  Trace analysis of radionuclides is an essential and versatile tool in modern
science and technology. Due to their ideal geophysical and geochemical
properties, long-lived noble gas radionuclides, in particular, 39Ar (t1/2 = 269
yr), 81Kr (t1/2 = 2.3x10^5 yr) and 85Kr (t1/2 = 10.8 yr), have long been
recognized to have a wide range of important applications in Earth sciences. In
recent years, significant progress has been made in the development of
practical analytical methods, and has led to applications of these isotopes in
the hydrosphere (tracing the flow of groundwater and ocean water). In this
article, we introduce the applications of these isotopes and review three
leading analytical methods: Low-Level Counting (LLC), Accelerator Mass
Spectrometry (AMS) and Atom Trap Trace Analysis (ATTA).
