Ion mobility spectrometry peak width data are fitted by a least-squares procedure to a semiempirical model having three adjustable parameters. Peaks are wider than contributions from initial pulse width and diffusion predict, and it is suggested that the additional width is due mainly to electric field inhomogeneity and Coulombic repulsion. The effects of operating conditions and instrument dimensions on resolving power are discussed. It is proposed that increased inhomogeneity of the electric field results in lower measured mobility values, as well as lower resolving power.
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A new analytical methodology combining on-line supercritical fluid extraction with high-resolution capWary gas chromatography for automated sample preparation and analysis Is described. Analytical-scale supercritical fluid extraction utilizes the variable solvating power of a supercritical fluid to selectively extract and Isolate discrete fractions from a sample matrix. The supercritical fluid extract Is decompressed through a restrictor to deposit and concentrate the analytes at the Inlet of a standard capillary gas chromatography column for subsequent analysis. This methodology allows several modes of operation Including quantitative extraction of all analytes from a sample matrix, quantitative extraction and concentration of trace analytes, selective extractions at various solvating powers to obtain specific fractions, or multiple-step extractions at various pressures for qualitative characterizations. This Initial report describes the later two modes of operation and demonstrates the potential usefulness of this methodology for sample extraction and selective fractionation using a standard polycyclic aromatic hydrocarbon mixture and two complex sample matrices.Until recently the use of supercritical fluid extraction (SFE) has been generally confined to relatively large-scale chemical processing applications (1-3). However, the use of SFE methods for analytical applications is attracting increased attention (4-8). The potential advantages of SFE accrue from the physical properties of supercritical fluids. The compressibility of supercritical fluids is large above the critical temperature, and small changes in pressure result in large changes in the density (and solvating power) of the fluid (2).At higher densities molecular interactions increase due to shorter intermolecular distances and solvating characteristics approaching that of a liquid are imparted. However, the viscosity and solute diffusivity can remain similar to those of a gas (2), thus allowing more rapid mass transfer of solutes than feasible with liquids. Many fluids have comparatively low critical temperatures that allow extractions to be conducted at relatively mild temperatures, e.g., 31 °C for carbon dioxide. In addition to using pressure and/or temperature to control the density or solvating power, various fluids or fluid mixtures that exhibit different specific chemical interactions can be used to obtain the desired selectivity.Recent studies have shown that analytical SFE provides comparable or better extraction efficiencies than conventional Soxhlet extraction and with over an order of magnitude increase in the rate of extraction (6). Other important potential advantages of SFE include the capability of selective extraction as a function of fluid solvating power, fractionation during collection (9), and the compatibility with on-line analysis of the extraction effluent. Various modes of on-line
Abstract. Coronaspray ion mobility spectrometry was used in sample detcction following reversed-phase liquid chromatographic separation. Samples were introduced by coronaspray using a fused silica transfer line that was inserted through a stainless steel needle. Ion mobility spectra and chromatographic responses via monitoring of reactant-ion depletion in the software second gate mode were shown for esters of para-hydroxy benzoic acid, isomers of nitroaniline, and a mixture of acetaminophen, caffeine, and phenacetin. Key words: ion mobility detection. ion mobility spectrometry, HPLC, coronaspruy INTRODUCTIONAlthough many different methods have been employed to detect components following liquid chromatographic separation, ultraviolet (UV) detection is still used most often, even though its response is limited to compounds with appropriate chromophores. Thus, the search continues for sensitive liquid chromatographic detectors that perform reliably and respond to non-light absorbing species, Yeung has summarized the current status of research with regard to detectors that are available for LC (1). As attention shifts to microcolumn LC, the need is increasing for detectors that can detect small quantities of material, are compatible with low flow rates, and will respond to compounds lacking chromophores. Ion mobility spectrometry (IMS) potentially satisfies these three requirements.An ion mobility spectrometer contains an ambient pressure region where a neutral gas is ionized, typically by beta emission from a b3Ni source (2). These ions, called reactant ions, drift through a uniform electric field at velocities that are based on their gas phase mobilities. When small quantities of organic compounds are present near the ionization source, charge transfer processes occur between these neutral organics and the reactant ions and produce a second set of ions called product ions that change the nature of the charge-carrying species in the spectrometer. Use of an orthogonal electric field as a "gate" at either end of a section of the spectrometer allows the drift time of the ions to be determined.
Significant modification of the traditional curriculum for science majors at Gonzaga University.
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