放射性示踪Radioactive tracer

放射性探测灵敏度极高，可检测到 10⁻¹⁵ 摩尔甚至更少的物质。选择示踪核素要考虑半衰期、射线类型和化学性质：³H、¹⁴C、³²P、³⁵S 常用于生化研究；¹³¹I、⁹⁹ᵐTc、¹⁸F 用于医学诊断。需注意同位素效应（尤其是氢同位素）和标记位置可能随反应改变。稳定同位素（²H、¹³C、¹⁵N、¹⁸O）配合质谱或核磁共振也是重要的示踪手段。

Radiation detection is extraordinarily sensitive, reaching 10⁻¹⁵ mol or less. Tracer choice depends on half-life, radiation type and chemistry: ³H, ¹⁴C, ³²P and ³⁵S are common in biochemistry, and ¹³¹I, ⁹⁹ᵐTc and ¹⁸F in medical diagnosis. Isotope effects, especially for hydrogen, and label scrambling during reactions must be considered. Stable isotopes such as ²H, ¹³C, ¹⁵N and ¹⁸O, detected by mass spectrometry or NMR, are also important tracers.

1913 年匈牙利化学家赫维西与帕内特用铅的天然放射性同位素测定铬酸铅的溶解度；1923 年赫维西用 ²¹²Pb 研究铅在植物中的吸收与迁移，开创了示踪法，获 1943 年诺贝尔化学奖。1950 年代卡尔文用 ¹⁴CO₂ 阐明光合作用的碳固定途径（卡尔文循环），获 1961 年诺贝尔化学奖。

In 1913 the Hungarian chemist George de Hevesy and Paneth used a natural radioactive isotope of lead to measure the solubility of lead chromate; in 1923 Hevesy used ²¹²Pb to follow lead uptake and movement in plants, founding the tracer method, and he received the 1943 Nobel Prize in Chemistry. In the 1950s Calvin used ¹⁴CO₂ to map the carbon-fixation pathway of photosynthesis, the Calvin cycle, earning the 1961 Nobel Prize in Chemistry.

化学中用于研究反应机理（如用 ¹⁸O 证明酯化反应中酸脱羟基、醇脱氢）；生物学中用于研究代谢途径、DNA 复制（梅塞尔森-斯塔尔实验用 ¹⁵N）和放射自显影；医学中用于甲状腺功能检查、PET/SPECT 显像；工业中用于检测管道泄漏、测量流量和磨损；环境中用于追踪污染物迁移和地下水年龄。

In chemistry tracers reveal mechanisms, for example ¹⁸O shows that in esterification the acid loses OH and the alcohol loses H; in biology they map metabolic pathways, DNA replication (the Meselson–Stahl experiment used ¹⁵N) and autoradiography; in medicine they test thyroid function and enable PET and SPECT imaging; in industry they detect pipe leaks and measure flow and wear; in environmental science they follow pollutants and date groundwater.