Sample, don't trample
Historical, archaeological, and paleontological artifacts are precious. And often preciously small: a 500-millimeter fossil fragment, 2 milligrams of charcoal from a prehistoric fire. Decoding the chemical composition of a material—especially things like bone, shell and teeth—can yield a wealth of information about the organism and time to which it belonged. But often studying something means dismantling it, and the thought of grinding some part of these tiny treasures into a fine powder for analysis makes museum curators cringe.
In a lab at the Smithsonian Institution's Museum Conservation Institute in early spring, scientist Odile Madden fingers tortoise shell hair combs and samples of elephant tusks. She explains that one technique can differentiate between an object made out of ivory from...
Raman spectroscopy can peer into the molecular interstices of many materials, fingerprinting their composition and the nature of their chemical bonds in great detail without harming the object it's probing. Other nondestructive techniques, such as infrared spectroscopy, analyze molecular structure with less resolution. "Infrared spectroscopy can tell you that you have a protein. It can't tell you if you have keratin, which is the protein of horns and hair and turtle shells," Madden says.
Raman spectroscopy entails shooting a beam of laser light at an object, then collecting and analyzing the light that bounces back. By calculating tiny amounts of energy lost or gained by the electron cloud or vibrating chemical bonds of the sample—a phenomenon known as the Raman effect, discovered and demonstrated with a crude spectrometer in 1928 by Nobel Prize-winning physicist Sir C.V. Raman—researchers can characterize the chemical nature of the material without disassembling it.
Though it was invented more than 70 years ago, Raman historically played second fiddle to other chemical analysis techniques, such as nuclear magnetic resonance and fluorescence spectroscopy. Then in the mid 1990s, Raman spectrometers incorporated newer lasers as their light sources, and featured more sensitive detectors and optics.
Though it is not cheaper than other technologies, Raman spectroscopy has recently become extremely lightweight and portable. Madden proudly displays her six-pound, lunchbox-size, portable Raman unit—"The iPod" of Raman, she calls it—that can take the analysis directly to the artifact. Most museums have Raman, and use is slowly catching on elsewhere.
Madden, who works at the Smithsonian's sprawling storage facility in Suitland, Md., can often be found hunkered over one of the institute's 137 million artifacts, using Raman spectroscopy to determine its composition and thus the best way to preserve it in a museum setting.
Madden tumbles a nondescript flake from a small vial into her hand. "This is a piece of mammoth bone," she says. The fragment chipped off of a near-complete, 13,000-year-old mammoth skeleton residing at the Arizona State Museum in Tucson. Madden used her Raman spectrometer on the fragment and found that the bone chip lacks intact collagen, a protein that lends bone flexibility and strength. "So if you were going to do, say, some proteomics work, or look for DNA, this would probably not be the sample to do it on."
Nancy Odegaard, a University of Arizona conservator and anthropologist, studies the mammoth skeleton and says that Raman spectroscopy could open new doors into exploring the fossil. "[Raman spectroscopy] looks really promising as a way to go in and identify some of the things we haven't been able to identify previously," she says. "I think we're kind of forging some new territory here."
Already, it seems that biologists are starting to use Raman spectroscopy in biological contexts. In 2008, Purdue University biological engineer Joseph Irudayaraj used Raman to detect gold nanoparticles (which Raman picks up particularly well) hooked to DNA strands complementary to BRCA1 gene variants (Anal Chem, 80:3342–49, 2008). "We can detect very very low quantities of DNA that other techniques may or may not be able to track," he says. Irudayaraj adds that one of his labs' goals is to make Raman a feasible and more widespread technology that could be used to track biological phenomena, such as monitoring the influx and efflux of specific proteins, in living cells.