Radiocarbon Dating & Urea Breath Test - Lecture Notes

# Radiocarbon Dating Lec. I (5 pages) Radiobiology Radiocarbon dating is one of the most widely used scientific dating methods in archaeology and environmental science. It can be applied to most organic materials and spans dates from a few hundred years ago right back to about 50,000 years ago....

# Radiocarbon Dating Lec. I (5 pages) Radiobiology Radiocarbon dating is one of the most widely used scientific dating methods in archaeology and environmental science. It can be applied to most organic materials and spans dates from a few hundred years ago right back to about 50,000 years ago. Radiocarbon dating is based on the detection of the radioactive isotope of carbon, $14C$, in the remains of organisms. This isotope ($^{14}C$) is formed in the upper atmosphere about 15 km above the ground. Cosmic radiation from space interacts with atmospheric gas molecules, generating neutrons. The newly formed neutrons, $n$, are highly reactive and combine with atmospheric nitrogen; the result is $^{14}C$ and a proton $p$: $n + ^{14}_7N \rightarrow ^{14}_6C + p$ Only a small amount of the total carbon in the world is radioactive; only one in a trillion ($1 \times 10^{12}$) carbon atoms is $^{14}C$. The stable isotope of carbon, $^{12}C$, is much more common, forming 1% (1 in 100) of all carbon. Radioactive carbon reacts with atmospheric oxygen to form heavy carbon dioxide, $^{14}CO_2$, that incorporated directly into plants through photosynthesis and then indirectly into the herbivores and carnivores that consume them. The nucleus of a $^{14}C$ atom contains six protons and eight neutrons, making the ratio 1 to 1.33. This represents a significant deviation from the ideal ratio of 1:1 for small atoms and causes the isotope to be unstable. The radioactive decay enables atoms to restore a favorable balance between protons and neutrons. In $^{14}C$, this is achieved through a $\beta$-decay process during which the nucleus emits a $\beta$-particle, (an electron, $e^-$) and an electron antineutrino ($\bar{\nu}_e$), so one of the neutrons in the $^{14}C$ nucleus changes to a proton and the $^{14}C$ nucleus reverts to the stable $^{14}N$ (i.e. the additional proton changes the atomic number of the element from six to seven, converting it from carbon to nitrogen). The atomic mass of the element is unchanged but with seven protons and seven neutrons, instead of six protons and eight neutrons, and the nucleus is now stable. The chemical equation for the radioactive decay of $^{14}C$ is: $^{14}_6C \rightarrow ^{14}_7N + e^- + \bar{\nu}_e$ Living organisms constantly exchange carbon with their surroundings, so the levels of stable and radioactive carbon they contain closely match their environment (i.e. contain the same ratio of $^{14}C$ to $^{12}C$ as the atmosphere). Once an organism dies, no more $^{14}C$ will be acquired, but the $^{14}C$ within its biological material at that time will continue to decay, and so the ratio of $^{14}C$ to $^{12}C$ in its remains will gradually decrease. Because $^{14}C$ decays at a known rate, the proportion of radiocarbon can be used to determine how long it has been since a given sample stopped exchanging carbon - the older the sample, the less $^{14}C$ will be left. The difference between the amount in the environment and the amount in the remains of the organism increases over time and forms the basis of radiocarbon dating. The half-life of $^{14}C$ is 5,730 years, this means that after 5,730 years, only half of the initial $^{14}C$ will remain; a quarter will remain after 11,460 years; an eighth after 17,190 years; and so on. | Amount of stable $^{12}C$ | Amount of unstable $^{14}C$ | Ratio | Years | # Half-lives | |---|---|---|---|---| | 100 Trillion | 100 | 1-T to 1 | 0 | 0 | | 100 Trillion | 50 | 2-T to 1 | 5,730 | 1 | | 100 Trillion | 25 | 4-T to 1 | 11,460 | 2 | | 100 Trillion | 12 | 8-T to 1 | 17,190 | 3 | | 100 Trillion | 6 | 16-T to 1 | 22,920 | 4 | | 100 Trillion | 3 | 32-T to 1 | 28,650 | 5 | # Urea Breath Test The urea breath test is used to detect the presence of the bacteria *Helicobacter pylori*, the major cause of peptic ulcer disease. * *H. pylori* is a Gram-negative, spiral shaped bacterium. It uses its flagella to escape the harsh luminal acidity by burrowing into the mucus layer that covers the gastric mucosa and so reside in close proximity to the more neutral pH of the epithelial cell surface of the gastric mucosa. * The urease enzyme which has been located on the surface of this bacterium, is important and likely to be vital for bacterial survival and colonization in the highly acidity milieu of the stomach. Its urease breaks down the luminal urea normally produced by the gastric mucosa, yielding carbon dioxide and ammonia. * Ammonia then accepts a proton ($H^+$), lessening the nearby acidity and forming protective surroundings that allow its survival. The urea breath test provides a reliable noninvasive method for *H. pylori* detection with high sensitivity and specificity. This test is based on the fact that *H. pylori* is powerful urease producer. Urease is an enzyme that converts urea to ammonium and carbon dioxide ($CO_2$). The way the test works is as follows: - The patient is given a certain amount of urea with some of the carbon being $^{14}C$. - If *H. pylori* is present, then this bacteria will break down the urea in the mucus layer and produce $CO_2$. - The labelled $CO_2$ diffuses to the epithelial cells and then is carried in the bloodstream and ultimately is released in the exhale. - The labelled $CO_2$ will be exhaled by the patient and can be measured. - The amount of the labeled $CO_2$ is related to the urease activity, which indicates the presence or absence of *H. pylori* infection. The more $^{14}C$ exhaled, the more *H. pylori* present. The breath test can be repeated to determine the success of the treatment. Conventionally, patient preparation for the test requires fasting for at least 4 h and oral ingesting of 5 µCi $^{14}C$-urea in 20 ml water. Breath is collected 20 min postdosing and radioactivity in the sample is measured by a scintillation counter, and the result is expressed as counts per minute (cpm). $NH_3$ *H. pylori* *C=O urease 2 $NH_3$ + *CO$_2$ $NH_3$