Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane

Mixtures of 50% tetramethylsilane (TMS) and methane have been found to give [M+73]⁺ adduct ions and structurally useful fragment ions for many oxygen- and nitrogen-containing organic compounds. All of the reagent ions in TMS react with polar compounds. The high-pressure TMS chemical ionization spectra of many simple oxygenated compounds are in agreement with predictions from ion chemistry of (CH₃)₃Si⁺ obtained by ion cyclotron resonance experiments at very low pressures, but differences are noted. Sensitivities for oxygen-, nitrogen-, and sulfur-containing compounds with TMS as the reagent gas appear to be approximately the same.     

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Diffusion based analysis in a sheath flow microchannel: the sheath flow T-sensor

This paper describes a microfluidic channel that allows for diffusion-based analysis of adsorbing species without passivation of the channel surfaces. The sheath flow configuration was used to measure the diffusion coefficient of fluorescently labeled species from their spatial distribution within the microchannel by analyzing the derivative of the intensity profile at the interface between two distinct core fluids. Measurements for both a small molecule (rhodamine B) and an intermediate-sized protein (wheat germ agglutinin) were made, demonstrating the utility of the sheath flow T-sensor.     

Association reactions of trimethylsilyl ions

Adduct ions, [M + (CH₃)₃Si]⁺, were produced by bimolecular association reactions of trimethylsilyl ions, (CH₃)₃Si⁺, with acetone, cydohexaoone, anisole, dimethyl ether, 2,5-dimethylfuran, 2-methylfuran and furan in ion cyclotron resonance experiments at 300 K and at pressures of ～10^?7 Torr (1 Torr = 133.3 Pa). The rate constants, k_a, for the association reactions varied from 100% to 2% of the collision rate constants, k_c. The rate constants were independent of pressure, except for furan. Measurements were also made of bond dissociation energies for these adduct ions, D[(CH₃)₃Si⁺–X], from equilibrium measurements. The association efficiency, k_a/k_c, increased with increasing bond dissociation energy and with increasing numbers of degrees of freedom, in qualitative agreement with theoretical predictions. Observations pertinent to the dependence of k_a on reactant temperature and relative kinetic energy are discussed. The possibility of determining ion-neutral complex binding energies from radiative association rate constants is considered.     

Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane with aliphatic alcohols

Mixtures of tetramethylsilane and helium have been found to form [M + 73]⁺ adducts, hydrated trimethylsilyl ions and alkyl ions with aliphatic alcohols. The adduct ions were found to be formed by displacement of water from the hydrated trimethylsilyl ion. Ratios of the abundances of the adduct ions to the hydrated trimethylsilyl ion can be used to differentiate among primary, secondary and tertiary alcohols. Sensitivities for a number of alcohols with the tetramethylsilane/helium chemical ionization reagent system are approximately equal.     

Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane with ethers

Chemical ionization mass spectra of several ethers obtained with He/(CH₃)₄Si mixtures as the reagent gases contain abundant [M + 73]⁺ adduct ions which identify the relative molecular mass. For the di-n-alkyl ethers, these [M + 73]⁺ ions are formed by sample ion/sample molecule reactions of the fragment ions, [M + 73 ? C_nH_2n]⁺ and [M + 73 ? 2CnH_2n]⁺. Small amounts of [M + H]⁺ ions are also formed, predominantly by proton transfer reactions of the [M + 73 ? 2CnH_2n]⁺ or [(CH₃)₃SiOH₂]⁺ ions with the ethers. The di-s-alkyl ethers give no [M + 73] ⁺ ions, but do give [M + H]⁺ ions, which allow the determination of the relative molecular mass. These [M + H]⁺ ions result primarily from proton transfer reactions from the dominant fragment ion, [(CH₃)₃SiOH₂]⁺ with the ether. Methyl phenyl ether gives only [M + 73]⁺ adduct ions, by a bimolecular addition of the trimethylsilyl ion to the ether, not by the two-step process found for the di-n-alkyl ethers. Ethyl phenyl ether gives [M + 73]⁺ by both the two-step process and the bimolecular addition. Although the mass spectra of the alkyl etherr are temperature-dependent, the sensitivities of the di-alkyl ethers and ethyl phenyl ether are independent of temperature. However, the sensitivity for methyl phenyl ether decreases significantly with increasing temperature.     

