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1.
2.
We show that the racemic states of the B 2 phase of liquid crystals composed of banana-shaped molecules do not satisfy the Curie principle. Thus it is argued that these states cannot exist in bulk samples and the homochiral states constitute the only stable microscopic structures. A reinterpretation of the racemic states with the same macroscopic optical behaviour is proposed in terms of mixtures of the homochiral structures. 相似文献
3.
Antolín R Borge G Posada T Raposo JC Seara I Etxebarria N 《Journal of AOAC International》2004,87(5):1218-1223
A validation study was carried out in order to evaluate the efficiency of inductively coupled plasma-optical emission spectrometry (ICP-OES) for the analysis of minor elements (manganese, chromium, copper, iron, and titanium) in aluminium alloys. Aluminium casting samples were obtained by adding compressed powder compacts of each alloying element and aluminium (minitablets) to aluminum baths in a laboratory crucible furnace. Digestion of solid samples was performed using concentrated HCI and H202 35% (v/v) previous to analysis by ICP-OES without any matrix separation. This solution-based method was validated considering direct current arc spectrometry as the reference method based on direct analysis without any pretreatment of the solid samples considered. Univariate statistical procedures were carried out, for which precision <3% and trueness of the analytical results were taken into account. 相似文献
4.
Hixson KK Rodriguez N Camp DG Strittmatter EF Lipton MS Smith RD 《Electrophoresis》2002,23(18):3224-3232
A method for the complete peptide mapping of the model integral membrane protein bacteri-orhodopsin is demonstrated. Utilizing more effective enzymatic digestion, procedures with capillary liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) and tandem mass spectrometry (MS/MS), all predicted tryptic digestion products were detected, as well as peptides from all previously reported post-translational modifications of bacteriorhodopsin. A significant contribution of chymotryptic-like digestion products was also observed. A characterization of the behavior of hydrophobic integral membrane peptides in a reversed-phase liquid chromatographic separation is also provided. The method reported here offers improved compatibility of the solubilizing reagents with both the chromatography and mass spectrometry, rendering it suitable for high-throughput proteomic applications. 相似文献
5.
R. Pérez-Olmos M. B. Etxebarria J. L. F. C. Lima M. C. B. S. M. Montenegro 《Fresenius' Journal of Analytical Chemistry》1998,362(2):230-233
A high dispersion FIA system with a fluoride selective and a double junction reference electrode in cascade arrangement was
developed for the determination of the fluoride content in chromium electroplating baths. The analyses of seven synthetic
baths prepared in the laboratory and two samples provided by the industry gave results with a mean recovery value of 100.5%,
and a mean variation coefficient of 1.6%. These data were better than those obtained by the EDTA complexometric titration,
the reference technique, with a mean recovery of 94.0% and a mean variation coefficient of 7.5%. The sampling rate for the
FIA manifold was 20 samples per hour.
Received: 27 November 1997 / Revised: 24 February 1998 / Accepted: 1 March 1998 相似文献
6.
A simple and very efficient protocol for the preparation of highly enantioenriched 2-alkylpiperidines has been set up, which allows the preparation of the final heterocycles with any wanted configuration at the stereogenic center starting from the same starting material. The key step of the synthesis relies on a diastereodivergent aza-Michael reaction protocol using the readily available and cheap reagent (+)-(S,S)-pseudoephedrine as chiral auxiliary. 相似文献
7.
N. Etxebarria G. Arana R. Antolín G. Borge T. Posada J. C. Raposo 《Accreditation and quality assurance》2007,12(11):575-580
Electrolytic manganese is an important alloying element for aluminium and steel melts. It is mainly added to melts of aluminium
in the holding furnace as tablets or minitablets (compressed compacts of manganese and aluminium powders). Selenium derivates
are usually added during the production of electrolytic manganese, so some selenium is present in the alloys produced when
electrolytic manganese is added to the aluminium furnace. Since the selenium contents of many alloys are of concern from health
and environmental perspectives, their values should be provided. In this work, a laboratory reference material (LRM) based
on electrolytic manganese was produced to assure our routine quality control method, where selenium is analysed by hydride
generation followed by optical emission spectrometry with inductively coupled plasma (HG–ICP–OES). Therefore, the present
paper describes in detail the preparation procedure for and the results from homogeneity and stability studies performed on
electrolytic manganese LRM. For this purpose, a commercial electrolytic manganese lot was selected and the main factors involved
in the preparation of the material (pretreatment step, homogenization, bottling and storage) were carefully studied and established
in order to guarantee the long-term stability of the LRM. The results obtained showed that the LRM developed was a fit-for-purpose
material for the quality control of the routine analysis of selenium. 相似文献
8.
A computer program called SPECA and written in compiled BASIC (Microsoft QuickBasic Ver. 4.5) has been written for the calculation of thermodynamic constants in solution equilibria studies from spectrophotometric measurements. In order to calculate the thermodynamic constants it is necessary to introduce different theories to estimate the activity coefficients of the system involved under investigation. In this version of the program the equations proposed in the models of Debye-Hückel. Specific Interaction Theory (SIT) or the Modified's Bromley Theory can be chosen. To achieve its purpose, the program uses absorbance and activity or free concentration data of one of the components, normally H(+), and it is not limited with respect to the number of components that can be studied. Minimization of the error square sum in the absorbance, which can be absolute or relative, is carried out using the Levenberg-Marquardt-Nash algorithm at three different levels: equilibrium constants in the first level, molar absorptivities in the second and interaction coefficients in the third for those systems using the SIT or the Modified Bromley's Theories. 相似文献
9.
Pygmalion S. Lianis Nestor A. Rodios Nicholas E. Alexandrou 《Journal of heterocyclic chemistry》1988,25(4):1099-1102
Mesitonitrile oxide reacts with C ≡ N bond of (aryl)(9-fluorenylidene)acetonitriles 1 forming in very good yields the corresponding 1,2,4-oxadiazoles 2. Analogously also proceeds the reaction of mesitonitrile oxide with 9-fluorenylidenemalonodinitrile (3) with the formation of bis-1,2,4-oxadiazole derivative 4. 相似文献
10.
Experimental dipole moments of a number of the title compounds are given and theoretical dipole moments have been calculated by vector addition for all possible conformations. The graphical method has been used to correlate experimental and calculated dipole moments. The more probable conformation in solution of the title compounds and especially of the imide group is proposed. 相似文献