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ANALYTICAL CURRENTS Imaging covalent bonds Franz Giessibl and colleagues at Augsburg University (Germany) have achieved sub-angstrom resolution with a scanning probe microscope (SPM) by imaging covalent bonds. This breakthrough in spatial resolution could have a significant impact on nano- and picoscale analyses in all areas of science. Atoms with closed electron shells are spherically symmetric. But atoms with partially filled shells can form covalent bonds. The charge distribution between the atoms creates pointed lobes. Tungsten is a transition metal that, at a certain surface, has a square arrangement of four lobes within 100 pm of each other. To image the charge distribution between tungsten atoms, the investigators reversed the conventional roles of the SPM tip and the surface so that the surface imaged the tip. They used highly oriented pyrolytic graphite as the surface. The small carbon atoms in the graphite traced the large atoms of tungsten in the SPM tip.
The tungsten tip was vibrated at a certain frequency and brought close to the graphite surface. As the tip moved over the surface, the researchers measured the interactions between the tungsten tip and the graphite surface. Higher harmonics in the tip–surface interactions were used to improve the spatial resolution of the scanned image and accurately map the tungsten atoms in the tip. Using this approach, the investigators were able to image the covalent bonds between the tungsten atoms with a lateral distance of 77 pm. Giessibl and colleagues had to operate the microscope at liquid-helium temperature (4.2 K) to minimize thermal vibrations between the atoms. The low temperature also permitted low-noise measurements. The investigators believe future innovations lie in the use of different types of tips. Diamond, beryllium, or hydrogen-terminated materials could reveal the properties of substances at the pi-
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Tungsten
Small carbon atoms on a graphite surface are used to image the covalent bonds of tungsten atoms located in an SPM tip. (Adapted with permission. Copyright 2004 American Association for the Advancement of Science.)
cometer-length scale. (Science 2004, 305, 380–383)
To determine whether a painting is a van Gogh or a van Faux, art experts often analyze a small piece of paint from the work. Methods such as FTIR or GC/MS that are currently used for studying proteinaceous paint components either require large amounts of sample or are unable to pinpoint the exact identities of the proteins. To overcome these limitations, Radovan Hynek and colleagues at the Institute of Chemical Technology, Charles University, the Academy of Fine Arts in Prague, and the Czech Academy of Sciences (all in the Czech Republic) used MALDI-TOF MS for paint analysis.
© 2004 AMERICAN CHEMICAL SOCIETY
The researchers removed a small piece of a 19th-century painting from the National Gallery in Prague for the experiment. The sample was digested with trypsin and run on a MALDI-TOF mass spectrometer. The mass spectrum obtained from the paint sample was then compared with spectra produced by standard mixtures of proteinaceous paint components. On the basis of these results, Hynek and colleagues concluded that the painting contains rabbit glue. This information could be useful for detecting forgeries. (Rapid Commun. Mass Spectrom. 2004, 18, 1896–1900)
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Is that really a van Gogh?
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