科研之友
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摘要
With its fiery hue, amber has been valued for millennia for ornaments. Amber pieces, often hundreds of millions of years old, are even more treasured when they entomb plants, insects or other invertebrates or, more rarely, vertebrates such as amphibians and reptiles. Occasionally, the globules contain evidence of birds or mammals, such as a feather or fur. These inclusions were trapped in ancient sticky tree resin that then hardened and polymerized over the eons to become amber.
But whenever an object fetches a high price, fakes are soon to follow. Precious family heirloom jewelry from the early 1900s often turns out to be celluloid or Bakelite plastics. In Sir Arthur Conan Doyle's books, Sherlock Holmes investigated the fake amber industry in England in the 1800s. Even Chinese writers in the fifth century A.D. warned against forgeries made of eggs and fish oil.
More recent counterfeits-made of resins, plastics or both-can imitate amber's refractive index, specific gravity and ability to generate a static charge when rubbed. Fortunately, there does not yet seem to be one material that can duplicate all of amber's natural properties. Melted resin from contemporary trees, usually poured over some unfortunate creature, possesses amber's scent and its ability to retain warmth, but not its hardness. Modern polymers mimic amber's property of floating in salt water, but have not yet managed to give off the same piney scent when touched with. a hot needle.
Determining a sample's botanical provenance is of interest to the scientist as well as the gemstone dealer and purchaser. Every day on the Internet commerce site eBay, unscrupulous sellers list numerous "amber" pieces that are obviously a sham to even a casual observer, yet unwary buyers still pay hundreds of dollars for these items. But if more skillful simulations were able to fool scientists, the consequences would be greater than just financial loss.
Amber is renowned as a preserver of ancient organisms, so paleobiologists study amber samples in an effort to learn more about Earth's past history. Amber specialists, ourselves included, are often asked to evaluate samples. If we were somehow erroneously to give our stamp of approval to a false piece of amber, any inclusion it contained might skew the direction of scientific investigation into that species' lineage. With such a serious outcome on the line, we began to wonder, could we be fooled?
Scientists can use more-modem tools and tests than a simple smell check to aid in their verification of a sample. Nuclear magnetic resonance (NMR) spectroscopy uses the magnetic properties of nuclei such as hydrogen or carbon to characterize different substances. The chemical environment of the nuclei influences their interaction with a magnetic field, as registered by what is called the chemical shift. The result is a spectrum of frequencies, reflecting the array of chemical species present in the material. This creates a sort of fingerprint for each compound in the sample.
But not all amber is the same. Deposits are found in several regions around the world and likely from more plant families than currently known from the fossil record. Therefore it's necessary to create NMR catalogs of many different kinds of amber. Besides helping to discern true from spurious samples, such a library has the added advantage of possibly indicating what kind of tree the amber may have come from, which could give us a better idea of the prehistoric landscape.
But to tell what tree amber came from, we may need to have something to compare it with as a starting point. Our approach has been to start with the exudates that modern plants ooze. There is no comprehensive library that relates NMR signatures to the taxonomy of modern plants that produce exudates. For the past 10 years, one of us (Santiago-Blay) has been amassing samples from all kinds of plants. We have now begun to outline the chemical picture of these gathered specimens of plant exudates.
- 出版日期2007-4
- 单位西北大学
