Stacy K. Seeley

Multidimensional Gas Chromatography: Fundamental Advances and New Applications

■ CONTENTS Main Types of MDGC 558 Heart-Cutting Two-Dimensional Gas Chromatography 558 Comprehensive Two-Dimensional Gas Chromatography 558 Fundamental Advances 558 New Modes of Implementation 558 Hybrid MDGC 558 Smart MDGC Interfaces 559 High-Speed GC × GC 559 High Temperature GC × GC 559 Coupling SFC to GC × GC 559 GC × GC Hardware Development and Method Optimization 559 New GC × GC Modulators 559 Comparison of Thermal and Flow Modulation 560 Studying the Precision of Low Duty Cycle Modulation GC × GC 560 Influence of Experimental Conditions on GC × GC Separations 560 Improved Mass Spectrometric Detection for GC × GC 561 Ionic Liquid Stationary Phases and MDGC 562 Unique Combinations of GC × GC Stationary Phases 562 GC × GC Retention Models 562 Predicting GC × GC Retention Times 562 Retention Indices and GC × GC 562 GC × GC Data Analysis 563 Improved 2-D Peak Integration 563 Multivariate Analysis of GC × GC Chromatograms 563 Applications of MDGC 564 Fuels and Industrial Chemicals 564 Examining Low Volatility/High Complexity Mixtures 564 Petroleum Biomarker Determination 565 Composition of Coal-Derived Materials 565 Alternative Fuel Characterization 566 GC × GC Separations of Miscellaneous Fuels and Industrial Mixtures 567 Environmental Analysis 567 Atmospheric Analysis 567 Water Analysis 567 Soil and Sediments 568 Biological Matrices 568 Commercial Products 569 Foods, Flavors, and Fragrances 569 Essential Oils 569 Personal Care Products 570 Agricultural Products 570 Wine Analysis 571 Biological Studies 572 Drug Analysis 572 VOC Emissions Resulting From Decaying Flesh 572 VOC Emissions From Specific Organisms 572 Analysis of Cells and Biofluids 573 Metabolomics 573 Summary of Instrument Configurations used for Applications 574 Conclusions 575 Author Information 575 Corresponding Author 575 Notes 575 Biographies 575 References 575

Solvation parameter model of comprehensive two-dimensional gas chromatography separations

A solvation parameter model was used to generate comprehensive two-dimensional gas chromatography (GC x GC) retention diagrams for 54 solutes on four different stationary phase combinations. Retention diagrams are plots used to predict the relative position of solutes in GC x GC chromatograms. In this study, retention diagrams were based entirely on solute and stationary phase descriptors taken from the literature. The temperature-averaged values of the stationary phase descriptors were used to further simplify the model. The relative positions of the solutes in the retention diagrams were compared with experimentally obtained GC x GC chromatograms. Excellent agreement was observed for each column combination. The model was found to generate primary retention time predictions with standard errors that were approximately 1% of the range of the experimental values and secondary retention time predictions with standard errors that were approximately 5% of the range of the experimental values. It is concluded that the GC x GC solvation parameter model is sufficiently accurate to aid in the identification of optimal column combinations.

Characterization of gaseous mixtures of organic compounds with dual‐secondary column comprehensive two‐dimensional gas chromatography (GC×2 GC)

A comprehensive two-dimensional gas chromatograph with dual secondary columns (GC × 2 GC) was used to characterize gaseous mixtures of volatile organic compounds (VOCs). Samples were collected on multi-layer sorbent tubes and introduced into the gas chromatograph using a thermal desorption apparatus. Differential flow modulation was used to couple the primary column to the secondary columns. Each GC × 2 GC analysis produced a pair of two-dimensional gas chromatograms. The chromatograms provided complementary information due to the unique selectivities of the secondary columns. The additional information was especially useful in separating and identifying oxygenated and aromatic compounds. Samples of outdoor air, indoor air, and exhaled breath were analyzed with the GC x 2GC system. More than 100 volatile organic compounds could be separated in less than 10 minutes. The identities of approximately 50 peaks were determined for each sample.

