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991.
The influence of sample matrix composition, absorption behavior and laser aerosol particle size distribution on elemental fractionation in laser ablation inductively coupled plasma mass spectrometry was studied for nanosecond laser ablation at a wavelength of 266 nm. To this end, lithium tetraborate glass samples with different iron oxide contents and trace amounts of a group of 11 elements were prepared synthetically. The samples were characterized in terms of optical absorbance, melting points, trace element concentrations and homogeneity. UV/VIS spectra showed that sample absorption rises with increasing Fe2O3 content. Crater depths and time-dependent particle size distributions were measured, and ablated and transported sample volumes were estimated. Furthermore, the laser aerosol was filtered using a particle separation device and transient ICP-MS signals were acquired with and without filtering the aerosol. The results demonstrate that the amount of ablated sample is related to the absorption coefficient of the sample and therefore to the optical penetration depth of the laser beam into the sample. The higher energy densities resulting from the shorter penetration depths result in smaller average particle sizes for highly absorbing samples, which allows more efficient transport to and atomization and excitation of the ablated material within the ICP. The particle size distribution changes continuously with ablation time, and larger particle fractions occur mainly at the beginning of the ablation, which leads to particle-related fractionation processes at the beginning of the transient signal. Exceeding a critical depth to diameter ratio, laser-related elemental fractionation processes occur. Changes in the volatile to non-volatile element intensity ratio after the aerosol is filtered indicate that particle size-related enrichment processes contribute to elemental fractionation. 相似文献
992.
993.
Robert A. Whiteside Raghavachari Krishnan John A. Pople Mary-Beth Krogh-Jespersen Paul Von Ragúe Schleyer Gottfried Wenke 《Journal of computational chemistry》1980,1(4):307-322
The geometries and energies of beryllium clusters up to Be5 are examined using ab initio molecular orbital theory. Allowances are made for electron correlation with Møller—Plesset perturbation theory to fourth order. Correlation is found to have a dramatic effect on the relative energies of the several structures examined for Be4 and Be5. Furthermore, the effect of d-type basis functions on the correlation energy results in an increased binding energy for the clusters. Be2 is only weakly bound. For Be3, the best estimate of the binding energy is 6 kcal/mole for the singlet equilateral triangle. Be4 is tetrahedral in its ground state and the estimated binding is 56 kcal/mole. The best structure for Be5 is a singlet trigonal bipyramid, and the binding energy is 88 kcal/mole at the highest level of theory used. 相似文献
994.
Titration curves for the acid—base–titration of polyvalent acids and bases and their mixtures can be created in a simple way and presented graphically. The method, an alternative to the existing procedures, shall be presented here for discussion. With the described method it is possible in an easy manner and for didactic purposes to show how the boundary conditions, such as concentrations, pKa values, Kw values (temperature–dependent), and mixing ratios influence the shape of titration curves.The method comes from closed mathematical formulas, which can be developed in the form: titration degree, T = f(pH). An increase in the titration volume is, therefore, not considered. It can be neglected in practice by choice of suitable volumes and concentrations. The graphic presentation can be realized with the help of a customary mathematic program, such as MathCAD 7. The titration curves are derived in the familiar form of pH = f(T). During the derivation of the equations for the titration of polyvalent acids and bases, difficult expressions appear and approximations are suggested with only small deviations from the exact formulas, which nevertheless are both simple and manageable.The procedure is presented in a series of practical examples where computed and measured titration curves are compared. When using polyvalent acids, such as malonic acid or citric acid, the appropriate activity coefficients have to be considered. If the ionic strength is constant, which can be accomplished by addition of electrolytes like KCl, this is easily accomplished and is demonstrated by examples. 相似文献
995.
David T. Connor Patricia A. Young Maximillian Von Strandtmann 《Journal of heterocyclic chemistry》1977,14(1):143-146
The synthesis of pyridine N-oxide substituted chromones, chromanones, coumarins, quinolines, dihydroquinolines and cinnolines from l-(2-hydroxyphenyl)-2-(2-pyridinyl)ethanone N-oxide, 1-(2-aminophenyl)-2-(2-pyridinyl)ethanone N-oxide and 1-[2-(methylamino)phenyl]-2-(2-pyri-dinyl)ethanone N-oxide is described. 相似文献
996.
