The disclosure features methods of analyzing a fluid extracted from a reservoir, the methods including introducing a first composition featuring a first complexing agent into a reservoir at a first location, extracting a fluid from the reservoir at a second location different from the first location, combining the fluid with a second composition featuring a concentration of a lanthanide ion to form a third composition featuring a concentration of a complex formed by the first complexing agent and the lanthanide ion, exposing a quantity of the complex to electromagnetic radiation for a first time period ending at a time t0, detecting fluorescence emission from the quantity of the complex for a second time period starting at a time t1>t0, where t1−t0 is greater than 2 microseconds, and determining information about a fluid flow path between the first location and the second location.