Autoradiography using Radiotracers:

In this technique, a radiotracer or a radiolabelled solute sample is used. After development of chromatogram, it is kept in contact along with a photographic film of the similar size. After a few times while a film is developed then dark spots are observed in place of spots corresponding to the movement of solute components. This is called autoradiography. It has been found to be especially useful for the study of distribution and metabolism of compounds administered to the plants or   animals. Within a unique experiment, the Nobel Laureate Calvin and coworkers identified intermediate steps involved in the photosynthesis of carbohydrates from atmospheric CO₂ in the presence of light and chlorophyll by using ¹⁴C, ³²P and ³H. Plants were placed in the atmosphere containing ¹⁴C labeled CO₂ and irradiated with light as shown in Figure.

The plants were removed after different irradiation periods of light and the molecular components were separated using paper chromatography. The presence of radioactive atoms in a compound by following autoradiography was taken as a proof of the involvement of that compound in the photosynthesis.

Figure:  Illustration of Autoradiography method used for the determination of compounds involved in photosynthesis