The effects of substituents (X) on the structures and stabilities of CH2X? anions for groups comprised of fourth- and fifth-period main group elements (X = K, CaH, GaH2, GeH3, AsH2, SeH, Br, Rb, SrH, InH2, SnH3, SbH2, TeH, and I) have been investigated by ab initio pseudopotential calculations. Full geometry optimizations have been carried out on the CH2X? anions and the corresponding neutral parent molecules, CH3X, at HF/DZP + and MP2/DZP + levels. Results for substituents from the second (X = Li? F) and third (X = Na? Cl) periods provide comparisons of substituent effects of the main group elements of the first four rows of the periodic table on methyl anions. Frequency calculations characterize the nature of stationary points and show pyramidal CH2X? anion structures to be the most stable unless π acceptor interactions (e.g., with BH2, AlH2, GaH2, and InH2 favor planar geometries. The CH2X? stabilization energies [at QCISD(T)/DZP + /MP2/DZP + + ZPE level for X = K? I and QCISD(T)/6?31 + G*/MP2/6?31 + G* + ZPE level] for X = Li? Cl) also show strong π-stabilizing effects for the same substituents. With the exception of CH3 and NH2, all substituents stabilize methyl anions, although the σ stabilization by OH and F is small. The SiH3? PH2? SH? Cl, GeH3? AsH2? SeH? Br, and SnH3? SbH2? TeH? I sets of substituents give stabilization energies between 19 and 30 kcal/mol. The stability of methyl anions substituted by the halogens and the chalcogens (X = OH, SH, SeH, and TeH) increases down a group in accord with the increasing substituent polarizability, while for π acceptors (BH2, AlH2, GaH2, and InH2) the stability decreases down a group in line with their π-accepting ability. © 1994 by John Wiley & Sons, Inc. 相似文献
997.
Helmut Bode und Horst Fabian 《Fresenius' Journal of Analytical Chemistry》1959,170(2):387-399
Zusammenfassung Die Empfindlichkeit der flammenphotometrischen Bestimmung von Gallium, Indium und Thallium wird durch Zusatz organischer, mit Wasser mischbarer Lösungsmittel zur wäßrigen Analysenlösung beträchtlich gesteigert. Es wird vorgeschlagen, 80 Vol-% Aceton enthaltende Lösungen zu verwenden. Beim Arbeiten mit dem Spektralphotometer PMQ II mit Flammenzusatz der Fa. Zeiss erzielt man eine 40 bis 50fache Steigerung der Empfindlichkeit. Der Einfluß der Konzentration verschiedener Säuren und der Anwesenheit von Fremdmetallen in der Analysenlösung wird geprüft. Die Zahl der Störungen ist gering. Bei extraktiver Anreicherung der genannten Elemente sind so gut wie keine Störungen zu erwarten. Die Anwendung des Verfahrens auf die Bestimmung von Gallium in verschiedenen Materialien wird beschrieben.Unsere Arbeit wurde durch die Deutsche Forschungsgemeinschaft und den Verband der chemischen Industrie — Fonds der Chemie — gefördert, wofür auch an dieser Stelle gedankt sei. 相似文献
998.
The synthesis of the 7-deaza-2′-deoxy-adenine derivatives 7b–3 with chloro, bromo, or methyl substituents at C(5) is described. Glycosylation of the 5-substituted 4-chloropyrrolo[2,3-d]pyrimidines 4b–d with 2-deoxy-3,5-di-O-(4-toluoyl)-α-D -erythro-pentofuranosyl chloride ( 3 ) gave the β-D -nucleosides 5b–d , exclusively. They were deblocked (→ 6b–d ) and converted into the tubercidin derivatives 7b–d . 相似文献
999.
1000.