鉴别葡萄酒新法：放射性CO2检测

Two decades of atomic bomb testing in the atmosphere are yielding（出产，让步） an unexpected bonus for consumers, scientists reported here today at the 239th National Meeting of the American Chemical Society (ACS). It's a new test to determine whether that Bordeaux（波尔多葡萄酒） or burgundy（深紫红色） is from a fine vintage year（好酒酿成的年份，成绩卓著的一年） and commands premium price（溢价） or actually is a counterfeit（假冒的，伪造的） vin ordinaire（廉价葡萄酒） or cheap plonk（劣质酒） worth much less. Graham Jones, Ph.D., who headed the research, said that misrepresenting（歪曲，误传） the vintage — the year the wine was made — is an ongoing problem. He is with the University of Adelaide, Australia. Some years bring perfect growing conditions for vineyards in a country or region of a country and the grapes produce exceptional wines that command premium prices. In other years, bad weather, such as heavy late-season rains that bloat（膨胀，肿胀） grapes with water or long hot, dry spells（干旱期） at harvest, means poor quality wines. Some wine experts estimate that up to 5 percent of the fine wine sold today is fake.

"The problem goes beyond ordinary consumers being overcharged for a bottle of expensive wine of a famous winery with a great year listed on the label," Jones pointed out. "Connoisseurs（鉴赏家，内行） collect vintage wines and prices have soared with 'investment wines' selling for hundreds of thousands of dollars a case at auction."

Jones said the wine industry is fighting forgeries（伪造） with special seals and high-tech labels. The method for authentication（证明，鉴定） of a wine's vintage may provide added confidence that the vintage on the label is the vintage in the bottle.

In their leading research, Jones and colleagues found that radioactive carbon dioxide produced from atomic bomb tests in the atmosphere absorbed by grapes can be used to accurately determine wine vintages. The new technique is similar to radio-carbon dating, used for years to estimate the age of prehistoric（陈旧的，史前的） objects. It works by comparing the amount of carbon-14 (C-14), a less common form of atmospheric carbon, to carbon-12 (C-12), which is more stable and abundant. The ratio（比率） of these two carbon forms, or isotopes（核素，同位素） , has remained constant in the atmosphere for thousands of years.

"Until the late 1940's all carbon-14 in the Earth's biosphere（生物圈） was produced by the interaction between cosmic rays and nitrogen in the upper atmosphere," Jones says. "This changed in the late 1940's up to 1963 when atmospheric atomic explosions significantly increased the amount of C-14 in the atmosphere. When the tests stopped in 1963 a clock was set ticking — that of the dilution（稀释，冲淡） of this "bomb-pulse" C-14 by CO2 formed by the burning of fossil fuels."

He explains that traces of radioactive carbon are captured by the grape plants through the absorption of carbon dioxide and eventually transformed into alcohol and other carbon-based components of the wine. The "bomb-pulse" of the atmosphere is eventually absorbed into the wine.

"The year that the grapes were grown fixes the age or vintage of the wine," Jones says.

"The carbon-14 isotope ratio of the wine alcohol can therefore be used to determine the vintage of a wine."

The scientists used a highly-sensitive analytical device called an accelerator mass spectrometer to determine the C-14 levels in the alcohol components of 20 Australian red wines with vintages from 1958 to 1997 and then compared these measurements to the radioactivity levels of known atmospheric samples. They found that the method could reliably determine the vintage of wines to within the vintage year.

In addition to testing alcohol, measuring the age of other wine components, such as tartaric acid（酒石酸） and various phenolic（酚的） substances, can help improve the reliability of the technique for detecting fraud, Jones notes.