News
Real-time size and composition of aerosol particles
Electropherograms of the Fmoc-DDXX library (a) without vancomycin and (b) with 104 pM vancomycin.
pushed the detection limits of the MS/MS detection system, which required 100 fmol per component for reliable fragment ions. To avoid sensitivity problems with larger libraries, the most active ligands could be preselected before ACE/MS/MS analysis. The authors point out several advantages of ACE/MS in screening combinatorial libraries. First, the receptor binding studies are conducted in solution, thus eliminating the ambiguities of interaction on an adsorbent surface. Second, the small volumes make the method economical, and third, the separation mechanism easily eliminates noninteracting compounds. Unlike other screening methods, ACE/MS does not require equimolar concentrations of the library compounds to avoid false negatives. (J. Am. Chem. Soc. 1996,118, 7827-35)
Surfactants at the surface Next time you wash your hands or clothes, consider this: Despite the widespread commercial use of surfactants in detergents and soaps, little is known about how water and surfactants interact at liquid surfaces and interfaces at the molecular level. This lack of information limits chemists' ability to predict the behavior of surfactants under various conditions and to design new molecules for specific tasks. By using vibrational sum-frequency generation, G. L. Richmond and colleagues at the University of Oregon were able to measure the vibrational spectra of surfactants and water molecules at the water's air/water interface. 654 A
The U.S. Environmental Protection Agency is currently contemplating tougher regulations on airborne particulates because of recent studies showing a correlation between high levels of particulate pollution and adverse health effects. Information on the chemical speciation occurring within an aerosol particle and data on particle size are crucial for determining the original particle source and for making predictions on the ultimate fate and reactivity of different particle types. However, collecting such data is a challenge because the concentrations of gaseous atmospheric species can vary over time frames as short as a minute. Analysis techniques do exist for real-time measurement of gas-phase species in these time frames, but most chemical analysis methods for particulates rely on filter collection, which
precludes the measurement of hourly or diurnal time-resolved changes. Furthermore, in the time it takes to go from sampling to analysis, these offline techniques for aerosol particles lose analytes to evaporation, condensation, or reaction. Christopher Noble and Kimberly Prather of the University of California-Riverside have developed an instrument that combines a duallaser aerodynamic particle sizing and tracking system with laser desorption/ ionization TOFMS to provide real-time, in situ measurements of the aerodynamic size and chemical composition of individual aerosol particles. At typical ambient concentrations, the aerosol TOFMS analyzes 50-100 particles per minute and, at high concentrations, up to 600 per minute. Coupling this kind of dynamic monitoring of individual particles with data obtained from conventional ambient aerosol sampling devices could aid in understanding complex field data on atmospheric processes. (Environ. Sci. Technol. 1996,30, 2667-80)
Data showing chemical speciation as a function of particle size for two commonly detected cations.
They found that the water molecules show a high degree of orientation in the presence of a charged surfactant, with the directionality dependent on the nature of the surfactant headgroup. The opposing alignment manifests itself by an interference between CH and OH stretching modes, which is constructive for anionic surfactants and destructive for cationic surfactants. Uncharged surfactants show relatively minimal impact on the ordering of interfacial water. (J. Phys. Chem. 1996,100,14272-75)
Analytical Chemistry News & Features, November 1, 1996
Sum frequency spectra of 14 nm dodecylammonium in H20 (•) and D20 (o).
