Have never heard of detecting hydrocarbons in the coolant as a test for
a leaking head gasket. Although it sounds plausable... it also sounds
rather exotic... and would require some highly specialized costly gear.
I think the old fashioned method... of pressurizing the system... and
watching for leak-down is good. I would be satisfied with that
diagnostic if your confident it was performed correctly.

Professor
www.telstar-electronics.com