tracers

Tracers are substances or signals used to reveal the movement or fate of a component within a system by leaving a detectable trace. They enable visualization of flow, transport, or interactions without requiring destructive measurement.

In fluids and environments, physical tracers include colored dyes, fluorescent markers, smoke, particles, and inert gases. They visualize flow patterns and aid in measuring diffusion and residence times. Tracer gases such as sulfur hexafluoride or helium are common in building ventilation studies and groundwater hydrology. Choice depends on detectability, toxicity, and how closely it matches the system.

In biology, chemistry, and medicine, tracers use radioactive or stable isotopes and fluorescent or luminescent labels. They track metabolic pathways, nutrient uptake, drug distribution, or environmental sources. Medical imaging relies on radiotracers for PET and SPECT to visualize tissues and functions.

In computing, tracing records events or execution paths. Network tracing tools diagnose routes and latency (for example, traceroute). Distributed tracing in software collects request data across services to analyze performance. In computer graphics, ray tracing uses the notion of tracing light paths to render images, but it is a distinct application of the term.

Limitations include detection limits, tracer bias, and perturbation of the system. Some tracers are toxic, radioactive, or environmentally persistent, requiring careful handling and regulatory oversight. Proper calibration and modeling are typically needed to interpret tracer signals. See also tracer dyes, traceroute, radiotracer, and tracer methods in environmental science.