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This paper compares static (i.e., temporally unchanging) thermal gradient gas chromatography (GC) to isothermal GC using a stochastic transport model to simulate peak characteristics for the separation of C12-C14 hydrocarbons resulting from variations in injection bandwidth. All comparisons are made using chromatographic conditions that give approximately equal analyte retention times so that the resolution and number of theoretical plates can be clearly compared between simulations. Simulations show that resolution can be significantly improved using a linear thermal gradient along the entire column length. This is mainly achieved by partially compensating for loss in resolution from the increase in mobile phase velocity, which approximates an ideal, basic separation. The slope of the linear thermal gradient required to maximize resolution is a function of the retention parameters, which are specific to each analyte pair; a single static, thermal gradient will not affect all analytes equally. A static, non-linear thermal gradient that creates constant analyte velocities at all column locations provides the largest observed gains in resolution. From the simulations performed in this study, optimized linear thermal gradient conditions are shown to improve the resolution by as much as 8.8% over comparative isothermal conditions, even with a perfect injection (i.e., zero initial bandwidth).

This article presents a method of simulating molecular transport in capillary gas chromatography (GC) applicable to isothermal, temperature-programmed, and thermal gradient conditions. The approach accounts for parameter differences that can occur across an analyte band including pressure, mobile phase velocity, temperature, and retention factor. The model was validated experimentally using a GC column comprised of microchannels in a stainless-steel plate capable of isothermal, temperature-programmed, and thermal gradient GC separations. The parameters governing retention and dispersion in the transport model were fitted with 12 experimental isothermal separations. The transport model was validated with experimental data for three analytes using four temperature-programmed and three thermal gradient GC separations. The simulated peaks (elution time and dispersion) give reasonable predictions of observed separations. The magnitudes of the maximum error between simulated peak elution time and experiment were 2.6 and 4.2% for temperature-programmed and thermal gradient GC, respectively. The magnitudes of the maximum error between the simulated peak width and experiment were 15.4 and 5.8% for temperature-programmed and thermal gradient GC, respectively. These relatively low errors give confidence that the model reflects the behavior of the transport processes and provides meaningful predictions for GC separations. This transport model allows for an evaluation of analyte separation characteristics of the analyte band at any position along the length of the GC column in addition to peak characteristics at the column exit. The transport model enables investigation of column conditions that influence separation behavior and opens exploration of optimal column design and heating conditions.

This paper reports the first results of a robust, high-performance, stainless-steel microchip gas-chromatography (GC) column that is capable of analyzing complex real-world mixtures as well as operating at very high temperatures. Using a serpentine design, a 10 m column with an approximately semicircular cross-section with a 52 μm hydraulic diameter (Dh) was produced in a 17 × 6.3 × 0.1 cm rectangular steel chip. The channels were produced using a multilayer-chemical-etch and diffusion-bonding process, and metal nuts were brazed onto the inlet and outlet ports allowing for column interfacing with ferrules and fused silica capillary tubing. After deactivating the metal surface, channels were statically coated with a ≈0.1 μm layer of 5% phenyl–1% vinyl–methylpolysiloxane (SE-54) stationary phase and cross-linked with dicumyl peroxide. By using n-tridecane (n-C13) as a test analyte with a retention factor (k) of 5, a total of 44 500 plates (≈4500 plates per meter) was obtained isothermally at 120 °C. The col...

A compact ultrahigh-pressure nanoflow liquid chromatograph (LC) was developed with the purpose in mind of creating a portable system that could be easily moved to various testing locations or placed in close proximity to other instruments for optimal coupling, such as with mass spectrometry (MS). The system utilized innovative nanoflow pumps integrated with a very low volume stop-flow injector and mixing tee. The system weighed only 5.9 kg (13 lbs) or 4.5 kg (10 lbs) without a controller and could hold up to 1100 bar (16000 psi) of pressure. The total volume pump capacity was 60 μL. In this study, the sample injection volume was determined by either a 60 nL internal sample groove machined in a high-pressure valve rotor or by a 1 μL external sample loop, although other sample grooves or loops could be selected. The gradient dwell volume was approximately 640 nL, which allowed significant reduction in sample analysis time. Gradient performance was evaluated by determining the gradient step accuracy. A low RSD (0.6%, n = 4) was obtained for day-to-day experiments. Linear gradient reproducibility was evaluated by separating a three-component polycyclic aromatic hydrocarbon mixture on a commercial 150 μm inner diameter capillary column packed with 1.7 μm particles. Good retention-time reproducibility (RSD0.17%) demonstrated that the pumping system could successfully generate ultrahigh pressures for use in capillary LC. The system was successfully coupled to an LTQ Orbitrap MS in a simple and efficient way; LC-MS of a trypsin-digested bovine serum albumin (BSA) sample provided narrow peaks, short dwell time, and good peptide coverage.

