Density-driven transport of gas phase chemicals in unsaturated soils.

Variations of gas phase density are responsible for advective and diffusive transports of organic vapors in unsaturated soils. Laboratory experiments were conducted to explore dense gas transport (sulfur hexafluoride, SF6) from different source densities through a nitrogen gas-dry soil column. Gas pressures and SF6 densities at transient state were measured along the soil column for three transport configurations (horizontal, vertically upward and vertically downward transport). These measurements and others reported in the literature were compared with simulation results obtained from two models based on different diffusion approaches: the dusty gas model (DGM) equations and a Fickian-type molar fraction-based diffusion expression. The results show that the DGM and Fickian-based models predicted similar dense gas density profiles which matched the measured data well for horizontal transport of dense gas at low to high source densities, despite the pressure variations predicted in the soil column were opposite to the measurements. The pressure evolutions predicted by both models were in trend similar to the measured ones for vertical transport of dense gas. However, differences between the dense gas densities predicted by the DGM and Fickian-based models were discernible for vertically upward transport of dense gas even at low source densities, as the DGM-based predictions matched the measured data better than the Fickian results did. For vertically downward transport, the dense gas densities predicted by both models were not greatly different from our experimental measurements, but substantially greater than the observations obtained from the literature, especially at high source densities. Further research will be necessary for exploring factors affecting downward transport of dense gas in soil columns. Use of the measured data to compute flux components of SF6 showed that the magnitudes of diffusive flux component based on the Fickian-type diffusion expressions in terms of molar concentration, molar fraction and mass density fraction gradient were almost the same. However, they were greater than the result computed with the mass fraction gradient for >24% and the DGM-based result for more than one time. As a consequence, the DGM-based total flux of SF6 was in magnitude greatly less than the Fickian result not only for horizontal transport (diffusion-dominating) but also for vertical transport (advection and diffusion) of dense gas. Particularly, the Fickian-based total flux was more than two times in magnitude as much as the DGM result for vertically upward transport of dense gas.

Self-assembly formation of the magic ion of (H2O)20O+: observation of nanoscale cages of oxygenated water clusters induced from iron nanoparticles.

For the first time, we observed a stable and intense ion (m/z 376) of the oxygenated water cluster ion ((H(2)O)(20)O(+)) produced from simply spraying an aqueous solution of iron nanoparticles (Fe NPs) into an electrospray mass spectrometry (ESI-MS) system. Tandem mass spectrometric (MS/MS and MS/MS/MS) results were applied to identify the assignments of the fragment ions of m/z 376 in order to explore the possible structures of this cluster ion. The possible structures of the (H(2)O)(20)O(+) ions are proposed as pentagonal dodecahedron water clathrate cages from the results of tandem mass spectrometry since eliminations of five water molecules were frequently observed in the MS/MS results for many subsequent fragment ions of m/z 376. The formation of this oxygenated water cluster ion ((H(2)O)(20)O(+)) in ESI-MS is attributed to the high surface reactivity and surface energy of Fe NPs during ESI processes (under high temperature and high voltage (5 kV) of ESI spray environment). We believe that the observation of self-assembly formation of oxygenated water clusters is an important issue in nanoscience as well as in the fields of water clusters.

Revolving hollow fiber-liquid phase microextraction coupled to GC/MS using electron ionization for quantification of five aromatic hydrocarbon isomers.

A dynamic liquid phase microextraction method using a revolving hollow fiber was demonstrated for coupling to GC/MS [using EI (electronic ionization) and selected ion monitor (SIM)] as a concentrating probe for rapid analysis and quantitative determination of five aromatic hydrocarbon isomers (cumene; propylbenzene; 2-ethyltoluene; 1,2,3-trimethylbenzene; and 1,2,4-trimethylbenzene) from biological matrices (human urine and human plasma). This technique was named as revolving hollow fiber-liquid phase microextraction (RHF-LPME). The optimized parameters of RHF-LPME coupled to GC/MS experiments were extraction solvent, toluene; extraction time, 2 min; sample agitation rate, 700 rpm; salt concentration, 0%; rotating speed for motor driving rotator, 250 rpm; and the rotator was operated in a reversed direction with the stirrer. The linear range of calibration curve of RHF-LPME was from 0.002 to 0.4 microg/mL with R(2) > 0.9916 and the RSD values were from 4.5 to 5.2%. Additionally, comparing to single drop microextraction (SDME), this method offers better limits of detection (LODs) and EF (enrichment factor). This approach exhibits many advantages including simplicity, rapid detection with high reproducibility and high extraction efficiency, easy to operate and fast to reach equilibrium for analyzing biological samples. This approach has the potential to be widely used because it only requires simple devices to perform all extraction processes. We believe that this technique can be a powerful tool for GC/MS analysis of biological samples and clinical applications in the near future.

Probing the non-covalent binding interaction of the Na+ channel inactivation gate peptide in a linker between domain III and IV with 5,5-diphenyhydantoin in electrospray ion trap tandem mass spectrometry.

The Na(+) channel-subunit containing an Ile1488, Phe1489 and Met1490 (IFM) motif is critical for a fast inactivation process. BL-1, a model IFM-containing peptide with a sequence of acetyl-GGQDIFMTEEK-OH, was observed as a doubly charged potassium-adduct ion by electrospray ionization mass spectrometry (ESI-MS) and a singly charged ion by atmospheric-pressure matrix-assisted laser desorption/ionization mass spectrometry (AP-MALDI-MS). Two crown ethers were applied to demonstrate their desalting ability and then to confirm the potassium-adduct assignments. In order to probe the best binding condition for BL-1 with a local anesthetic drug, 5,5-diphenyhydantoin (DPH), a series of experiments were performed and the parameters affecting complexation were carefully investigated including molar ratios, reaction time and reaction temperature. The most effective conditions for the observation of the complex by ESI-MS were molar ratio of BL-1 and DPH of 1:28 after 18 h of incubation at 40 degrees C. In addition, collision-activated dissociation (CAD) was successfully applied to confirm the formation of the complex between BL-1 with DPH that is via a weak non-covalent bonding with a 1:1 stoichiometry.