Showing papers in "Journal of Microcolumn Separations in 1993"
TL;DR: In this paper, a comprehensive two-dimensional gas chromatography is applied to the analysis of complex petroleum mixtures, where the chromatogram is orthogonal such that the second dimension separation is independent of the first.
Abstract: Comprehensive two-dimensional gas chromatography is applicable to the analysis of complex petroleum mixtures. A kerosene sample analyzed by this method generates over six thousand chromatographic peaks. The chromatogram is orthogonal such that the second dimension separation is independent of the first. Aliphatics, aromatics, and naphthalene derivatives form distinct bands of peaks on the chromatographic plane with further subdivisions within each band based on chemical structure. With a nonpolar first column and a moderately polar second column, the first dimension separation is based largely on substance volatility while the second dimension separation is based on polarity. Each chromatographic peak has a pair of characteristic retention times, providing a more reliable identification. Peak capacity is much greater than that obtainable from any one-dimensional separation.
103 citations
TL;DR: A reproducible method for the preparation of highly efficient fused silica packed capillary columns using pressure programmed liquid or supercritical carbon dioxide to carry the packing material into the capillary is described in this paper.
Abstract: A new, reproducible method is described for the preparation of highly efficient fused silica packed capillary columns using pressure programmed liquid or supercritical carbon dioxide to carry the packing material into the capillary. The method allows facile preparation of long, uniformly packed capillary columns capable of producing over 240,000 theoretical plates in SFC. Columns of up to 10 meters in length were prepared using LC packing materials of different chemical nature and particle size. The new method does not require a separate step for preparing the slurry, and it is free from the drawbacks inherent in conventional slurry-packing methods. The lower viscosity and surface tension of carbon dioxide compared to conventional slurry-forming liquids, as well as the use of ultrasonic vibration and pressure programming in the packing process, provide favorable conditions for achieving packing uniformity over greater column lengths than is achievable by other packing techniques. Proper restrictors and appropriate decompression rates during the use of these columns in SFC were found to be very important in achieving optimum column performance. In comparison to the use of open tubular columns, higher effciencies, greater sample capacities, and faster analysis speeds can be obtained. These advantageous features make the newly developed columns suitable for trace analysis, separation of very complex mixtures, and analysis of difficult-to-separate solutes.
90 citations
TL;DR: In this article, a micellar solution and its CMCs and partial specific volumes at different temperatures were measured and the relationship between the molecular structures of solutaes and the resulting thermodynamic quantities was investigated.
Abstract: Micellar electrokinetic chromatography (MEKC) allows the calculation of distribution coefficients between a micelle and the surrounding aqueous phase from the measurement of the migration time, provided the critical micelle concentration (CMC) and the phase ratio are known. Thermodynamic quantities such as enthalpy and entropy changes of micellar solubilization were calculated from the temperature dependence of the distribution coefficients. Sodium dodecyl sulfate (SDS) was employed to prepare a micellar solution and its CMCs and partial specific volumes at different temperatures were measured. The relationship between the molecular structures of solutaes and the resulting thermodynamic quantities was investigated. The temperature inside the capillary was calculated from the temperature dependence of the distribution coefficient.
85 citations
TL;DR: In this paper, a mixture of β-cyclodextrin and deoxycholate was used to resolve the enantiomers of the 1-cyano-2-substituted-benz[L]isoindole and dansyl derivatives of a variety of amino acids.
Abstract: A mixture of sodium deoxycholate and β-cyclodextrin has been used to resolve the enantiomers of the 1-cyano-2-substituted-benz[L]isoindole and dansyl derivatives of a variety of amino acids. The enantiomers of CBI-histidine and glutamic acid were resolved with this system unlike the case of γ-cyclodextrin in combination with SDS reported in the literature. Dansyl derivatives have been generated in situ to provide information about the migration behavior of dansylic acid, a major by-product in the dansylation.
65 citations
TL;DR: In this article, a combined capillary electrophoresis/mass spectrometry (CE/MS) method is described that provides an increase in the efficiency of analysis by decreasing the analyte elution rate into an electrospray ionization (ESI) interface.