Polyethylene glycol diacrylate monoliths prepared using different amounts of monomer, porogen ratio, and capillary dimensions were characterized using capillary flow porometry (CFP) and scanning electron microscopy (SEM). Our results reveal good agreement between SEM and CFP measurements for through-pore size distribution. The CFP measurements for monoliths prepared by the same procedure in capillaries with different diameters (i.e., 75, 150, and 250 μm) clearly confirmed a change in through-pore size distribution with capillary diameter, thus, certifying the need for in-column measurement techniques over bulk measurements (e.g., mercury intrusion porosimetry). The mean through-pore size varied from 3.52 to 1.50 μm with a change in capillary diameter from 75 to 250 μm. Consistent mean through-pore size distribution for capillary columns with the same internal diameter but with different lengths (1.5, 2, and 3 cm) confirms the high interconnectivity of the pores and independence of CFP measurements with respect to capillary length. CFP and SEM measurements not only allow pore structure analysis but also prediction of relative column performance. Monoliths with narrow through-pore size distribution (0.8-1.2 μm), small mean through-pore size, and thin skeletal size (0.55 μm) gave the best performance in terms of efficiency for polyethylene glycol diacrylate monoliths.

In spite of an earlier report to the contrary and a different attempt that appears to have been unsuccessful, here we show an improved method that allows functionalization of hydrogen- and deuterium-terminated diamond with a dialkyl peroxide. In particular, hydrogen-/deuterium-terminated diamond particles were treated with neat di-tert-amyl peroxide (DTAP, C2H5C(CH3)2OOC(CH3)2C2H5) at elevated temperature. Surface changes were followed by X-ray photoelectron spectroscopy (XPS), diffuse reflectance Fourier transform infrared spectroscopy (DRIFT), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). After these reactions, the oxygen signal in the XPS spectra increased, the deuterium peak in the negative ToF-SIMS spectra decreased, and DRIFT showed C−H stretches, which were not previously present and which were similar to those of the precursor. In the C−H stretching region, the IR spectrum of adsorbed di-tert-amyl peroxide shows features that are red-shifted with respect to the IR spectrum of the ...

Recently, we reported the synthesis, fabrication, and preliminary evaluation of poly(ethylene glycol) (PEG)-functionalized polymeric microchips that are inherently resistant to protein adsorption without surface modification in capillary electrophoresis (CE). In this study, we investigated the impact of cross-linker purity and addition of methyl methacrylate (MMA) as a comonomer on CE performance. Impure poly(ethylene glycol) diacrylate (PEGDA) induced electroosmotic flow (EOF) and increased the separation time, while the addition of MMA decreased the separation efficiency to approximately 25% of that obtained using microchips fabricated without MMA. Resultant improved microchips were evaluated for the separation of fluorescent dyes, amino acids, peptides, and proteins. A CE efficiency of 4.2 x 10(4) plates for aspartic acid in a 3.5 cm long microchannel was obtained. Chiral separation of 10 different D,L-amino acid pairs was obtained with addition of a chiral selector (i.e., beta-cyclodextrin) in the running buffer. Selectivity (alpha) and resolution (R(s)) for D,L-leucine were 1.16 and 1.64, respectively. Good reproducibility was an added advantage of these PEG-functionalized microchips.

We report the design and performance of a novel radio-frequency (RF) ion-trap mass analyzer, the planar Paul trap, in which a quadrupolar potential distribution is made between two electrode plates. Each plate consists of a series of concentric, lithographically deposited 100-microm-wide metal rings, overlaid with a thin resistive layer. A different RF amplitude is applied to each ring, such that the trapping field produced is similar to that of the conventional Paul trap. The accuracy and shape of the electric fields in this trap are not limited by electrode geometry nor machining precision, as is the case in traps made with metal electrodes. The use of two microfabricated plates for ion trap construction presents a lower-cost alternative to conventional ion traps, with additional advantages in electrode alignment, electric field optimization, and ion-trap miniaturization. Experiments demonstrate the effects of ion ejection mode and scan rate on mass resolution for several small organic compounds. The current instrument has a mass range up to approximately 180 Thomsons (Th), with better than unit mass resolution over the entire range.

