The purveyors of fake wines probably never dreamed that they could be caught by a carbon atom. A special kind of carbon atom. One that in addition to the six protons in its nucleus has eight neutrons instead of the usual six. It鈥檚 called carbon 14. It鈥檚 very rare but can be a powerful tool in analytical chemistry. Let鈥檚 start at the beginning. In fact, the very beginning, the Big Bang. Over 13 billion years ago the universe began to expand from an extremely dense and hot core, spewing out the basic units of matter that would eventually form the chemical elements. These in turn would become the building blocks of everything in the universe.聽
One of the fundamental particles jettisoned during the Big Bang was the proton, a basic unit of mass. The universe is still permeated with protons scooting all over the place, generated either by the original Bang, or by chemical reactions in stars, which themselves were products of that amazing event thirteen billion years ago. Our own atmosphere is constantly showered by these protons, some of which bang into the oxygen and nitrogen molecules that make up our atmosphere. As a result of these collisions, neutrons, the other fundamental units of mass, are released, and they in turn are absorbed by the nuclei of nitrogen atoms, converting these in carbon-14. The carbon-14 produced reacts with oxygen to form carbon dioxide. This is a rare event, so that very few of the carbon dioxide molecules in nature contain carbon-14, the vast majority contain the most abundant form of carbon, which is carbon-12. In fact carbon 14 makes up only about one part per trillion of the carbon in the atmosphere. But that is enough to make what we call radiocarbon dating possible. The term 鈥渞adiocarbon鈥� comes about because carbon-14 is radioactive, meaning that it is unstable and spontaneously converts to nitrogen-14 by emitting an electron with a half life of 5730 years. This means that of any sample containing carbon-14, only half of the carbon-14 atoms will remain after that period of time. A living plant absorbs carbon dioxide from the air, so its concentration of carbon-14 will be the same as that of the atmosphere. But once the plant is no longer photosynthesizing, it is no longer absorbing carbon dioxide, and the carbon-14 content will start to decay. From the residual carbon-14 content, the time that photosynthesis stopped, that is when the plant was harvested, can be determined. In the case of wine, this refers to the time the grapes were picked. The problem is that radiocarbon dating is only accurate to within a few hundred years, which is fine for archeological studies, but not for determining the specific year that grapes were picked. But amazingly, this can actually be done thanks to a little human intervention called the atomic bomb.
From the 1940s to the 1960s, atmospheric tests of nuclear weapons were carried out. These reactions generated neutrons which in turn caused a significant increase in the carbon-14 level of the atmosphere. For thousands of years the ratio of carbon-14 to carbon-12 in the atmosphere had been essentially constant, but now became skewed. As grapes grow, they take up carbon dioxide, so their ratio of C-14 to C-12 reflects that in the atmosphere. Since during the testing years the atmospheric ratio was well documented, scientists are now able to determine the exact vintage of a wine by measuring this ratio. As a consequence, there would be no problem in determining whether that bottle of Chateau Laffite 1787 that sold for a record $156,000 was indeed authentic. Of course wine connoisseurs maintain that their palate is a better instrument than any found in an analytical chemistry lab.