Abstract: A combined capillary electrophoresis/mass spectrometry (CE/MS) method is described that provides an increase in the efficiency of analysis by decreasing the analyte elution rate into an electrospray ionization (ESI) interface. The method is readily implemented by instituting a step change in the CE electric field strength for desired periods of time during a separation. As a result, the m/z range or the number of times an m/z range can be scanned while a solute elutes is increased by a factor proportional to the decrease in CE electric field strength. In addition, the method is shown not to cause any significant loss in separation quality as demonstrated for mixtures of proteins and peptides, while providing an effective gain in sensitivity. This allows an enhancement in the quality of mass spectra recorded while scanning wide m/z ranges. Mass spectra for a set of standard proteins were obtained for injections of 60 femtomole/protein, while analysis of an albumin tryptic digest was obtained for injections corresponding to 40 femtomoles of protein.
63 citations
TL;DR: A post-column reactor was designed for chemiluminescence detection in the capillary electrophoresis separation of isoluminol thiocarbamyl derivatives of amino acids.
Abstract: A post-column reactor was designed for chemiluminescence detection in the capillary electrophoresis separation of isoluminol thiocarbamyl derivatives of amino acids. This system was characterized with respect to hydrogen peroxide concentration and flow rate, surfactant concentration, microperoxidase concentration, and mixing distance. A linear calibration curve (R >0.999) was constructed for isoluminol ranging from at least 30 femtomoles to the detection limit of 40 attomoles injected; the dynamic range of the instrument is at least three orders of magnitude. Detection limit for the isoluminol thiocarbamyl derivative of valine was 500 attomoles. The theoretical plate counts for the raw data are typically 1 × 105 for labeled amino acids, and a factor of two better for isoluminol.
60 citations
TL;DR: In this paper, the preparation and performance of an online preconcentrator for capillary electrophoresis (CE) is described, where Borosilicate glass microspheres are dry-packed into the fused silica column terminus and sintered in place with a micro electric.
Abstract: The preparation and performance of an on-line preconcentrator for capillary electrophoresis (CE) is described. Borosilicate glass microspheres were dry-packed into the fused silica column terminus and sintered in place with a micro electric are developed for this application. Reversed-phase LC packing material was slurry-packed onto this frit, in lengths of 1-3 mm. An additional frit of borosilicate beads behind the preconcentrator was used to fix the packed bed in place. The performance of the packed bed preconcentrator for the enhancement of UV and fluorescence detection limits was evaluated. Construction of the micro electric are device, the packing procedures, and practical applications are described.
57 citations
TL;DR: In this paper, the effect of dilution of the chiral stationary phase octakis (3-Obutyryl-2, 6-di-O-pentyl)-γ-cyclodextrin in the achiral polysiloxane OV-1701 on the separation factor was investigated over a wide concentration range.
Abstract: The effect of dilution of the chiral stationary phase octakis (3-O-butyryl-2, 6-di-O-pentyl)-γ-cyclodextrin in the achiral polysiloxane OV–1701 on the separation factor was investigated over a wide concentration range. The loss in enantioselectivity was only negligible down to a dilution of 50% (w/w), since the separation factor vs. concentration curves levels off at higher concentrations. Because of the strong temperature dependence of the separation factors, the properties of short columns (10 m and 4.5 m) were evaluated. These short columns, which allow operation at low temperatures and lead to enhanced separation factors were found to be appropriate for the separations of simple mixtures, because the gain in selectivity exceeded the loss in separation efficiency. For complex mixtures, the higher efficiencies of columns of at least 25 m in length were still necessary. A plot of In α vs. 1/T did not yield straight lines, but showed a significant curvature, leading to considerable relative errors if thermodynamic data were derived from these plots.
47 citations
TL;DR: In this paper, polysiloxane-anchored permethylated β-cyclodextrin (Chirasil-Dex) was used for enantiomeric separation by GC.
