Mechanisms to detect the presence of isotopic labels

mechanisms to detect the presence of isotopic labels; through their difference in mass, vibrational
modes, or by radioactive decay products. Mass Spectroscopy or Nuclear Magnetic Resonance can
detect the difference in an isotope's mass, while Infrared Spectroscopy can detect the difference in
the isotope's vibrational modes. If the radiolabeled molecule was given to a plant or animal, the fate
or location of the molecule can be followed by taking a tissue slice and looking for the C-11 by
detecting its radioactive decay using ionization analyzers, or by using film to make autoradiographs
(pictures). Alternatively, electrophoretic gels can be used to first separate the components and then
detecting the radioactive decay.
Metabolic pathways were discovered using radiolabeling techniques. How do we know that the
carbon atoms in glucose end up in carbon dioxide in our breath ? Here is an example using C-11. C-
11 is produced in a cyclotron and has a half-life of 20.4 minutes, and emits positrons that are
detected as gamma radiation. Plants were given carbon dioxide radiolabeled with C-11. The plants
then assimilated the carbon dioxide into glucose, which becomes labeled with C-11. The radiolabeled
glucose was fed to rats and the rats exhaled
11
CO
2
. Yes, these were very messy experiments and
were a tremendous amount of work. We should be grateful to those who came before us and made
these discoveries.
glycolysis Citric Acid Cycle
11
C-glucose ¾¾¾¾¾®
11
C-pyruvate ¾¾¾¾¾¾¾¾¾®
11
CO
2
Diagnosis and Imaging
• X-rays and CT scans use radiation
An X-ray is the standard in imaging, especially for extremities. The radiation is equivalent to
about 3 hours of background radiation.
A CAT scan , or CT scan, means computer assisted tomography and is a computerized composite
of hundreds of x-rays. Tomography is the method of converting data into images with a computer. It
gives much more data but also uses much more radiation, about 8 months of background radiation.
• Ultrasound and MRI do not use radiation
Ultrasound gives excellent detail without radiation (Figure 2c). It is used in pregnancies and liquid-
filled organs such as the heart and gall bladder, but cannot penetrate air or bone.
An MRI gives excellent detail without radiation. It is good for soft tissues of joints and brain but
not good for air-filled spaces such as lungs. It also is costly and time consuming.
How Does MRI Work ?
Hydrogen atoms in molecules are paramagnetic, meaning that they will align themselves in a
strong magnetic field. Normally, a patient’s hydrogens are random, in other words, not aligned in a
magnetic field. An MRI produces a magnetic field about 20,000 times stronger than that of earth.
Superconductivity is needed because wires would melt. The primary coil is kept at -450
o
F using
liquid helium. The MRI machine aligns the patient’s hydrogens in the head to toe direction. You would
expect that there would be equal numbers of hydrogens facing the head and the feet, but there is a
small excess (a few ppm) of hydrogens facing either the head or the feet.