A novel gas chromatograph-mass spectrometer (GC-MS) based on a miniature toroidal ion trap mass analyzer (TMS) and a low thermal mass GC is described. The TMS system has an effective mass/charge (m/z) range of 50–442 with mass resolution at full-width half-maximum (FWHM) of 0.55 at m/z 91 and 0.80 at m/z 222. A solid-phase microextraction (SPME) fiber mounted in a simple syringe-style holder is used for sample collection and introduction into a specially designed low thermal mass GC injection port. This portable GC-TMS system weighs

We report the formation of a highly stable amino stationary phase on diamond and demonstrate its use in solid-phase extraction (SPE). This process consists of spontaneous and self-limiting adsorption of polyallylamine (PAAm) from aqueous solution onto oxidized diamond. Thermal curing under reduced pressure or chemical cross-linking with a diepoxide was shown to fix the polymer to the particles. The resulting adsorbents are stable under even extreme pH conditions (from at least pH 0-14) and significantly more stable than a commercially available amino SPE adsorbent. Coated diamond particles were characterized by X-ray photoelectron spectroscopy (XPS) and diffuse reflectance Fourier transform-infrared spectroscopy (DRIFT). Model silicon surfaces were characterized by spectroscopic ellipsometry and wetting. Solid-phase extraction was demonstrated using cholesterol, hexadecanedioic acid, and palmitoyloleoylphosphatidylcholine as analytes, and these results were compared to those obtained with commercially available materials. Breakthrough curves indicate that, as expected, porous diamond particles have higher analyte capacity than nonporous solid particles.

Electric field gradient focusing (EFGF) is an equilibrium gradient focusing technique that depends on an electric field gradient and a hydrodynamic counterflow to focus, concentrate, and separate charged analytes. In this work, EFGF devices were fabricated from poly(ethylene glycol) (PEG)-functionalized acrylic plastic. The separation channel was formed in an ionically conductive and protein-resistant PEG-functionalized hydrogel, which was cast in a changing cross-sectional cavity in the plastic device. A linear electric field gradient was obtained by applying a voltage lengthwise across the shaped hydrogel. Standard proteins were used as analytes to demonstrate the performance of these EFGF devices. With an increase in counterflow rate or decrease in applied voltage, analyte bands broadened, but resolution increased in agreement with theory. To reduce analyte band dispersion and improve focusing performance, a protein-compatible PEG-functionalized monolith was incorporated in the EFGF channel. Compared with focusing in an open channel, protein bands in the monolith-filled EFGF channel were significantly narrower.

A new electron ionization source was developed for orthogonal acceleration time-of-flight mass spectrometry (TOFMS) based on the superimposition of a magnetic field around a radio frequency-only (rf-only) ion guide. The cylindrically symmetric magnetic field compresses the electron beam from the electron source into a long narrow volume along the ion guide axis. The magnetic field also helps to maintain a narrow energy distribution of electrons that penetrate the full length of the ion guide despite the influence of the radial rf field. Ionization occurs inside the ion guide with improved efficiency resulting from efficient use of electrons, prolonged interaction time, and nontraditionally large ionization volume. At the same time, the rf field effectively focuses ions radially and confines them to the axis of the ion guide by collisional focusing, leading to high ion transmission efficiency. Furthermore, the source can also be operated in a trap-and-pulse mode to improve the ion sampling duty cycle of orthogonal acceleration TOFMS. To validate the design concept of this new ion source, a simple prototype using a single set of cylindrical rods was constructed and retrofitted to an orthogonal acceleration TOFMS. A significant increase in ion signal intensity was observed by operating the source in a pulsed ion extraction mode. Low detection limits (for example, 12 fg for toluene) were determined at 12.5 spectra s(-1) in the full spectrum mode.

Electric field gradient focusing (EFGF) is a separation technique that uses an electric field gradient and an opposing hydrodynamic flow to separate and concentrate charged analytes. This work describes miniaturized EFGF devices that are used for protein analysis. These devices employ a unique ionically conductive polymer that enables the required electric field gradient to be established. This polymer has good protein compatibility and allows the transport of small buffer ions while retaining large analytes such as proteins. With the use of an EFGF device, green fluorescent protein was concentrated 10 000-fold and the separation of a protein mixture was demonstrated. The development of these ionically conductive polymer-based devices represents a step toward making EFGF a useful analytical tool for proteomics investigations.