Stationary phase selection and comprehensive two-dimensional gas chromatographic analysis of trace biodiesel in petroleum-based fuel

The GC×GC solvation parameter model has been used to identify effective stationary phases for the separation of fatty acid methyl esters (FAMEs) from petroleum hydrocarbons. This simple mathematical model was used to screen the 1225 different combinations of 50 stationary phases. The most promising pairs combined a poly(methyltrifluoropropylsiloxane) stationary phase with a poly(dimethyldiphenylsiloxane) stationary phase. The theoretical results were experimentally tested by equipping a GC×GC instrument with a DB-210 primary stationary phase and an HP-50+ secondary stationary phase. This instrument was used to analyze trace levels of FAMEs in kerosene. The FAMEs were fully separated from the petroleum hydrocarbons on the secondary dimension of the 2-D chromatogram. The resulting GC×GC method was shown to be capable of accurately quantifying FAME levels as low as 2 ppm (w/w). These results demonstrate the utility of the solvation parameter model for identifying optimal stationary phases for high resolution GC×GC separations. Furthermore, this work presents an effective method for determining the level of biodiesel contamination in aviation fuel and other petroleum-based fuels.

Volume and surface area study of tobramycin-polymethylmethacrylate beads.

Polymethylmethacrylate bone cement beads impregnated with antibiotic are a common treatment for patients with persistent articular joint infections or osteomyelitis. They also are used as a prophylaxis for infection in patients with large soft tissue wounds. The current study was designed to evaluate the relationship between bead geometry and elution of the antibiotic tobramycin by methodically varying the shape of the beads for a given set of volumes. Beads of five shapes (spherical to ovoid) and two volumes were prepared and studied. Only 0.9% to 3.3% of the total amount of tobramycin present actually eluted from the beads in a 96-hour period and of this amount, approximately ⅓ eluted within the first 4 hours. The elution mass data indicate the benefit of numerous, small and elliptically shaped beads for maximal antibiotic availability. Additionally, a mathematical model is presented that describes these findings and can be used to predict tobramycin delivery rates from bone cement beads. This model assumes that the antibiotic is delivered through two mechanisms: fast dissolution of tobramycin initially adhering to the bead surface and slow release by diffusion through the polymer. The results generate diffusion coefficients for tobramycin in polymethylmethacrylate bone cement on the order of 2 × 10−11 cm2/s.

Conjugation of the linoleic acid oxidation product, 13-oxooctadeca-9,11-dienoic acid, a bioactive endogenous substrate for mammalian glutathione transferase

The oxidation of linoleic acid leads to the generation of several products with biological activity, including 13-oxooctadeca-9,11-dienoic acid (13-OXO), a bioactive 2,4-dienone that has been linked to cell differentiation. In the current work, the conjugation of 13-OXO by human glutathione transferases (GSTs) of the alpha (A1-1, A4-4), mu (M1-1, M2-2) and pi (the allelic variants P1-1/ile, and P1-1/val) classes, and a rat theta (rT2-2) class enzyme has been evaluated. The kinetics and stereoselectivity of the production of the 13-OXO-glutathione conjugate (13-OXO-SG) have been examined. In contrast to many xenobiotic substrates, the endogenous substrate 13-OXO does not exhibit an appreciable non-enzymatic rate of conjugation under physiological conditions. Therefore, the GST-catalyzed conjugation takes on greater significance as it provides the only realistic means for formation of 13-OXO-SG in most biological systems. Alpha class enzymes are most efficient at catalyzing the formation of 13-OXO-SG with kcat/Km values of 8.9 mM(-1) s(-1) for GST A1-1 and 2.14 mM(-1) s(-1) for GST A4-4. In comparison, enzymes from the mu and pi classes exhibit specificity constants from 0.4 to 0.8 mM(-1) s(-1). Conjugation of 13-OXO with glutathione at C-9 of the substrate can yield a pair of diastereomers that can be resolved by chiral HPLC. GSTs from the mu and pi classes are the most stereoselective enzymes and there is no apparent relationship between catalytic efficiency and stereoselectivity. The role of GST in the metabolic disposition of the bioactive oxidation products of linoleic acid has implications for the regulation of normal cellular functions by these versatile enzymes.

