Quantitative relationship between chromatographic retentions and molecular structures of polychlorinated dibenzo-p-dioxins (PCDDs)

A new approach to study the quantitative relationships between chromatographic retentions and molecular structures of polychlorinated dibenzo-p-dioxins (PCDDs) is described. The retention equations of PCDDs log k' = A + B/T in gas chromatography (GC) are used to evaluate the properties of the regression coefficients A and B, which have been widely accepted as highly reliable chromatographic retentions. The quantitative relationships between the A, B values and the molecular structures are found. The molecular descriptors given for the first time in this article are very effective. As a result, the regression equations are derived with correlation coefficients greater than 0.9995. The A, B values of PCDDs with no standards available have been predicted according to these relationships. They are very useful in chromatographic identification. The retention times of all PCDDs can be conveniently predicted at any temperature program. Compared with the data obtained from the relevant experiments, the results of prediction are very accurate. (C) 2000 Elsevier Science Ltd. All rights reserved.

Cleanup and quantification of polychlorinated dibenzo-p-dioxins/furans and polychlorinated biphenyls

By using modern techniques of isotope dilution, high-resolution gas chromatography/high-resolution mass spectrometry and multiple ions detection, an effective cleanup, qualitative and quantitative method was developed for polychlorinated dibenzo-p-dioxins/furans (PCDD/F) and polychlorinated biphenyls analysis. Based on the chromatographic relative retentions of PCDD/F, a software was established for automatic peak recognition of all the isomers from tetra- to octachlorine PCDD/F. It ensured good reliability and accuracy of the analytical data.

UV-sensitive P compounds: Release mechanism, seasonal fluctuation and inhibitory effects on alkaline phosphatase activity in a shallow Chinese freshwater lake (Donghu Lake)

Filtrable phosphorus compounds in a shallow Chinese freshwater lake (Donghu Lake) were fractionated by Sephadex G-25 gel-filtration chromatography. Some portions of those compounds released soluble reactive phosphorus upon irradiation with low dose ultraviolet light. Catalase and a hydroxyl radical scavenger (mannitol) markedly prevented photosensitive phosphorus release. The observed effects may be explained by the action of oxidizing reagents such as hydroxyl radicals, produced in photochemical reactions between UV irradiation and humic substances in the water. There was a strong seasonality in UV-sensitive P (UVSP) release. Michaels constants (K-m) of total alkaline phosphatase in the lake water showed a direct positive relation to UVSP. Plot of K-m against the UVSP/phosphomonoester ratio reveals a strong relationship between the two variables. These results suggest that in some situations UVSP may be a competitive inhibitor of alkaline phosphatase activity in the lake. The competitive inhibition of fractionated UVSP on alkaline phosphatase reagent (Sigma) apparently supports this hypothesis.

Comparative extraction of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F-s) from a variety of solid samples

Extraction experiments with spiking of C-13(12)-PCDD/Fs were performed with a variety of PCDD/Fs contaminated samples. The extraction recovery of PCDD/Fs was mainly influenced by PCDD/Fs concentration and the sample matrix. Generally, the first soxhlet extraction with toluene has suitable recovery. From the selected samples, only FAMS4 and 5 which are fly ashes with high concentration, the recovery of the first soxhlet extraction with 24 hr. is low, but PCDD/Fs were almost completely removed after 72 hr. Copyright (C) 1996 Elsevier Science Ltd

Computer assisted peak recognition for polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) in environmental samples

A software has been developed for the peak recognition of 136 polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) after high resolution gas chromatography coupled with mass spectrometry (HRGC/HRMS). Based on the retention times of C-13 labelled 2,3,7,8-substituted PCDD/F internal standards, the retention times of all PCDD and PCDF can be calibrated automatically and accurately. Therefore, it is very convenient to identify the peaks by comparing the retention of samples and the calibrated retention times of their chromatograms. Hence, this approach is very significant because it is impossible to obtain always a standard chromatogram and PCDD/F analysis are very expensive and time consuming. The calibration results can be transferred to Excel for calculation. The approach is a first step to store costly and environmentally relevant data for future application.

a-Si:H���P+/i/n+结构器件

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射频磁控溅射法制备p 型ZnO 薄膜及其特性研究

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基于F-P���滤波器的光纤光栅热疗测温系统的研究

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p-GaN欧姆接触电极及GaN蓝光激光器窄条工艺研究

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纳米硅P+层在非晶/微晶叠层电池隧道结中的应用

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半导体磁性掺杂和p-型掺杂的第一性原理研究

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Hole concentration test of p-type GaN by analyzing the spectral response of p-n(+) structure GaN ultraviolet photodetector

A new method to test the hole concentration of p-type GaN is proposed, which is carried out by analyzing the spectral response of p-n(+) structure GaN ultraviolet photodetector. It is shown that the spectral response of the photodetector changes considerably with reversed bias. It is found that the difference between photodetector's quantum efficiency at two wavelengths, i.e. 250 and 361 nm, varies remarkably with increasing reversed bias. According to the simulation calculation, the most characteristic change occurs at a reversed voltage under which the p-GaN layer starts to be completely depleted. Based on this effect the carrier concentration of p-GaN can be derived.

Metamorphic InGaAs p-i-n Photodetectors with 1.75 mu m Cut-Off Wavelength Grown on GaAs

Top-illuminated metamorphic InGaAs p-i-n photodetectors (PDs) with 50% cut-off wavelength of 1.75 mu m at room temperature are fabricated on GaAs substrates. The PDs are grown by a solid-source molecular beam epitaxy system. The large lattice mismatch strain is accommodated by growth of a linearly graded buffer layer to create a high quality virtual InP substrate indium content in the metamorphic buffer layer linearly changes from 2% to 60%. The dark current densities are typically 5 x 10(-6) A/cm(2) at 0 V bias and 2.24 x 10(-4) A/cm(2) at a reverse bias of 5 V. At a wavelength of 1.55 mu m, the PDs have an optical responsivity of 0.48 A/W, a linear photoresponse up to 5 mW optical power at -4 V bias. The measured -3 dB bandwidth of a 32 mu m diameter device is 7 GHz. This work proves that InGaAs buffer layers grown by solid source MBE are promising candidates for GaAs-based long wavelength devices.