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排序方式: 共有110条查询结果,搜索用时 31 毫秒
1.
Problems associated with interferences in the analysis of serum for polychlorinated biphenyls 总被引:2,自引:0,他引:2
V W Burse M P Korver P C McClure J S Holler D M Fast S L Head D T Miller D J Buckley J Nassif R J Timperi 《Journal of chromatography. A》1991,566(1):117-125
During a recent survey to determine serum concentrations of polychlorinated biphenyls (PCBs) among people living around New Bedford, MA, U.S.A., an unidentified contaminant precluded the quantification of some early eluting Webb and McCall peaks. Loss of data is estimated to have reduced reported serum levels by 12%. Efforts to identify the contaminant by gas chromatography with an electron-capture detector, a Hall electrolytic condutivity detector, and mass spectrometer were not successful. Researchers ascertained, however, that the contaminant is not a PCB, it does not contain halogens, but it may contain phthalates. Vacutainer tubes and closures for serum storage bottles are suspected sources of contamination. 相似文献
2.
V W Burse D F Groce M P Korver P C McClure S L Head L L Needham C R Lapeza A L Smrek 《The Analyst》1990,115(3):243-251
Serum for reference pools of in vivo polychlorinated biphenyls (PCBs) was obtained from four goats that had received one dose (100 mg kg-1) of a selected technical Aroclor (AR) (1016, 1242, 1254 or 1260) and were allowed to recover for 30 d. These pools were used to assess the differences in an analytical method that determines and quantifies PCBs using packed-column gas chromatography (PCGC) (quantified on the basis of mean mass percent. data for grouped PCB peaks) and capillary-column gas chromatography (CCGC) (quantified on the basis of percent. composition data for specific congeners). With CCGC, results were statistically significantly different (p less than or equal to 0.0002) from results with PCGC for ARs 1016, 1242 and 1254 but not for AR 1260 (p = 0.23). When comparing these gas chromatographic methods using bovine serum spiked in vitro with the same ARs at 17-25 p.p.b., it was found that the methods were not statistically significantly different for any of the ARs (p = 0.30-0.92). Levels of serum PCB determined by the two methods for 12 persons, divided into two groups according to exposure, were compared using the paired t-test. Group 1 consisted of three persons with dietary and/or environmental exposure; one with dietary and/or environmental exposure in addition to occupational exposure dating back 20 years. Group 2 consisted of eight persons with recent occupational exposure.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
3.
G. Ayrey B. C. Head R. C. Poller 《Journal of polymer science. Part A, Polymer chemistry》1975,13(1):69-75
14C-Azoisobutyronitrile was used to initiate polymerizations of methyl methacrylate in the presence of the organotin compounds: tetrabutyltin, dibutyltin diacetate, dibutyltin dilaurate, dibutyltin di(ethyl mercaptide), dibutyltin di(dodecyl mercaptide), and dibutyltin dichloride. Only dibutyltin dichloride affected rates of polymerization significantly, and this was ascribed to an increase in the velocity constant kp for the propogation reaction. No evidence was obtained for radical displacement reactions of the polymer radicals with bonds between tin and carbon, oxygen, sulfur, or chlorine. Transfer activity exhibited by the mercaptides was ascribed to traces of thiol impurity, possibly formed during storage. The relevance of these results to the mechanism of stabilization of poly(vinyl chloride) is briefly discussed. 相似文献
4.
5.
Martínez-Junza V Rizzi A Jolliffe KA Head NJ Paddon-Row MN Braslavsky SE 《Physical chemistry chemical physics : PCCP》2005,7(24):4114-4125
Photophysical studies with semi-rigid, 1, and flexible, 2, donor-bridge-acceptor (D-b-A2+) molecules with D a porphyrin and A2+ a methyl viologen moiety, were performed in neat polar solvents as well as included in surfactant (DTAB) aqueous and in reverse AOT/n-alkane micelles. The micelles acted as nanoreactors for the photoinduced electron transfer reaction upon laser excitation. In spite of the longer lifetime of the charge separated (CS) state in the semi-rigid tetrad 1(ca. 200 ns vs. ca. 100 ns for the flexible dyad 2), the CS formation quantum yield, for example in acetonitrile, was lower for the former (phi(CS) = 0.13) than for the latter (0.58). Comparison of the time-resolved fluorescence data in neat solvent and in the micelles yielded the phi(CS) values in the dilute micellar solutions. Application of laser-induced optoacoustic spectroscopy at various temperatures to 1 dissolved in a polar organic solvent (benzonitrile, BZN) included in aqueous DTAB nanoreactors afforded structural volume changes for the production in hundreds of ps of the CS state upon excitation of a polar molecule. The contraction during CS formation upon excitation of the collapsed conformer in BZN is attributed to the entering of solvent into the open molecular cavity. The opening upon formation of the CS state due to photoinduced electron transfer in the 1 collapsed conformation arises from the repulsion of the two positively charged ends in this state, as previously calculated. Inclusion of 1 in reverse AOT micelles in various n-alkanes also led to a contraction upon excitation, but the data had much more error due to the limited range of variability of the ratio of thermoelastic parameters. The data obtained with the more flexible "supermolecule" 2 showed the predicted large conformation flexibility of these molecules. 相似文献
6.
Napper AM Head NJ Oliver AM Shephard MJ Paddon-Row MN Read I Waldeck DH 《Journal of the American Chemical Society》2002,124(34):10171-10181
A systematic determination of electronic coupling matrix elements in U-shaped molecules is demonstrated. The unique architecture of these systems allows for the determination of the electronic coupling through a pendant molecular moiety that resides between the donor and acceptor groups; this moiety quantifies the efficiency of electron tunneling through nonbonded contacts. Experimental electron-transfer rate constants and reaction free energies are used to calibrate a molecular-based model that describes the solvation energy. This approach makes it possible to experimentally determine electronic couplings and compare them with computational values. 相似文献
7.
Mian Chin Gregory L. Durst Simone R. Head Paul L. BockJohn A. Mosbo 《Journal of organometallic chemistry》1994,470(1-2):73-85
Molecular mechanics (MM2) calculations were performed on 54 conformations of 18 phosphines (PH3; PH3−nRn, where n = 1,…3, and R = Me and Et, n = 1 or 2 and R =iPr, and n = 1 and R =tBu, PMe2Et, PMeEt2, and PPhMe2, and PPh2R where R = Me, Et, iPr, tBu and Ph). The results are compared to those previously obtained from MINDO/3 and MNDO calculations, and to experimental data. Single conformer cone angles and weighted average cone angles were calculated from MM2 optimized geometries employing Tolman's general definition, and they are compared to Tolman's values, MINDO/3 results, and T.L. Brown's ER values. Of the cone angle definitions used, the weighted average values are suggested as the best single representation of phosphine ligand sizes. The steric parameters (cone angle and ER values) alone, and in conjunction with electronic parameters, are correlated with experimental data. 相似文献
8.
9.
Functionalized Graphene as a Gatekeeper for Chiral Molecules: An Alternative Concept for Chiral Separation
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Andreas W. Hauser Narbe Mardirossian Julien A. Panetier Martin Head‐Gordon Alexis T. Bell Peter Schwerdtfeger 《Angewandte Chemie (International ed. in English)》2014,53(37):9957-9960
We propose a new method of chiral separation using functionalized nanoporous graphene as an example. Computational simulations based on density functional theory show that the attachment of a suitable chiral “bouncer” molecule to the pore rim prevents the passage of the undesired enantiomer while letting its mirror image through. 相似文献
10.