管理提醒： 本帖被 chenshuuu 执行加亮操作(2010-12-27)
Experts can easily spot most counterfeit money by its odd texture or missing security features. In recent years, however, skilled counterfeiters have circulated a new generation of "supernotes"—counterfeit bills so realistic that they evade detection by standard methods. Now researchers at the University of Hong Kong have developed a mass spectrometry technique that recognizes fakes and forgeries by chemically mapping the dyes and inks on a document—without destroying the evidence in the process (. Chem., DOI: 10.1021/ac1020485).
In addition to physical features such as color pattern, texture, or handwriting, currency and written documents have rich chemical information hidden on their surfaces that could help authorities identify forgeries, says Hong Kong chemist Kwan-Ming Ng. Unfortunately, most chemical ysis methods destroy the sample. Ng and colleagues wanted to devise a non-destructive method that extracts both physical and chemical information from suspicious documents.
The investigators turned to a technique called imaging mass spectrometry that others had developed to visualize the chemical composition of biological tissues. The technique allows researchers to scan a surface and collect data on chemicals at specific locations. This method could produce a map of the types of ink molecules on a bank note. But existing protocols require applying chemicals called organic matrices or solvents to the sample and therefore could destroy documents or ear ink to produce low-resolution images.
To get around these destructive steps, Ng and colleagues deposited a thin layer of gold nanoparticles on banknotes and checks. When the researchers irradiated the nanoparticle-coated documents with a nitrogen laser, the gold nanoparticles helped transfer energy to chemicals in the documents' visible and fluorescent inks. The energy ionized ink chemicals, and the researchers then detected the ions with a mass spectrometer.
The scientists scanned a 3 x 3.5 cm area of a Hong Kong $20 banknote to reveal that a layer of red and blue ink sat on top of a background ink layer. The upper layer shielded the lower layer from ionization, resulting in an interrupted pattern of background ink signals where the inks overlapped. The researchers say that authorities could detect fake bills with this method even if they look authentic on the surface because counterfeiters may not know how to properly order the ink layers, especially for complex printing patterns.
The researchers next applied the technique to detect a simulated check forgery. With a black ballpoint pen, a researcher wrote the amount of $15,000 on a check. Then, using a black ballpoint pen made by a different company, the person changed the amount to $48,000. Imaging mass spectrometry identified an ion found only in ink from the second pen, exposing the forgery.
Marcos Eberlin, a mass spectrometrist at the State University of Campinas in Brazil, compliments the researchers on the high-quality images produced by their technique. Good methods for forgery detection are in high demand, he says, so the next step should be to produce "miniature instrumentation that is cheap and simple enough to be used in police labs."