Electrospray ionization has grown to be one of the most commonly used ionization techniques for mass spectrometry, and efforts continue to improve its performance. Typically, the sprayer tip must be very close to the entrance orifice of the mass spectrometer in order to maximize the conduction of ions from the sprayer into the mass spectrometer. However, because of space-charge repulsion, most ions never reach the sampling orifice. In this work, an industrial air amplifier, for which the working mechanism is based on venturi and coanda effects, was added between an electrospray ionization source and a time-of-flight mass spectrometer. When a series of reserpine solutions (0.5, 1.0, 5.0, and 10.0 microM) were monitored using mass spectrometry, an over 5-fold increase in m/z 609.3 ion intensity was measured for a separation distance of 14 mm between the electrospray tip and interface capillary inlet, as compared to when the electrospray tip was in its normal position 1 mm in front of the inlet without the amplifier. When a voltage was applied to the air amplifier to further assist in focusing the electrosprayed ions, an approximately 18-fold increase in m/z 609.3 ion intensity was obtained. In addition, a 34-fold reduction in method detection limit was observed.

Equilibrium gradient methods belong to a family of separation techniques in which analytes are forced to unique equilibrium points by a force gradient and a counter force along the separation pathway. The basic theory for equilibrium gradient methods where the force gradient is induced by a field gradient is developed in this paper. The results indicate that peak capacity can be dynamically improved by using a nonlinear field-intensity gradient in which the first section is steep, and the following section is shallow. Using electromobility focusing (EMF) as an example, a separation model is established. EMF is an equilibrium gradient method that uses an electric field intensity gradient to induce a force gradient on charged analytes, such as proteins, and a constant hydrodynamic flow as an opposing force. Equations relating operating parameters with separation performance are given. Although simulation results show that a peak capacity of over 10,000 is theoretically possible using a single channel in a separation time just under 2 months, if 100 parallel separation units are utilized in an array format under the same operating conditions, the same peak capacity can be obtained in just over 12 h.

Experimental peak capacities for capillary gas chromatography (GC), capillary liquid chromatography (CLC), and capillary electrochromatography (CEC) were compared. To obtain a meaningful comparison, the following constraints were applied. First, the same sample (mixture of alkylbenzenes) was used as a test mixture for all three techniques; second, the same packing material and column diameter were used in CLC and CEC; and third, isothermal conditions were used in GC, while isocratic conditions were used both in CLC and in CEC. Comparison of peak capacities for the same total column efficiency (approximately 36000 plates) showed that the peak capacity of GC is greater than those of the liquid-phase separation techniques. Comparison of CEC and CLC for constant retention factor was also carried out. For this condition, the results depend on the particle size used; for 3-microm porous particles, CEC had a peak capacity larger than CLC due to higher efficiency from the flow profile generated by electroosmotic flow. However, when 1.5-microm nonporous particles were used, the peak capacities were approximately the same for both techniques. The effect of linear velocity on peak capacity was also studied for all three techniques. Practical conditions aimed at increasing peak capacities of liquid-phase separation techniques are discussed.

Kinetic data from the literature were used to predict formation rates and product yields of oil and gas at typical low-temperature conditions of coal maturation. These data indicate that gas formation rates from hydrocarbon thermolysis are several orders of magnitude too low to have generated known coal-seam natural gas reserves, assuming bulk first-order kinetics defined by a single activation energy and preexponential factor. By assuming distributed activation energies, thermal cracking of liquid hydrocarbons and coal kerogen to methane can occur at sufficiently high rates to produce commercial quantities over long periods of geologic time. Acid-mineral-catalyzed cracking, transition-metal-catalyzed hydrogenolysis of liquid hydrocarbons, and transition-metal-catalyzed CO2 hydrogenation form gas at very high rates at geologic temperatures. Rates of gas production in these reactions are orders of magnitude higher than those predicted from thermolysis; moreover, the gaseous products for metal-catalyzed hyd...