Abstract: Polysiloxane-anchored permethylated β-cyclodextrin (Chirasil-Dex) was used for enantiomeric separation by GC. Four types of Chirasil-Dex, containing up to 36% permethylated β-cyclodextrin by weight, were synthesized and compared with 10% permethylated β-cyclodextrin dissolved in OV–1701. The influence of the cyclodextrin weight percentage on the chiral separation factor α and on polarity, as expressed by the Kovats indices, was studied. The chiral separation factors α did not increase significantly above a threshold of ˜30% permethylated β-cyclodextrin by weight. The plots of α vs. weight percentage depend on the complexation strength of the chiral solutes, as do the Kovats indices; for weakly complexing solutes, changes in the cyclodextrin concentration result in larger changes in α, but in less pronounced changes in the Kovats indices, as compared to strongly complexing solutes. Column performances were investigated for immobilized and nonimmobilized stationary phases. As expected, immobilization properties were found to depend on the content of Si-H groups in the polysiloxane backbone. Immobilized columns exhibited a much better long-term stability of all chromatographic variables investigated.
47 citations
TL;DR: In this paper, retention processes in reversed-phase liquid chromatography were examined in situ along an optically transparent column by means of laser-induced fluorescence, using a homologous series of derivatized fatty acids as model solutes.
Abstract: In this study, retention processes in reversed-phase liquid chromatography were examined in situ along an optically transparent column by means of laser-induced fluorescence. Using a homologous series of derivatized fatty acids as model solutes, the capacity factors were systematically measured as a function of temperature in the range from 10 to 60°C and pressure in the range from 980 to 4,430 psi. A phase transition was observed for the octadecylsilica stationary phase as temperature was decreased and as pressure was increased. From evaluation of its thermodynamic properties during this phase transition, the stationary phase appears to be more similar to a randomly oriented alkane solvent than a highly ordered system such as a phospholipid membrane. Because this phase transition occurs within the range of routine operating conditions in liquid chromatography, it has important implications for both fundamental and practical applications.
42 citations
TL;DR: In this paper, Butyldimethylsilylated cyclodextrin derivatives dissolved in polysiloxanes were shown to be versatile chiral stationary phases for the differentiation of important chiral food and flavor ingredients and chiral acids of biochemical and pharmaceutical interest.
Abstract: tert-Butyldimethylsilylated cyclodextrin derivatives dissolved in polysiloxanes were shown to be versatile chiral stationary phases for the differentiation of important chiral food and flavor ingredients and chiral acids of biochemical and pharmaceutical interest.
TL;DR: In this article, an automated procedure for the analysis of aqueous samples by membrane disk extraction coupled on-line to capillary gas chromatography (GC) is presented, which is applied to the determination of a series of organophosphorus pesticides in tap water and water from European rivers.
Abstract: An automated procedure for the analysis of aqueous samples by membrane disk extraction coupled on-line to capillary gas chromatography (GC) is presented. Organophosphorus pesticides are preconcentrated from aqueous samples on three 0.5 mm thick, 4.0 mm diameter XAD-2 membrane extraction disks which are mounted in a polymer holder for use in a PROSPEKT sample processor. The layers are dried by a stream of nitrogen (30 min at ambient temperature). Desorption of the analytes is carried out with ethyl acetate which is directly introduced into a retention gap under partially concurrent solvent evaporation conditions, using an early solvent vapor exit. The method is a fully software-controlled automated system, and includes sample preparation, sample transfer and GC analysis. The final analysis is carried out by GC with nitrogen-phosphorus (NPD) or mass-selective detection. The technique is applied to the determination of a series of organophosphorus pesticides in tap water and water from European rivers. With a sample volume of only 2.5 mL, the detection limits achieved with the NPD were 20–50 ng L−1 in tap water and 20–100 ng L−1 in river water.
TL;DR: In this article, the authors compared the lead time and effectiveness of CZE with LC and showed that CZE can be much more rapid in CZE but with a sufficiently diverse group of compounds, CZE cannot provide a full analysis without gradient control.