Energy and substituent effects on the mass spectra of substituted benzophenones

For two competing decompositions of the same molecular ion to give products [A₁⁺] and [A₂⁺], the ratio [A₁⁺]/[A₂⁺], is equal to the ratio of rate constants for the formation of the stable ions. Thr ratios, [Y C₇H₄O⁺]/[C₇H₅O⁺], were determined for several benzophenones for electron energies from 15 to 70 eV. Plots of log [Y C₇H₄O⁺]/[C₇H₅O⁺] vs.[ω⁺] gave good straight lines at all energies. Similar correlations have been reported for log [Y C₇H₆⁺]/[C₇H₇⁺] from substituted diphenyl ethanes and are also true for substituted acetophenones, log [YøCO⁺]/[CH₃CO⁺]. A few charge exchange data were obtained which showed the same general trend as the electron-impact data and emphasize the contribution of low energy ions in the 70 eV mass spectra. Relatively poor correlations were obtained for the [Y C₆H₄⁺] and [C₆H₅⁺] ions that are formed by both one-step and two-step decompositions.     

The local microenvironment surrounding dansyl molecules attached to controlled pore glass in pure and alcohol-modified supercritical carbon dioxide

We report on the local microenvironment surrounding a free dansyl probe, dansyl attached to controlled pore glass (D-CPG), and dansyl molecules attached to trimethylsilyl-capped CPG (capped D-CPG) in pure and alcohol-modified supercritical CO2. These systems were selected to provide insights into the local microenvironment surrounding a reactive agent immobilized at a silica surface in contact with pure and cosolvent-modified supercritical CO2. Local surface-bound dansyl molecule solvation on the CPG surface depends on the dansyl molecule surface loading, the surface chemistry (uncapped versus capped), the bulk fluid density, and the alcohol gas phase absolute acidity. At high dansyl loadings, the surface-bound dansyl molecules are largely "solvated" by other dansyl molecules and these molecules are not affected significantly by the fluid phase. When the dansyl surface loading decreases, dansyl molecules can be accessed/solvated/wetted by the fluid phase. However, at the lowest dansyl loadings studied, the dansyl molecules are in a fluid inaccessible/restrictive environment and do not sense the fluid phase to any significant degree. In uncapped D-CPG, one can poise the system such that the local concentration of an environmentally less responsible cosolvent (alcohol) in the immediate vicinity of surface-immobilized dansyl molecules can approach 100% even though the bulk solution contains orders of magnitude less of this less environmentally responsible cosolvent. In capped C-CPG, the surface excess is attenuated in comparison to that of uncapped D-CPG. The extent of this cosolvent surface excess is discussed in terms of the dansyl surface loading, the local density fluctuations, the cosolvent and surface silanol gas phase acidities, and the silica surface chemistry. These results also have implications for cleanings, extractions, heterogeneous reactions, separations, and nanomaterial fabrication using supercritical fluids.     

Sensitivity of Stokes flow to geometry

An experimental investigation of low Reynolds number flow in a rotating tank indicates that in some cases closed eddies in Stokes flow are very sensitive to geometry, especially if the eddies are slender. It is not known if such flows are merely extremely sensitive to the bounding geometry, or if they are actually geometrically unstable.     

Interfaces to PATH 3.0: Design, Implementation and Usage

Several new interfaces have recently been developed requiring PATH to solve a mixed complementarity problem. To overcome the necessity of maintaining a different version of PATH for each interface, the code was reorganized using object-oriented design techniques. At the same time, robustness issues were considered and enhancements made to the algorithm. In this paper, we document the external interfaces to the PATH code and describe some of the new utilities using PATH. We then discuss the enhancements made and compare the results obtained from PATH 2.9 to the new version.