruce Hamilton