Abstract: Twenty-two sulphonamides and three dihydrofolate reductase inhibitors (DHFR) were included in this study. At pH 7.5, eighteen drugs were resolved in 22 min. Above pH 7.5 the DHFR are not resolved and the pH range of 7-7.5 was found to be optimal. Around pH 7, where the degree of ionization of the SFA is highly variable, pKa and migration time are strongly correlated. Increasing phosphate buffer concentration from 25-100 mM generally improves resolution but increases analysis times. A good correlation between analyte mobility and the ratio of net charge to molecular weight indicates that this ratio may be used as a quick guide to the possibility of separation of related compounds. Our secondary objective was to compare the lead time and effectiveness of CZE with LC. It is clear that method development is much more rapid in CZE but with a sufficiently diverse group of compounds, CZE can be incapable of providing a full analysis without gradient control. Nonionized compounds are not separated at one extreme, and excessively mobile species may not emerge in a reasonable time, or, may migrate to the wrong electrode.
TL;DR: In this article, a combined solid-phase extraction and thermal desorption (SPETD) module was incorporated into a system for automated GC analysis, which was carried out after injection of an aqueous sample of 100 μL either by a loop using the carrier gas for sample introduction or manually with a syringe.
Abstract: A module for combined solid-phase extraction and thermal desorption (SPETD) was incorporated into a system for automated GC analysis. Solid-phase extraction of analytes from water could be carried out after injection of an aqueous sample of 100 μL either by a loop using the carrier gas for sample introduction or manually with a syringe. After drying of the stationary phase by the carrier gas at a high flow rate, thermal desorption could be carried out at temperatures up to 300–350°C. Suitable packing materials for the liner in the SPETD module, i.e., materials which efficiently trap the organic compounds from an aqueous solution and are sufficiently thermostable, include Tenax and carbon-based phases, as well as a silylated alkylmodified silica. With the latter, however, the problem of efficient drying after sorption still must be solved. Each phase has its own application range which, from among the analytes which are trapped during sorption, is determined at the volatile end by losses occurring during drying by helium purging, and at the nonvolatile end by losses due to incomplete release during thermal desorption. From this study (using n-alkanes, chlorobenzenes, and chlorophenols as test solutes), it appears that the Tenax phases, Tenax-TA and Tenax-GR, have the widest application range; essentially quantitative recoveries were obtained for C10 through C26n-alkanes. The carbon-based phases appear to be useful for relatively volatile compounds, i.e., up to C19n-alkanes. Each phase gave good recoveries for two test mixtures containing chlorobenzenes and chlorophenols dissolved in 100 μL of water at the 10 ppb level.
TL;DR: In this article, a fused-silica capillary with specially designed charged surface coatings was developed for the control of electroosmotic flow and for the separation of peptides and proteins in capillary electrophoresis.
Abstract: Fused-silica capillaries with specially designed charged surface coatings were developed for the control of electroosmotic flow and for the separation of peptides and proteins in capillary electrophoresis. Cryptand moieties were immobilized onto the capillary surface through a crosslinking reaction of a cryptand-containing polymer and copolymerization of a cryptand-containing monomer. The resultant coatings provided switchable electroosmotic flow from low to high pH due to the complexing ability of the cryptand moieties with the metal ions from the buffer solution. A positively charged quaternary amine surface-coated column was found to be very effective in reversing the electroosmotic flow compared to an untreated fused silica capillary, and to be suitable for separation of proteins whose pI values are greater than the buffer pH. Also, a negatively charged sulfonic acid surface-coated column could be used to stabilize the electroosmotic flow from pH 3 to 9, and the magnitude of the electroosmotic flow could be controlled by varying the concentration of sulfonic acid groups in the coating solution. This negatively charged surface coating gave excellent reproducibility for the separation of a tryptic digest of β-casein.
TL;DR: In this paper, a simple method for the preparation of polymer-coated fused silica capillary columns for electrophoresis is presented, where the column is subjected to heat treatment to immobilize the polymer film.