An automated comprehensive isotachophoresis−capillary zone electrophoresis (ITP−CZE) system is described. The sample is focused in the first capillary by ITP and injected repeatedly into the second smaller diameter capillary for more rapid CZE separation. Since only small portions of the concentrated zones are sequentially injected for CZE separation, overloading was not observed. Moreover, the sensitivity is enhanced because all of the concentrated zones are analyzed and the results are summed. A single detector (only for the CZE dimension) is required, and accurate timing for CZE injection is not necessary. The system was evaluated using a mixture of angiotensins. The effect of addition of leading electrolyte at the junction of the ITP and CZE capillaries before each CZE run on comprehensive ITP−CZE peak area was studied, and leading electrolyte volumes between 7 and 11 μL led to the best sensitivity. Under optimized conditions, a detection limit of approximately 5 nM could be achieved by injecting 10 μ...

In this paper, the efficiencies and peak capacities of columns packed with nonporous and porous particles were investigated under typical solvating gas chromatography (SGC) conditions using a carbon dioxide mobile phase. The contributions of mass-transfer resistance (Cstag) to the plate height from the stagnant mobile phase were compared for liquid chromatography (LC), supercritical fluid chromatography (SFC), and SGC using 5-μm nonporous and porous particles. The results show that the contribution of the Cstag term to the total mobile-phase mass-transfer resistance for porous particles decreased markedly from LC to SFC to SGC. In SGC, mass-transfer resistance in the stationary phase and longitudinal diffusion are more significant. The total mass-transfer terms for nonporous silica (5 and 10 μm) encapsulated with SE-54 (e.g., 1% w/w loading) were 30−40% higher than those for porous silica (5 μm, 120 A, and 10 μm, 80 A) encapsulated with SE-54 (e.g., 10% w/w loading). Polymer-encapsulated porous particles ...

Counterflow isotachophoresis−capillary zone electrophoresis (ITP−CZE) on directly coupled columns of different diameters was explored to increase the injection volume and lower the detection limits in CZE. A solution of neostigmine bromide and propantheline bromide was used to evaluate the system; the relative standard deviations for four injections of 3.3 μL of 0.1 μM neostigmine bromide and propantheline bromide were 1.4 and 4.0%, respectively. The correlation coefficients for volumetric calibration curves over the range of 1.7−8.3 μL of the neostigmine bromide and propantheline bromide solution were 0.9979 and 0.9975, respectively, and a 2.5 nM solution could be detected easily from 8.3 μL of sample. Approximately 40 min was required for focusing.

In this study, the peak capacities for packed capillaries used in various forms of chromatography were investigated. Experiments found that the peak width at half-height (w1/2) linearly increased with increasing retention time when operating under isothermal, isobaric, and isocratic conditions in capillary gas chromatography, solvating gas chromatography, supercritical fluid chromatography, and liquid chromatography. From this relationship, a general peak capacity (n) expression was obtained. This expression covers conventional packed, packed capillary, and open tubular column chromatographies. With this general expression, it is possible to directly compare the separating capabilities of the various column types and chromatographic techniques.

The compositional traits of C9-34 n-alkanes were measured in supercritical CO2 extracts from 14 U.S. coals of varied geologic ages and wide thermal maturity range (lignite through low-volatile bituminous). The analysis data exhibit no unique component depletion pattern signatures diagnostic of known types of postgeneration physical, chemical, or microbiological degradation processes that commonly affect crude oil in sedimentary rocks. The C9-34 n-alkanes in Paleocene and Upper Cretaceous age coals exhibit bimodal carbon-number distribution profiles that strongly resemble those of the biogenic n-alkanoic acids present in brown coals. The compositional trait similarities between these n-alkanes and n-alkanoic acids and the covariance of the bulk coal organic matter atomic oxygen-to-carbon (O/C) ratios and carbon-preference index (CPI) values offer tangible evidence for the existence of a genetic linkage between these two series of compounds. Our analysis results indicate that the C2-5 alkanes and C6+ hydroc...

In this study, organic solvents at temperatures higher than their normal boiling points were used as mobile phases for packed capillary column chromatography. No restrictor or back pressure was imposed on the column outlet. As a result, a large difference in mobile-phase properties existed, and a phase transformation from liquid to gas occurred, along the column. Solvating gas chromatography (SGC) was used to characterize this chromatographic process because, near the column outlet, a gaseous mobile phase existed. Chromatographic performance characteristics including mobile-phase flow, column efficiency, mobile-phase solvating power, and solute retention were investigated using fused-silica capillary columns packed with microparticles (5- and 10-μm diameters). It was found that when the temperature was increased to values that were higher than the normal mobile-phase boiling point, the mobile-phase linear velocity increased rapidly at constant column inlet pressure. Although large differences in mobile-ph...

