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We've learned so much about radioactivity! Incredibly, there is a whole branch of chemistry unaffected by electrons. It's been fun to see the uniqueness of protons and neutrons! We started with nuclear stability. We learned that as the nucleus gained mass, it became more unstable. Why? Because the electrostatic repulsion overcomes the strong nuclear force. How do we know? The binding energy began to decrease after lead. So, it required less energy to rip apart the nucleus. Then, we studied how an atom increases its stability via decay. There were four kinds: alpha, beta, gamma, and electron capture. It's cool to learn, but it's been very theoretical. How do chemists apply these things?
What if we could detect radiation as it moves through a system? It turns out we can! The most common application of radioactivity is radiotracers. Radiotracers are compounds with one of their atoms replaced by a radioactive isotope. Why? So we can use the radioactive decay to follow them. Remember, we can't tell when an individual atom will decay. But, we do know that decay obeys statistics. That's the half-life. So, we can assume that there is continuous radiation. The radiation is detectable for a least a couple of half-lives. Let's study a few of the main areas where we use radiotracers!
Biochemistry
Have you ever wondered how we know about photosynthesis or the citric acid cycle? I mean, how do we follow carbon dioxide in plants? Or how do we know what happens to each carbon in a glucose molecule? Biochemists use a radioactive tracer. The tracer moves through the body, cells, and organelles, and we follow it. There are a couple of common tracers in biochemistry. Let's look at each one.
The first is an isotope of hydrogen called tritium. It has one proton and two neutrons for a total atomic mass of three amu. That's why we named it tritium. Remember, chemists are so creative! Its principal decay method is beta-minus decay. But, the generated electron has very low energy. It's challenging to detect. So, why do we use it? It's the only radioactive isotope of hydrogen that occurs in nature. Heavier isotopes do exist. But they are synthetic and have half-lives of seconds. That's a zeptosecond! The half-life of tritium is about 12 years. Also, hydrogen is one of the most abundant elements in biological systems. So, it's essential to track it.
The other principal tracer in biochemistry is carbon-14. We encountered it when we studied radioactive dating. Most biological systems focus on carbon. So, it makes sense to track it. Remember, carbon-14 undergoes beta decay. Let's say we want to follow an organic compound through a system. For example, we want to know what happens to glucose. We can replace one of the carbons with carbon-14. So, we can track the whole molecule. But, we can also observe the specific carbon atom.
Medicine
We use radiotracers all the time in modern medicine. Do you know how we find cancer? It has a higher rate of glucose use. So, the doctor injects a patient with radioactive glucose. The concentration will be higher in the cancer cells. So, the detector notices a high amount of radioactivity. A computer converts the signal into an image. Here, we're using carbon-11. Carbon-11 undergoes beta-plus decay. So it releases a positron. What's the technique that measures positrons? It's positron emission tomography or PET. You may have heard of a PET scan.
Using a radiotracer is a versatile technique for discovering diseases. Why? Because different areas of the body intake unique elements. For example, the thyroid is the only organ that consumes large amounts of the halogen iodine. So, radiologists use Iodine-131 to examine the thyroid. Do you want to study blood? Iron is the main component of hemoglobin. So, we can use Iron-59 to investigate red blood cells. If the body is not utilizing the element properly, it may indicate disease.
Environmental Chemistry
Environmental chemists employ radiotracers all the time! Often we are using it to study contaminants. So, for example, let's say we want to know the bioavailability of a pollutant in a system. We can track it using radiotracers. We use minuscule amounts. The concentration will be at or below what occurs in nature. So, even if our experiment is in an environmental system, it has a low impact. We can even trace a contaminant from the environment into an organism. We can then see how the organism processes it.
What's an example of a contaminant we might track? One of the gravest contaminants is the transition metal mercury. Specifically, methyl-mercury is particularly dangerous. Why? The methyl group helps to stabilize the molecule. So, it accumulates within organisms. We call this process bioaccumulation of biomagnification. For the most part, exposure remains low in humans. But, there have been a few incidences of high exposure. In those cases, the main symptoms were loss of neurological function. So, it's something to make sure we track.
