Radioisotope dilution is a technique used to measure the concentration of a substance in a sample by mixing it with a known quantity of a radioactive isotope. The radioactive isotope acts as a tracer, allowing scientists to track the distribution and quantity of the original substance. By measuring the radioactivity of the mixture, researchers can calculate the concentration of the non-radioactive substance. This method is commonly used in various fields, including environmental science, medicine, and biochemistry. It provides accurate and sensitive measurements, making it valuable for studying processes such as nutrient uptake in plants or the distribution of drugs in the body.