Abstract: A simple method for the preparation of polymer-coated fused silica capillary columns for electrophoresis is presented. It is shown that the static coating technique used in GC and SFC can be conveniently utilized for the preparation of highly efficient capillary electrophoresis columns. In this method, the coating solution contains appropriate proportions of three ingredients: a polymer, a surface derivatization reagent, and a crosslinking reagent dissolved in a suitable low-boiling solvent. After coating, the column is subjected to heat treatment to immobilize the polymer film. Simplicity, rapidity, high column efficiency, batch-to-batch and run-to-run reproducibility, and long column life-time are advantageous features of the new method. Column efficiencies of as high as 1.2 million theoretical plates were achieved for cytochrome c on a 96 cm long Ucon 75 H-90,000 coated column. Efficient electrophoretic separation of five cytochrome c proteins was achieved on a Superox 4 coated column. The possibility of CE separation of histones on the new columns was also explored.
TL;DR: In this article, the location of the fiber in the injector for programmable direct and splitsplitless injectors is described since fiber positioning varies with injector design, and the factors affecting desorption and cryofocusing are discussed.
Abstract: Thermal desorption of analytes from a solid phase microextraction (SPME) device is the ideal techniques for sample introduction into microcolumns because the solvent is entirely eliminated. SPME is compatible with various injector designs, is independent of the instrument manufacturer, and can be completely automated. It can be used to concentrate the analytes from liquid, gaseous, or headspace samples with equal success. The location of the fiber in the injector for programmable direct and splitsplitless injectors are described since fiber positioning varies with injector design. An SPME injection consists of two parts: desorption in the injector followed by cryofocusing in the gas chromatographic column. The factors affecting desorption and cryofocusing, and the thickness of the column stationary phase. Example of fiber injections of liquid and gaseous samples are discussed.
TL;DR: The analytical figures of merit of laser-excited fluorescence detection of DNA restriction fragments, based on the enhanced fluorescence of selected dyes upon intercalation, are presented in this article.
Abstract: The analytical figures of merit of laser-excited fluorescence detection of DNA restriction fragments, based on the enhanced fluorescence of selected dyes upon intercalation, are presented. The fragments present in a ϕX174 Hae III digest are efficiently separated by capillary electrophoresis employing methyl cellulose as a sizesieving reagent. On-column DNA labeling is accomplished by adding either ethidium bromide (EB) or POPO-3 iodide (POPO-3) to the electrophoretic running buffer. Experimental parameters that are evaluated include type of laser (helium-cadmium at 325 nm or helium-neon at 543 nm) and laser power, dye type and concentration, pre-injection labeling procedure, and applied field. Calibration plots are linear over two orders of magnitude in DNA concentration. Limits of detection are in the low-to-mid femtogram injected range for the individual fragments, with the POPO-3 providing slightly lower values. Both dyes exhibit large increases in fluorescence quantum efficiency upon intercalation and contribute little or nothing to the observed optical background. Detectability with the dyes is enhanced by a spectral shift upon intercalation. Signal intensities increase with DNA fragment size in approximately a linear fashion. Intercalation by the dyes also influences mobility and the resolution of the fragments studied.
TL;DR: In this paper, polymeric hollow fibers prepared from polypropylene and polybutylene terephthalate were used as CE columns to separate substituted pyridines, model proteins and standard ribonucleotides.
Abstract: In this paper, polymeric hollow fibers prepared from polypropylene and polybutylene terephthalate were used as CE columns. Surface modifications with polyacrylamide greatly improved the performance of the capillaries. Separations of substituted pyridines, model proteins and standard ribonucleotides were demonstrated. The column efficiency for model proteins was over 200,000 theoretical plates. Comparisons of electroosmotic flow, UV absorption properties, heat dissipation, and diameter variation between fused silica capillaries and polymeric hollow fibers were made. The practical potential of small diameter polymeric hollow fibers as CE columns was evaluated.
TL;DR: Chromatographic separation of C60 and C70 fullerenes was performed with phenylpropyl and three different methoxyphenyl propyl bonded phases in microcolumn liquid chromatography as mentioned in this paper.
