PCBs and OCPs in human milk and selected foods from Luqiao and Pingqiao in Zhejiang, China

This study was conducted to measure the levels of 23 PCB congeners and 6 organochlorine pesticides (OCPs) in human milk and three food types collected from Luqiao and Pingqiao in Zhejiang Province, China. An effort was also made to explore the potential health risk for the mothers and breast-fed infants living in these two localities. Luqiao was selected as the sampling site because it is the largest place for the disassembly of obsolete transformers and electrical waste in China. Pingqiao, located 100 kin NW of Luqiao, is not known to be a place for any electronic or electrical waste and hence was chosen as the control site. Both localities are important agricultural places in the province. The organochlorines were measured in the samples using the GC-PECD technique. Micro-EROD bioassay method was also used as a complement of the chemical analysis to estimate the TEQ levels of dioxin-like PCBs in human milk. The data showed that the human milk, rice, hen egg, and fish samples from Luqiao were more heavily contaminated with PCBs than those from Pingqiao, suggesting that the mothers and their breast-fed infants in Luqiao tended to receive greater exposure to PCBs than those living in Pingqiao. The OCP levels in the two localities were found comparable, suggesting that the major source of contamination with these pesticides was from their agricultural uses. Significant correlation (R-2 = 0.87, P < 0.001) of PCB TEQs was found between the bioassay and chemical analysis method, suggesting that micro-EROD is an effective method for comprehensive determination of TEQ levels in human milk. Comparison with literature data showed that the PCB levels in milk samples from Luqiao were significantly higher than those from localities in other Chinese provinces and comparable to those in developed or industrialized countries. (c) 2007 Published by Elsevier B.V.

Exciton Localization and Delocalization in GaNAs/GaAs Quantum Wells

We have studied exciton localization and delocalization effect in GaNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy (MBE) using photoluminescence (PL) and timeresolved PL measurements. Studied results suggest that, at low temperature and under a conventional CW excitation, measured PL spectra were dominated by localized exciton (LE) emission caused by potential fluctuations in GaNAs layer. However, under short pulse laser excitation, it is different. An extra high-energy PL peak comes out from GaNAs/GaAs QWs and dominates the PL spectra under high excitation and/or at high temperature. By investigation, we have attributed the new PL peak to the recombination of delocalized excitons in QWs. This recombination process competes with the localized exciton emission, which, we believe, constitutes the "S-shaped" temperature-dependent emission shift often reported in ternary nitrides of InGaN and AlGaN in the literature.

Decomposition of emergent macrophyte roots and rhizomes in a northern prairie marsh

Dry mass, nitrogen and phosphorus content in belowground litter of four emergent macrophytes (Typha glauca Godr., Phragmites australis (Cav.) Trin., Scolochloa festucacea (Willd.) Link and Scirpus lacustris L.) were followed for 1.2 years in a series of experimental marshes, Delta Marsh, Manitoba. Litter bags containing roots and rhizome materials of each species were buried in unflooded soil, or soil flooded at three water depths (1–30, 31–60, > 60 cm). There were few differences in dry mass loss in unflooded or flooded soils, and depth of flooding also had little effect on decomposition rates. In the flooded sites, Scolochloa and Phragmites roots lost more mass (48.9–63.8% and 59.2–85.5%, respectively) after 112 days than Typha and Scirpus (36.3–43.6 and 37.0–47.2%, respectively). These differences continued through to the end of the study, except in the shallow sites where Scirpus roots lost more mass and had comparable mass remaining as Scolochloa and Phragmites. In the unflooded sites, there was little difference between species. All litters lost nitrogen (22.9–90.0%) and phosphorus (46.3–92.7%) during the first 112 days, then levels tended to remain constant. Decay rates for our belowground root and rhizome litters were comparable to published literature values for aboveground shoot litter of the same species, except for Phragmites roots and rhizomes which decomposed at a faster rate (−k = 0.0014−0.0032) than shoots (−k = 0.0003−0.0007, [van der Valk, A.G., Rhymer, J.M., Murkin, H.R., 1991. Flooding and the decomposition of litter of four emergent plant species in a prairie wetland. Wetlands 11, 1–16]).