In this study, porous layer solid-phase microextraction (SPME) was investigated for the extraction of polycyclic aromatic hydrocarbons (PAHs) from water. The porous layer coatings were prepared using silica particles (5 μm diameter) bonded with phenyl, C8, and monomeric and polymeric C18 stationary phases for use in HPLC. It was found that several factors affected the selectivity for extraction of PAHs, including functional group in the bonded phase, alkyl chain length of the bonded phase, and phase type (monomeric or polymeric). The distribution coefficients of PAHs in the porous layer increased with an increasing number of carbon atoms, and greater selectivity toward solute molecular size and shape were obtained using a polymeric C18 porous layer. The effect of solution ionic strength on extraction was also investigated. A phenyl bonded phase porous layer fiber was used to extract PCB congeners from Arochlor 1254, and headspace SPME was applied to the analysis of contaminated soil samples.

Natural beta-cyclodextrin was treated with 3.5 equiv of tert-butyldimethylsilyl chloride and then with NaH and MeI in the same reaction vessel to give a mixture of oligosilyl-substituted permethylated beta-cyclodextrins. The mixture was treated with NH(4)F and the resulting 6(A),6(D)-(1), 6(A),6(C)-(2), and 6(A),6(B)-(3) dihydroxy permethylated beta-cyclodextrins were isolated in 7.6-8.9% yields by two silica gel chromatographic separations. The structures of 1-3 were verified by comparison of spectral properties with a known diol (2) or preparing known derivatives (1 and 3). This is a superior way to prepare and isolate dihydroxy permethylated beta-cyclodextrin intermediates for bifunctional enzyme mimics.

Conventional analytical techniques, such as headspace gas chromatography and Soxhlet extraction, can provide compositional information for the gaseous (C 1-5 ) and heavy (C 15+ ) hydrocarbon constituents, respectively. The volatile (C 6-14 ) hydrocarbons, if present, usually go undetected because of volatility fractionation and loss. In this study, supercritical CO2 was used to extract the C6-C14 volatile hydrocarbons from pulverized coal samples. Capillary column gas chromatography/mass spectrometry was used to identify the mixture components, and packed capillary column supercritical fluid chromatography was used to separate and quantify the aliphatic and aromatic hydrocarbon class fractions. It was found that the compositions of the light hydrocarbon fractions included several homologous series of normal and branched aliphatic hydrocarbons, cyclic and aromatic hydrocarbons, and alkyl-substituted benzenes and naphthalenes ; the concentrations of these volatile hydrocarbons ranged between 0.01 and 0.2 wt % of the bulk material for different coal and shale samples.

High-efficiency separation methods such as capillary electrophoresis (CE) interfaced with structural information-producing detection systems such as mass spectrometry (MS) represent invaluable tools in analytical chemistry. Most CE applications to date have been reported using UV absorption detection. To extend the analytical power of CE, new detectors which demonstrate increased speed, sensitivity, and specificity must be developed. In this paper, we describe the design of an electrospray ionization time-of-flight mass spectrometer and its performance as a detector for CE. Emphasis is placed on fulfillment of the speed and sensitivity requirements. Low attomole and low femtomole detection limits are demonstrated for continuous infusion and CE−MS operation, respectively.

In this study, nonporous silica (NPS) particles with diameters from 5 to 25 μm were packed into fused silica capillary columns of 250 μm i.d. using a CO2 slurry packing method. The column efficienc...

In this paper, solid phase microextraction (SPME) using porous layer-coated fibers is reported. Bonded silica particles (5 μm diameter), which are typically used for reversed-phase high-performance liquid chromatography, were fixed onto the surface of metal wires using high-temperature adhesives. The adsorption and desorption properties of the new porous layer SPME fibers were investigated using aromatic hydrocarbon test compounds, and the results were compared with those of commercially available polymer-coated fused silica fibers. It was found that the desorption time using the porous layer fibers was less than 10 s, which provides high chromatographic efficiency without cryogenic trapping. The total mass of toluene adsorbed from a 0.1 ppm aqueous solution on a fiber with a 30 μm C-8 porous layer was 8 times larger than that obtained using a fiber with a 100 μm thick polydimethylsiloxane coating. It was also found that the selectivity of the porous layer in SPME greatly depends on the properties of the ...