Abstract: Chromatographic separation of C60 and C70 fullerenes was performed with phenylpropyl and three different methoxyphenylpropyl bonded phases in microcolumn liquid chromatography. The results indicate that the 3,4-dimethoxyphenylpropyl phase possesses the best resolving power among the evaluated phases, producing a separation factor of 1.87 for C70 and C60 using n-hexane as the mobile phase. This observation could be explained by examining the energy minimized conformations of the bonded phases and their associative interactions with the fullerene molecules.
TL;DR: In this paper, a miniaturized evaporative light scattering detector for combination with packed microcolumns in supercritical fluid chromatography has been developed, and it is demonstrated that argentation SFC of triacylglycerols provides separation according to chain length and degree of unsaturation.
Abstract: Packed capillary columns provide excellent separation of neutral lipids by supercritical fluid chromatography. For these separations, it is necessary to add a polar modifier to the mobile phase; hitherto, a detector suitable for quantitative analysis has been lacking. For this purpose, a miniaturized evaporative light scattering detector for combination with packed microcolumns in supercritical fluid chromatography has been developed. Since the detector response is dependent on the mobile phase flow rate, a drop in response was found at flow rates above 16 mL min−1 of expanded mobile phase. However, below this flow rate, there was a region of relatively constant response. In this region, quantitative analysis could also be successfully performed when the pressure and/or the temperature of the separation were programmed. The detection limit was lower than 6 ng. With pressure programming, the relative standard deviation in peak area was less then 4%. Minor peaks gave higher RSD values.
Further, it is demonstrated that argentation SFC of triacylglycerols provides separation according to chain length as well as to the degree of unsaturation.
TL;DR: In this article, the combination of 50 pm i.d. columns with ion trap mass spectrometric detection is evaluated and the performance of the system is demonstrated by the analysis of some real-world samples.
Abstract: Fast gas chromatographic separations can be achieved by vacuum-outlet operation and by applying narrow-bore columns. In this work, the combination of 50 pm i.d. columns with ion trap mass spectrometric detection is evaluated. Detection limits in the electron ionization mode, and in the chemical ionization mode using CH4 as the reaction gas, are 1 pg and 5 pg, respectively. Owing to the high sensitivity of the ion trap mass spectrometer, a significant improvement in the working range in comparison with other detection systems is obtained. The small column flows cause no loss of mass spectral resolution and sensitivity. The performance of the system is demonstrated by the analysis of some real-world samples.
TL;DR: In this paper, the value of miniaturizing field-flow fractionation (FFF) channels is examined based on theoretical and experimental evidence, and the importance of the Brownian excursion distance (l) is discussed.
Abstract: The value of miniaturizing field-flow fractionation (FFF) channels is examined based on theoretical and experimental evidence. While FFF and chromatography are dynamically similar, the dimension analogous to capillary or particle diameter in chromatography is the Brownian excursion distance (l), a "soft" dimension controlled by field strength and thus subject to significant reduction by increasing the field strength. Optimization of the “hard” channel dimensions (length, breadth, thickness) is a more complex matter involving diverse factors including sample dilution and detectability, clog-free operation, edge effects, relaxation time, compatibility with pump and detector systems, and only in some cases the influence of these dimensions on the achievable l.
TL;DR: In this paper, a simple method of generating pH gradients for use in capillary isoelectric focusing was described and evaluated experimentally, where the cone shaped capillary filled with a conventional buffer was used to produce an effect of nonuniform heating which then generated temperature and corresponding pH gradient along the capillary axis.
Abstract: A simple method of generating pH gradients for use in capillary isoelectric focusing was described and evaluated experimentally. In the proposed scheme, the cone shaped capillary filled with a conventional buffer is used to produce an effect of nonuniform heating which then generate temperature and corresponding pH gradients along the capillary axis. Concentration and separation of proteins by applying a simple isoelectric focusing system based on this principle is demonstrated. An inexpensive absorption imaging detection system is used to continuously monitor progress of the separation and concentration processes.
