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1.
The conformational energies, rotational barrier heights and molecular structures in C(CH2X)4 molecules (X=F, Cl, Br) based on molecular-mechanics calculations have been obtained. The results from these calculations are compared with the experimental gas-phase results. 相似文献
2.
Analysis of the beta-blockers oxprenolol, atenolol, timolol, propranolol, metoprolol, and acebutolol in human urine by a combination of isotachophoresis (ITP) and zone electrophoresis (ZE) was investigated. Methods were developed with a conventional capillary electrophoresis (CE) apparatus and a poly(methyl methacrylate) (PMMA) microchip system. With CE the separation of oxprenolol, atenolol, timolol, and acebutolol from a standard solution containing 5 microg/mL of each compound was accomplished by performing ZE with transient ITP. The electrolyte system consisted of 10 mM sodium morpholinoethane sulfonate (pH 5.5) and 0.1% methylhydroxyethylcellulose as the leading electrolyte and 30 mM ortho-phosphoric acid (pH 2.0) as both the terminating and the ZE background electrolyte. With the microchip system the separation of oxprenolol and acebutolol from a standard solution containing 10 microg/mL of each compound was accomplished by a coupled-channel ITP-ZE device using the same leading electrolyte solution as the CE system but 5 mM glutamic acid (pH 3.4) as terminating and background electrolytes. The systems were used for analyses of patient urine samples. Water-soluble hydrophilic matrix compounds were removed from the urine samples by solid-phase extraction (SPE). Limits of quantification below 5 microg/mL could be achieved. The PMMA ITP-ZE chip has not earlier been used for analyses of any drugs from urine samples. 相似文献
3.
Pekka Kekäläinen Mikko Voutilainen Antti Poteri Pirkko Hölttä Aimo Hautojärvi Jussi Timonen 《Transport in Porous Media》2011,87(1):125-149
A model transport system is considered in which a pulse of tracer molecules is advected along a flow channel with porous walls.
The advected tracer thus undergoes diffusion, matrix-diffusion, inside the walls, which affects the breakthrough curve of
the tracer. Analytical solutions in the form of series expansions are derived for a number of situations which include a finite
depth of the porous matrix, varying aperture of the flow channel, and longitudinal diffusion and Taylor dispersion of the
tracer in the flow channel. Novel expansions for the Laplace transforms of the concentration in the channel facilitated closed-form
expressions for the solutions. A rigorous result is also derived for the asymptotic form of the breakthrough curve for a finite
depth of the porous matrix, which is very different from that for an infinite matrix. A specific experimental system was created
for validation of matrix-diffusion modeling for a matrix of finite depth. A previously reported fracture-column experiment
was also modeled. In both cases model solutions gave excellent fits to the measured breakthrough curves with very consistent
values for the diffusion coefficients used as the fitting parameters. The matrix-diffusion modeling performed could thereby
be validated. 相似文献
4.
When the quality system of the health care organization is certified and the laboratory accredited, two different types of
assessments are also performed. To reduce the extra work caused by these different assessments the Finnish Accreditation Service
FINAS and SFS-Certification have made a pilot experiment in combining the assessments. Joint assessments have been a positive
experience to the laboratory as well as to the assessing bodies, and we find this operating model worth developing. 相似文献
5.
The stereospecific total synthesis of (±) - 19,20 - dihydro - 20 - desformyl - 20 - methoxycarbonyl-vallesiachotamine and its 20-desethyl analogue has been carried out by sodium dithionite reduction of appropriate 1-[2-(3-indolyl)ethyl]-3-methoxycarbonyl pyridinium salts to the corresponding 1,4-dihydropyridine derivatives, followed by acid-induced cyclization. 相似文献
6.
Maleic anhydride has many uses in industry, but workers' exposure to it is poorly known. Our new method allows airborne maleic anhydride to be determined with a limit of quantification of 1 microg/m3 per 12 1 of air, i.e., the concentration of about 0.01 times the occupational exposure standard (0.4 mg/m3). Air samples are collected in Tenax tubes containing sodium sulfate as a drying agent. Maleic anhydride is eluted with methyl tert.-butyl ether containing 5% acetonitrile and 0.1% acetic anhydride, and determined by capillary gas chromatography with electron-capture detection without interference from generic anhydrides. The tested method suits both long-term and short-term measurements. 相似文献
7.
The molecular structures of gaseous cyclopentene oxide, cyclohexene oxide and cycloheptene oxide are investigated by electron diffraction. The boat conformation of cyclopentene oxide is confirmed, while a half-chair conformer is determined for cyclohexene oxide. Cycloheptene oxide is found to exist in a 66(2):34(2) conformational mixture of two chair forms. 相似文献
8.
Kalevi Pihlaja Frank G. Riddell Jorma Jalonen Pirkko Rinta-Panttila Marc Anteunis 《Magnetic resonance in chemistry : MRC》1974,6(5):301-302
The 300 MHz 1H NMR spectrum of 4-vinylbutyrolactone has been recorded and analysed. The results showed that the compound is a mixture of two rapidly interconverting envelope forms in which (C-3) is the flap atom. The vinyl group favours the pseudo-equatorial orientation by 1·9 ± 0.3 kJ mol?1, as shown by the dependence of the vicinal coupling constants on temperature. 相似文献
9.
Pirkko Pfäffli 《Fresenius' Journal of Analytical Chemistry》1992,342(1-2):183-185
Summary A new method for the determination of airborne methyl ethyl ketone peroxide (MEKP) is based on the reaction of peroxides with 4-tert-butyl catechol, in which the peroxide radicals oxidize the catechol to the corresponding benzoquinone. The yellow colour generated is measured at 395 nm. Sampling from factory air can be performed by aluminium oxide sampling tubes. Glacial acetic acid is used as a solvent and for the desorption of the peroxide from the aluminium oxide. The detection limit (0.05 mg/m3 for 4 h samples) for the MEKP in air is well below the current hygienic reference value (1.5 mg/m3). The method was used for MEKP determinations during lamination applications in the production of reinforced unsaturated polyester plastics, where the airborne concentrations of the peroxide were well below the current hygienic reference values. 相似文献
10.
Heikkinen S Toikka MM Karhunen PT Kilpeläinen IA 《Journal of the American Chemical Society》2003,125(14):4362-4367
A quantitative method to record (1)H-(13)C correlation NMR spectra (Q-HSQC) is presented. The suppression of (1)J(CH)-dependence is achieved by modulating the polarization transfer delays of HSQC. In addition, the effect of homonuclear couplings, as well as relaxation during the pulse sequence are discussed. We developed the Q-HSQC approach for the quantitative analysis of wood lignin, a complex polymer where it has been difficult to obtain reliable data on the relative amounts of different structural units. The current method is applicable to a variety of complex mixtures, where normal 1D (1)H- and (13)C-NMR methods fail. 相似文献