Elements with same atomic number (SAN) but different atomic weights (DAW).

There are two types of isotopes:

  1. Stable Isotopes
  • Naturally occurring
  • Do not emit radiations
  • Identified and quantitated by mass spectrometry or nuclear magnetic resonance.
  • Sometimes used in biochemical investigations.


  1. Unstable or Radioactive Isotopes (IMP SAQ)

Nucleus unstable → Decay → αβ and gamma rays emission

Curie : Defined as the amount of radioactivity equivalent to 1 gm of radium (2.22 x 10 raised to 12) disintegration per minute.

Millicurie = 2.2 x 10 raise to 9

Microcurie = 2.2 x 10 raise to 6

  • Geiger counters
  • Liquid scintillation counter
  • Autoradiography
  • Determine metabolic origin of complex molecules like heme , cholesterol , purines and phospholipids.


  • Determining the precursor-product relationship in metabolic pathways ex. Krebs cycle, urea cycle etc..


  • Study of metabolic pools and metabolic turnovers.
  • Endocrine and immunological studies


  • To elucidate drug metabolism.


  • Scanning of organs – thyroid 131I , bone 90Sr , kidney 131 I hippuran.


  • Treatment of cancer

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