TL;DR: In this article, different buffer compositions, pH, temperature, operating conditions, and capillary tubing treatments were compared for the optimal separation and best reproducibility of deoxynucleosides and deoxyynucleotide-3′-monophosphates.
Abstract: Simultaneous separation of ionic and neutral compounds in micellar electrokinetic capillary chromatography (MECC) is a consequence of a combination of charge/mass ratios, hydrophobicity and charge interactions at the surface of the micelles. The type of resolution can be chosen mainly by modification of the buffer composition and pH. Reproducibility depends on the capillary tubing characteristics, operating conditions, and temperature effects. We report here on the different buffer compositions, pH, temperature, operating conditions, and capillary tubing treatments for the optimal separation and best reproducibility of deoxynucleosides and deoxynucleotide-3′-monophosphates. The effects of different buffer compositions were tested when operating in a constant current mode at 38 μAmps. Lithium phosphate buffer improved the separation speed with little loss in resolution. The best concentration was found to be 20 mM. A concentration of 5% acetonitrile substantially improved the efficiency of the nucleoside peaks and therefore their resolution. The optimum SDS concentration was found to be at 100 mM. The pH has a significant influence on retention time and resolution, and was adjusted for maximum resolution at pH 9.6. Capillary surface treatments were also investigated. Effective heat dissipation occurred below an operating voltage of 15kV, and a temperature of 35°C was found to give a good compromise between efficiency, resolution, and time of analysis. Time of analysis can be decreased by using higher operating voltages without compromising the resolution. Efficiencies were 350,000 and 235,000 plates m−1 for the nucleosides and nucleotides, respectively. The reproducibility of retention times was good with coefficients of variation below 1%, which allows the possibility for automatic fraction collection.
TL;DR: In this article, the preparation of two new medium polar CH3O-terminated 50% diphenyl substituted polysiloxane phases by means of a one step condensation of bis(dimethylamino)alkylsilanes and diphenysilanediol is reported.
Abstract: The preparation of two new medium polar CH3O-terminated 50% diphenyl substituted polysiloxane phases by means of a one step condensation of bis(dimethylamino)alkylsilanes and diphenysilanediol is reported. Both phases can be used up to temperatures of 420°C. The unusual route of synthesis offers an easy and reliable way to prepare a variety of tailor-made high temperature stable polysiloxane phases.
TL;DR: In this article, the authors studied solvent mass transfer on short, uncoated columns for several solvents forming Type I binary mixtures with CO2, and found evidence in the solvent peak shapes of distinct liquid and vapor phases, with the liquid present as a dynamically formed film.
Abstract: Open-tubular supercritical fluid chromatography (SFC) is a useful technique for many analyses, but is somewhat limited, in current practice, with respect to trace analysis of liquid samples. Direct injection of the volumes required for trace analysis with ordinary detectors creates a meter or more length of flooded zone and may lead to unacceptable band broadening. We studied solvent mass transfer on short, uncoated columns for several solvents forming Type I binary mixtures with CO2, and found evidence in the solvent peak shapes of distinct liquid and vapor phases, with the liquid present as a dynamically formed film. This study suggests that selecting an injection temperature and pressure to reduce, but not eliminate, the surface tension of the liquidvapor interface, and increasing the mobile phase velocity during injection increase the liquid film thickness and reduce the length of the resulting flooded zone. When a separate uncoated inlet tube (that is, a retention gap) is used to connect the injector with the column, decreasing the inlet tube radius improves the ratio of maximum effective injection volume to inlet tube volume.
Just as in GC, dynamically formed films function as pseudo-stationary phases during injection. Without a liquid film, solutes can be transported by injection-solvent-modified mobile phase well beyond the length of a typical film, increasing the difficulty of refocusing the solutes before separation. Refocusing solutes from film-coated flooded zones is easily accomplished using either a solvent effect (specifically, solvent trapping) or phase-ratio focusing.
We recommend the use of solvents with phase behavior like toluene and carbon tetrachloride, both of which form films at convenient temperatures and pressures. We discourage the use of solvents with phase behavior like n-pentane which tend to make single-phase, supercritical mixtures with CO2 under typical SFC injection conditions.
