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71.
An ion trap mass spectrometer, equipped with a membrane/jet separator interface, is used for the direct detection of volatile and semi-volatile organic compounds in aqueous solutions. Aqueous sample is passed through a capillary membrane, the outside surface of which is continuously purged by helium. The permeate is pneumatically transported to the mass spectrometer via a jet separator which acts as an additional enrichment device. The performance and response characteristics of non-porous silicone and microporous polytetrafluoroethylene (PTFE) membranes are studied. The microporous membrane allows sufficient water to pass for it to be used as a reagent gas for chemical ionization. Both types of membranes provide detection limits in the parts per trillion (pptr) to parts per billion (ppb) range with a linear dynamic range of 3 orders of magnitude for some volatile organic compounds. Results show that there is no detectable matrix effect on response in the selected cases examined. The use of microporous membranes to analyze more polar compounds, such as 5-hydroxymethyl furfuraldehyde and lactic acid, is also demonstrated. The effects of other experimental parameters, such as membrane temperature and length, on sensitivity are also investigated. 相似文献
72.
Robin J. Marles James B. Hudson Elizabeth A. Graham Chantal Soucy-Breau Peter Morand R. Lilia Compadre Cesar M. Compadre G. H. Neil Towers J. Thor Arnason 《Photochemistry and photobiology》1992,56(4):479-487
The photoactivated antiviral and cytotoxic activities of the naturally occurring thiophene, alpha-terthienyl (1), and 15 synthetic analogues were evaluated against murine cytomegalovirus and Sindbis virus, and murine mastocytoma cells. After irradiation with near UV light, alpha-terthienyl and most of its analogues had significant toxicity, with minimum inhibitory concentrations in the range of 0.02-40 microM. In the absence of near UV irradiation, only one analogue had antiviral activity and five were cytotoxic. The most active analogues were those containing carboxylic acid, hydroxyl, or cyano substituents. Quantitative structure-activity relationship analysis of thiophene phototoxicity suggested that the rate of singlet oxygen production is the primary determinant of antiviral and cytotoxic activities. For phototoxicity against murine cytomegalovirus, a significant role for hydrophobicity was also demonstrated. Tricyclic thiophenes show significant potential for photochemotherapy of viral infections and cancer, and further evaluation in animal models is recommended. 相似文献
73.
The effects of a ready-made surfactant (sodium dodecyl sulfate) on the dynamic interfacial tension between a model acidic oil (linoleic acid dissolved in paraffin oil) and various aqueous alkaline (NaOH) systems have been studied using pendant drop tensiometry at surfactant concentrations both below and above the critical micelle concentration (CMC). Below the CMC the added surfactant contributes significantly to a further reduction of interfacial tension of the reacting acid/alkaline system, whereas above the CMC the added surfactant plays an important role in damping the dynamic trends observed for the reactive system alone. Copyright 2001 Academic Press. 相似文献
74.
The stereochemistry of istanbulin-B has been defined by X-ray crystal-structure analysis. Rings A and B are transfused, with the Me groups at C(4) and C(5) cis. The α,β-unsaturated,γ -lactone ring is essentially planar. Crystals of istanbulin-B are orthorhombic, space group P2121,21, with a = 6.729(2), b = 13.447(3), c = 14.721(3)Â,and Z = 4. The crystal structure was determined by direct phasing and the atomic parameters were subsequently adjusted by least-squares calculations that converged at R 38% over 1179 diffractometer |Fa| values 相似文献
75.
John A. Powell Jong J. Whang Frederick H. Owens Roger K. Graham 《Journal of polymer science. Part A, Polymer chemistry》1967,5(10):2655-2664
Copolymers of methyl methacrylate with methyl and ethyl α-hydroxymethylacrylate and with α-hydroxymethylstyrene have been prepared with free-radical initiators at temperatures below 80°C. At higher reaction temperatures or under extrusion conditions, alcohol was eliminated and the free hydroxyl content was greatly decreased. All evidence indicates the formation of six-membered lactone groups in this post-polymerization reaction: direct evidence for their formation is lacking, however, since neither infrared nor nuclear magnetic resonance spectra could be used to detect lactonization in this system. The loss of activity from 14C ester-labeled methyl methacrylate copolymer on heating could be correlated with the extent of lactonization. The degree of lactonization is relatively less with copolymers containing higher amounts of hydroxymethyl groups. The resulting polymers exhibit higher heat distortion temperatures and decreased impact resistance when compared to poly(methyl methacrylate). Attempts were made to incorporate similar lactone structures by cyclocopolymerization with methyl methacrylate of α-methacryloxymethylstyrene or ethyl α-methacryloxymethylacrylate, but only crosslinked polymers or polymers with pendent unsaturation were found. 相似文献
76.
Philip J. Cox Olga A. Melvin Simon J. Garden James L. Wardell 《Journal of chemical crystallography》1995,25(8):469-475
The title organotin carbohydrate, C31H36O6Sn, has been synthesized and its molecular structure has been determined in solution and in the solid state. NMR, infrared, mass and X-ray crystallographic techniques were used. The chiral molecules crystallize in the monoclinic space group P21 withZ=2. The triphenyltin and carbohydrate moieties are linked by a trans methylene-oxygen-methylene arrangement. The pyranosyl ring adopts a twist-boat conformation and the isopropylidene rings adopt different (half-chair and envelope) forms. Solution and solid-state conformations are similar as only three 13C shift values are greater than 2ppm; the 119Sn value is 12 ppm. 相似文献
77.
78.
Uphill transport of L-Dopa and phenylalanine (Phe) across cation-exchange membranes into micelle-containing receiver solutions is reported. With L-Dopa as the analyte in a sample solution at a pH where it is it a zwitterion, preconcentration by a factor of 3.2 +/- 0.2 (n = 5) is observed when 0.10M sodium dodecyl sulfate (SDS) is the receiver. When the SDS concentration is varied, preconcentration of L-Dopa is not observed until the critical micelle concentration is reached. Similar results were obtained with Phe as the analyte under conditions where it is protonated in both the sample and receiver. The transport is demonstrated to obey the assumptions required to quantify the results by the fixed-time kinetic method. That is, the amount of Phe transferred from a 200-ml sample across a 10-cm(2) membrane into a 5-ml receiver was directly proportional both to the dialysis time for up to 90-min and to the initial concentration of Phe in the sample when a 60-min dialysis time was used. The latter yielded a constant enrichment factor, 4.8 +/- 0.2 (n = 6), when the sample concentration of Phe was in the range 0.61 mM-6.0 muM. Means to increase the enrichments to practical values are discussed. 相似文献
79.
80.