Oxygen-18 facts for kids

Oxygen-18 (¹⁸O, Ω) is a special type of oxygen. It's a natural and stable (meaning it doesn't break down easily) version of oxygen. It's one of the important "environmental isotopes" that scientists study.

¹⁸O is very important for making a special substance called fluorodeoxyglucose (FDG). This FDG is used in a medical scan called positron emission tomography (PET). PET scans help doctors see what's happening inside the body.

To make FDG, scientists use water that has a lot of Oxygen-18 in it (called H₂¹⁸O). They shoot tiny particles called hydrogen ions at this water. This happens inside big machines like a cyclotron or a linear accelerator. This process creates fluorine-18. Fluorine-18 is then used to make FDG, which can be given to patients for their scans.

Oxygen-18 can also be used to make a very heavy type of water. This happens when it mixes with tritium (which is a form of hydrogen-3). This super heavy water, called ³H₂¹⁸O or T₂¹⁸O, is almost 30% heavier than regular water.

Scientists need to be very careful when measuring Oxygen-18. They follow special steps to prepare and store samples correctly. This helps them get accurate results.

Oxygen-18 and Earth's Past Climate

Scientists use Oxygen-18 to learn about Earth's climate long ago. They look at the ratio of ¹⁸O to ¹⁶O (another type of oxygen) in ice cores. These ice cores are long samples of ice taken from places like the Arctic and Antarctic. This ratio is called δ¹⁸O.

How Ice Cores Reveal Temperature

The δ¹⁸O in ice cores helps scientists figure out the temperature when the snow fell and turned into ice. Colder temperatures lead to different amounts of Oxygen-18 in the snow. This is because of something called equilibrium fractionation. It means that water molecules with different oxygen types freeze at slightly different rates depending on the temperature.

As water vapor moves from warm areas (like the equator) to cold areas (like the poles), it loses some of its heavier Oxygen-18 molecules. This process is called Rayleigh fractionation. It means that the further the water travels towards the poles, the less Oxygen-18 it will have.

In the 1950s, a scientist named Harold Urey did an experiment. He mixed regular water with water containing Oxygen-18. Then, he partly froze the mixture. This experiment helped show how Oxygen-18 can be used to study temperature changes.

Fossils and Ancient Ocean Temperatures

The ratio of ¹⁸O to ¹⁶O (δ¹⁸O) can also help scientists find out how warm oceans were in the past. This is called paleothermometry. They study certain types of fossils that grew over time, like shells.

For example, scientists can look at a single scallop shell. As a scallop grows, it adds new layers to its shell. Each growth layer can be measured. By looking at the Oxygen-18 in each layer, scientists can figure out the ocean temperature when that part of the shell formed. This can even show seasonal temperature changes.

To understand how ocean temperatures changed over very long periods, scientists compare fossils of the same species from different layers of rock. The differences in their Oxygen-18 levels show how temperatures changed over millions of years.

Oxygen-18 in Plants

Scientists also use Oxygen-18 to study how plants breathe and use light. This process is called photorespiration. They can add Oxygen-18 to the air around plants. This helps them measure how much oxygen the plants take in during photorespiration.

Plants produce oxygen during photosynthesis. But they also take in some oxygen through photorespiration. Studies using Oxygen-18 have shown that in the past, plants would reabsorb about half of the oxygen they made during photosynthesis. This means that the amount of food plants could make was cut in half because of the oxygen in the air.

Making Fluorine-18

Fluorine-18 is usually made by shooting high-energy protons at water that has a lot of Oxygen-18 (H₂¹⁸O). This happens in machines like a cyclotron or a linear accelerator. This process creates a liquid with Fluorine-18 in it.

This liquid is then quickly used to make special molecules for medical uses. Often, the Fluorine-18 replaces another part of the molecule, like a hydroxyl group. These special molecules, called radiopharmaceuticals, must be made after the Fluorine-18 is ready. This is because the strong proton radiation would destroy the molecules if they were already made.

Many positron emission tomography (PET) centers use large amounts of Oxygen-18 enriched water. They use it to make their own Fluorine-18 labeled fludeoxyglucose (FDG) right there.

After the water has been used to make Fluorine-18, it needs to be cleaned. This removes any unwanted substances. It also removes tiny amounts of tritium that might have formed. This cleaning makes sure the water can be used again to make more Fluorine-18.

See also

• Isotopes of oxygen