TL;DR: The presented approximate model represents the transition between classical models of IEF and ITP and is shown by its application to isoelectric focusing with electrophoretic mobilization, to isotachophoresis (ITP) with polyampholytic spacers, and to related methods.
Abstract: The physical and mathematical basic of a natural steady state, moving pH gradient is outlined. With certain approximations, a differential equation of the concentration of the component contributing to the natural steady state, moving pH gradient is formulated. Solutions of the differential equation and the relationships to the resolution calculations of both carriers and trace components present in the large excesses of carriers are presented. The steady state distribution profiles of both amphoteric and nonamphoteric components predict Gaussian distributions only if and when the conductivity and the pH gradient remain constant within the moving focused zone. The practical significance of the model is shown by its application to isoelectric focusing (IEF) with electrophoretic mobilization, to isotachophoresis (ITP) with polyampholytic spacers, and to related methods. It is shown that the presented approximate model represents the transition between classical models of IEF and ITP.
TL;DR: Fullerenes (C60 and C70) linked to small silica or polymer particles provide novel chromatographic stationary phases with electron donor/acceptor surface interactions that may parallel those of aromatic carbons as discussed by the authors.
Abstract: Fullerenes (C60 and C70) linked to small silica or polymer particles provide novel chromatographic stationary phases with electron donor/acceptor surface interactions that may parallel those of aromatic carbons. Separations of fullerenes and aromatic compounds frequently encountered as environmental contaminants were investigated using LC microcolumns containing either particles prepared from BrHC60 linked to amino-silica (Spherisorb-NH2, Phenomenex) or porous polymers containing ca. 12% fullerene by weight. The polystyrene divinylbenzene (PSDVB)-C60/70H column had greater capacity (k') than the SiNHC60/70H column. Toluene and polycyclic aromatic hydrocarbons (PAHs), naphthalene, anthracene, pyrene, and perylene, were retained in the order of increasing ring number; nitro-substituents increased retention; and chlorinated-dibenzo-p-dioxins and -dibenzofurans were more strongly retained than their parent compounds. PCB congeners with no chlorine in the ortho, ortho'-positions are co-planar and are strongly retained; for these isomers, retention increased with higher chlorination. We demonstrate that surface-linked fullerenes can also accommodate further chemical modification through the addition of 2-(4-nitrophenyl)ethyl or -NHCH2CH2NH2 moieties to yield altered electron donor/acceptor affinity. Separations appear to be derived primarily from aromatic electron induced dipole-dipole or electron-pair donor or acceptor interactions (charge transfer complexes). Except for the fullerenes, surface interactions with compounds are largely suppressed with methylene chloride as the mobile phase.
TL;DR: In this paper, the authors synthesized the magnesium salt of dodecyl sulfate (Mg(DS)2) to investigate how changes in the counter ion identity affect resolution via changes in selectivity (α) and retention.
Abstract: In micellar electrokinetic capillary chromatography (MECC) the most widely used surfactant for the pseudostationary phase is sodium dodecyl sulfate (SDS), although other types of surfactants have been investigated. We have synthesized the magnesium salt of dodecyl sulfate (Mg(DS)2) in order to investigate how changes in the counter ion identity affect resolution via changes in selectivity (α) and retention. Buffer solutions containing Mg(DS)2 exhibited operating currents and electroosmotic velocities one third to one half that of a comparable SDS buffer solution, and significantly larger methylene and functional group selectivities for the compounds studied. In addition, capacity factors, K', for netral solutes were between 1.5 and 2.5 times larger for the Mg(DS)2. The elution range, tmc/to, for the Mg(DS)2 was independent of the percentage of organic modifier in the buffer solution while the elution range for SDS increased as the percentage of organic modifier increased. Improvements in α and increased retention resulted in increased Rs for neutral solutes in Mg(DS)2. The relatively constant elution range for Mg(DS)2 also aided in the retention-mediated optimization of Rs and Rs per unit time.