Noble gases (He, Ne, Ar, Kr, Xe) are inert and highly volatile elements. As their inertness keeps them from complicated processes involving chemistry, they are the simplest (thus the best!) tracers of physical processes such as mixing and kinetic fractionation. Several radiogenic and radioactive isotopes also provide useful chronometers.
I am currently taking part in a research project involving 81Kr (half life = 229,000 yr) and 85 Kr (half life = 10.8 yr) with Neil Sturchio. Radiokryptons are ideal tracers of water ventilation ages, which can be used to constrain flows rates and/or mixings of subsurface waters. The recent development of the ATTA instrument (atomic trap trace analysis) at Argonne National Laboratory makes it possible to analyze these extremely low abundance (<10E-11) isotopes. We are in progress of constructing a system to extract and purify krypton dissolved in water for the analysis using ATTA.
My previous works are related to non-radioactive noble gases and nitrogen which is occasionally considered as "6th noble gas". Because primordial volatile elements are preserved in the Earth's mantle, analyses of noble gases in mantle-derived samples can shed light on the origin of the atmosphere. My favorite work among those is based on the xenon isotope systematics of mantle-derived samples from the Kola Peninsula, Russia, from which we proposed that the primitive degassing of the Earth continued over several hundreds of Myr, over Hadean.