News
Shpol'skii spectroscopy of amino- and nitro-PAHs Amino- and nitro-substituted PAHs are mutagenic and found in various common materials, including coalderived products, shale oil, and diesel and gasoline exhaust aerosols. Igor S. Kozin and colleagues at Free University (The Netherlands) used Shpol'skii spectroscopy to study the luminescence properties of individual amino-PAHs with twotofive-ringsystems in «-alkane solvents at room and low temperatures. For nitro-PAHs they used reference materials to optimize a procedure that consisted of 3. nor"mal-phase separation off-line chemical reduction with zinc powder and Shpol'skii spectroscopy They also investigated photostabilitv linearity of emission response and the influence of the solidification rate on spectral features Their data indicated that Shpol'skii spectroscopy was not generally applicable to detecting the whole range of amino-PAHs, and only 1-nitropyrene was identified and quantitated in a reference material that contained other nitro-PAHs at lower levels. Shpol'skii spectroscopy has potential as a complementary technique and, if sample solidification can be accelerated, its applicability could be improved. (Anall Chim. Acta 1996,333,193-204)
Going long NMR methods are among the best analytical tools for studying liquid crystalline materials at the molecular level. The recent introduction of 2-D techniques for measuring heteronuclear dipolar couplings has opened up applications to more complex crystalline molecules such as phospholipids. However, both 1-D and 2-D methods fail to yield long-range dipolar coupling data, which can provide additional molecular insights. Lyndon Emsley and colleagues from the Ecole Normale Superieure de Lyon (France) described, for the first time, an NMR-pulse sequence that allows the direct measurement and assignment of longrange heteronuclear dipolar couplings in oriented phases without geometrical assumptions. The researchers illustrated its use by determining chain-chain, chaincore, and intercore dipolar couplings in a nematic liquid crystal. Measuring longcouplings provided strong experimental constraints for models of the internal motions of liquid-crystal molecules in the nematic phase A 3-D Fourier transformation of the data from the new pulse sequence yields a pair-
Measuring the double layer Double-layer phenomena play a crucial role in interfacial systems, yet there have been few reported direct measurements of the structure and forces at die interface, particularly for electrodes. Allen J. Bard and co-workers at the University of Texas at Austin attacked this problem using an atomic force microscope (AFM). The researchers measured the force between a spherical probe tip placed on the end of an AFM cantilever and an electrode, as the
Room temperature (dotted line) and Shpol'skii fluorescence spectra (solid line) of (A) 2-aminonaphthalene and (B) 1,2-diaminonaphthalene. (Adapted with permission from Elsevier Science.)
wise dipolar field spectrum, as shown in the figure. For each distinguishable carbonproton pair, a single doublet splitting (irrespective of the multiplicity of the proton) represents the averaged dipolar coupling between the two nuclei. The meehod also provides assignment of short- and mediumrange splittings, which is not possible in 2-D experiments. The authors envision that the technique could be used to measure heteronuclear couplings in lipid samples, multicomponent liquid crystals, or liquid crystals in cholesteric or ferroelectric phases. (J) Am. Chem. .oc. 1996 111 12224-25)
Schematic of the pairwise local field spectrum obtained from a 3-D Fourier transformation.
probe passed through the double layer in an aqueous solution with various alkalihalide electrolytes. Experiments show that the interaction between two silica surfaces was repulsive and could be accurately predicted by standard theory, showing the surfaces to be negatively charged at pH 5.5. On the other hand, an attractive interaction was measured between silica and gold surfaces at neutral pH. Under electrochemical conditions, the silicagold interaction was attractive at positive potentials and repulsive at negative potentials. Force measurements as a function of the gold electrode potential indicated that the repulsive force increased when moving toward negative potentials which corresponds to a loss of adsorbed anions Force data were also compared with theoretical predictions of the forces between riissimilarlv charged surfaces Chem 1996 TOO 1880&-i
Schematic of modified force-measuring tip and cantilever. 78 A
Analytical Chemistry News & Features, February 1, 1997