Inverse supercritical fluid chromatography (ISFC) is used to measure partition coefficients of five polar and nonpolar organic solids between supercritical CO2 and cross-linked poly(dimethylsiloxane) (PDMS) swollen with CO2. Polymer−solute interaction energies, which are determined from the partition coefficients, correlate with the heats of vaporization of the pure solutes. These interaction energies can be determined at ambient temperature for solutes that are too nonvolatile to study by inverse gas chromatography. Binary and ternary phase equilibria are modeled with lattice fluid theory including a term to account for the degree of polymer cross-linking. Experimental results indicate that solute adsorption on the support phase, which can be severe in packed columns, is negligible with the use of capillary columns.

Chiral 2,16-diallyl-, 2,16-dimethyl-, and 2,16-di-tert-butylpyridino-18-crown-6 ligands have been prepared by treating the appropriate chiral α,α‘-disubstituted pyridinedimethanol with tetraethylen...

The syntheses, structures, and spectroscopic properties of 6(A),6(B)-bis-O-[p-(allyloxy)phenyl]-substituted beta-cyclodextrins have been investigated. Selective activation of the 6(A),6(B)-hydroxy groups was carried out by treating heptakis(2,3-di-O-methyl)-beta-cyclodextrin (1) with 2,4-dimethoxybenzene-1,5-disulfonyl chloride to give 6(A),6(B)-bissulfonate ester 2 in a yield of only 3%. This material was treated with sodium p-(allyloxy)phenoxide in DMF to form 6(A),6(B)-bis-O-[p-(allyloxy)phenyl]-heptakis(2,3-di-O-methyl)-beta-cyclodextrin (3), which had two isomers. One (3A) has the two p-(allyloxy)phenyl arms directed away from the cyclodextrin cavity, and the other (3B) has one of the p-(allyloxy)phenyl groups through the cavity to form a self-inclusion complex. When either 3A or 3B was treated with methyl iodide and sodium hydride, the resulting permethylated 6(A),6(B)-bis-O-[p-(allyloxy)phenyl]heptakis(2,3-di-O-methyl)-6(C),6(D),6(E),6(F),6(G)-penta-O-methyl-beta-cyclodextrin (4) was composed of two isomers, in which 4B is a self-inclusion complex. 3A and 3B also can be converted into a mixture of 3A and 3B in strong base but not when melted in the absence of base. 4A and 4B do not isomerize. Detailed 1D and 2D NMR spectroscopic studies were carried out to characterize the structures of these new compounds, and molecular mechanics techniques were used to explain the experimental facts.

Procedures were developed for the estimation of orthogonality in two-dimensional (2D) separations. The parameters evaluated include peak spreading angle, retention correlation, and practical peak capacity. Solute retention parameters, such as retention times and capacity factors on both dimensions, were used to establish a correlation matrix, from which a peak spreading angle matrix was calculated using a geometric approach to factor analysis. The orthogonality is defined by the correlation matrix with correlation coefficients that vary from 0 (orthogonal) to 1 (perfect correlation). Equations were derived for the calculation of practical peak capacity in 2D separations. The calculations are based on the peak capacities obtained on each dimension and the peak spreading angle in an orthogonal, 2D retention space. The equations and the procedures can be used to evaluate the performance of a comprehensive 2D separation. Using experimental data from a 2D GC separation, it is demonstrated that the equations are very useful for the comparison, evaluation, and optimization of 2D separations.

The ring-closure reaction of M(CO){sub 5}L-L (M = W, Mo) where L-L denotes a bidentate ligand was chosen for its comparative independence of solvent properties to demonstrate the repulsive (or intrinsic) part of the activation volume. The ring-closure reaction rate constant was determined in supercritical CO{sub 2} and in supercritical ethane over the range between 1000 and 3500 psi at several temperatures (35, 42, 48, and 60 {degree}C). The activation volume was observed to be as high as +7000 cm{sup 3}/mol just above the critical point in each of these solvents. The ring-closure activation volume was also determined in liquid CO{sub 2} and in several other liquid solvents for comparison. These studies demonstrate a huge repulsive contribution to the activation volume in these two supercritical fluids. 39 refs., 4 figs., 4 tabs.

Determination of the aromatic hydrocarbon content of diesel fuels by supercritical fluid chromatography (SFC) has been approved as an American Standard Test Method. Commercially available microbore columns usually used in this application suffer from poor stability and low resolution. In this work, 200 [mu]m i.d. packed capillary SFC columns were prepared, and their chromatographic performances were compared with commercial microbore columns. Various packing materials with different pore sizes were evaluated, and the effects of column temperature and pressure were carefully examined. It was found that the pore size of the packing material and, therefore, the surface area had a significant effect on elution order. Using a 1 m long column, a resolution of as high as 15 for n-hexadecane and toluene was achieved within 5 min at 45[degree]C. The column performance was very reproducible; day-to-day and month-to-month resolution variations were less than 3%, and retention time variations were less than 1%. In this method, no additional columns and valve switching were involved. The method is simple, fast (approximately 10 min), and very suitable for quality control analysis. 35 refs., 5 figs., 7 tabs.

Three series of basic proteins and peptides were selected for use in studying the adsorption behavior of fused-silica capillary electrophoresis columns. A comparative study of untreated and treated fused-silica columns was done. The polymers used as coating materials ranged from very hydrophobic to very hydrophilic. The average efficiency of weakly hydrophilic Ucon-coated capillaries for basic proteins was over 1 million theoretical plates at a buffer pH of 4.8. For this coating, it was found that the maximum column efficiency was obtained at a film thickness of approximately 100 nm. It was easily seen that both ionic and hydrophobic interaction forces can play important roles in protein adsorption onto the capillary walls. Furthermore, strongly hydrophilic polymer coatings may possibly lead to hydrogen-bonding interactions, which decrease column efficiency. Studies have been conducted using two series of polypeptides with varying chain lengths. One series was used to study the effect of polypeptide chain length on retention behavior. Efficient separations of bioactive peptide drugs on Ucon-coated columns were achieved. The results indicate that high efficiency and resolution of basic peptides tested can be realized on columns modified with weakly hydrophilic polymers at a pH value of about 5.

A coupled supercritical fluid extraction (SFE), high-speed gas chromatography (GC), and element-selective radiofrequency plasma detection (RPD) system was evaluated for the analysis of volatile organic compounds in soil matrices. The effect of GC carrier gas (carbon dioxide) and plasma reagent gas (oxygen) on the selectivity and sensitivity of the RPD was studied to establish the optimal operational conditions. The selectivities obtained were 3500 (Cl/C) for chlorine and 2600 (S/C) for sulfur. The detection limits were 24.8 and 9.2 pg s -1 for chlorine and sulfur, respectively. Complete quantitative analysis could be easily accomplished within 10 min

The design and use of a solid-phase injector for open tubular column supercritical fluid chromatography (SFC) are reported. For sample introduction, a liquid sample was loaded on a platinum wire. After the solvent had evaporated, the wire was inserted into, and sealed in, the injector. The chromatographic run was started by introducing the supercritical mobile phase into tie injector. Absolute and relative peak area reproducibilities, as well as retention time reproducibility, of the injection method were evaluated. The relative standard deviations (% RSDs) of absolute and relative peak areas were 2.0-4.5 and 0.4-1.8%, respectively, for both 1- and 2-μL injections

Liquid samples were introduced into an open tubular column supercritical fluid chromatograph (SFC) using a short packed capillary column as a solute focusing device after a typical sample loop valve injector. The packing materials were held inside the capillary by porous ceramic frits. Excess solvent was vented into the atmosphere through the packed capillary. Since the exit end of the packed capillary column was at atmospheric pressure, a density gradient was established along the length of the column and solutes precipitated at the point where the density dropped below the carbon dioxide solvating density for these compounds

A new method for the simultaneous deactivation and coating of porous silica particles for supercritical fluid chromatography (SFC) has been developed. This method is based on a dehydrocondensation reaction between polymeric silicon hydride reagents and the silanol groups on the surface of the particles. The procedure produces a less active surface than conventional silica packings, which results in less adsorption and improved peak shapes for polar analytes. In SFC, more polar analytes can be chromatographed without the need for mobile phase modifiers. Furthermore, the sensitive and universal flame ionization detector (FID) can be used, since modifiers are not necessary. To avoid splitting of the column effluent before FID detection, packed capillary columns were utilized in this study. The ability to use packed capillary columns for the analysis of polar compounds, while at the same time allowing the use of a wide range of detection methods, serves to expand the number of useful applications for packed column SFC.

Solanesol, a trisesquiterpenoid alcohol, has been shown to be present in environmental tobacco smoke. Results from the determination of particulate-phase solanesol in environmental tobacco smoke in both chamber and indoor environments show that solanesol is a suitable tracer for the particulate phase of environmental tobacco smoke

Une methode est presentee pour la regulation de la pression et de la vitesse lineaire en chromatographie a fluide supercritique