Comprehensive two-dimensional gas chromatography analysis of high-ethanol containing motor fuels

A comprehensive 2-D GC (GC x GC) instrument equipped with a flow-switching modulator was used to determine the concentration of ethanol and eight other alcohols in a retail pump sample of E85 fuel. E85 is a mixture of ethanol and gasoline where the ethanol concentration can range from 70 to 85 vol%. The increased peak capacity and selectivity generated by GC x GC analysis allowed the alcohols to be fully resolved from the gasoline hydrocarbons. GC x GC analysis was compared to the performance obtained with the standard analytical method for determining ethanol in fuel ethanol (ASTM D5501) and the standard method for determining oxygenate concentrations in gasoline (ASTM D4815). The GC x GC analysis required 14 min while the combined ASTM D5501 and ASTM D4815 analyses required more than 60 min. The ethanol concentration obtained by GC x GC was in excellent agreement with the value obtained by the D5501 method. Poorer agreement was observed between the GC x GC and D4815 concentrations for the other alcohols present in E85. In all cases, the differences could be attributed to deficiencies in the D4815 method that led to coelutions between the alcohols and gasoline hydrocarbons.

High speed Deans switch for low duty cycle comprehensive two-dimensional gas chromatography.

A new high-speed valve-based modulator has been designed and tested for use in comprehensive two-dimensional gas chromatography (GC×GC). The modulator is a Deans switch constructed from two micro-volume fittings and a solenoid valve. Modulator performance was characterized over a wide range of device settings including the magnitude of the switching flow, the gap between the tips of the primary and secondary column, the primary column flow rate, and the carrier gas identity. Under optimized conditions, the modulator was found to be capable of generating narrow pulses (<50ms) of primary effluent with a 2mL/min secondary column flow. This capability will ultimately allow the modulator to be used with GC×GC separations employing a wide range of detectors and secondary column geometries. The main disadvantage of this modulator is that it employs a low sampling duty cycle, and thus it produces separations with sensitivities that are lower than those produced with thermal modulators or differential flow modulators. The efficacy of the new high-speed Deans switch modulator was demonstrated through the GC×GC separation of a hydrocarbon standard and gasoline. Precise quantitation of individual components was possible provided the modulation ratio was kept greater than 2.0.

Comprehensive two-dimensional gas chromatography with pattern modulation

Comprehensive two-dimensional gas chromatography (GC×GC) modulators normally transfer primary column effluent to the head of the secondary column as a series of sharp pulses. Such pulses are produced with time-varying temperature gradients in thermal modulation or with time-varying flow patterns in flow modulation. Thermal modulators produce narrow peaks at optimal flow rates, but require large amounts of consumables or a highly engineered heating/cooling system. Flow modulators involve simpler hardware and no additional consumables. However, flow modulators require a large increase in secondary column flow or transfer only a small portion of the primary effluent to the secondary column. This study examines a new method of producing GC×GC separations with a flow modulator. Instead of injecting narrow pulses, the modulator transfers primary effluent to the secondary column in the form of an intricate injection pattern. The detector signal is deconvoluted and converted to a two-dimensional chromatogram. The high duty cycle of the technique (>50%) leads to deconvoluted peaks with twenty times greater intensity than those produced by conventional modulation with a Deans switch modulator. Pattern modulation can be produced without requiring elevated carrier flows. This study evaluates the efficacy of pattern modulation GC×GC by analyzing a standard mixture of 43 oxygenated organic compounds and an E85 fuel sample.

Analysis of siloxanes in hydrocarbon mixtures using comprehensive two-dimensional gas chromatography

A comprehensive two-dimensional gas chromatography (GC×GC) method for separating siloxanes from hydrocarbons has been developed using a systematic process. First, the retention indices of a set of siloxanes and a set of hydrocarbons were determined on 6 different stationary phases. The retention indices were then used to model GC×GC separation on 15 different stationary phase pairs. The SPB-Octyl×DB-1 pair was predicted to provide the best separation of the siloxanes from the hydrocarbons. The efficacy of this stationary phase pair was experimentally tested by performing a GC×GC analysis of gasoline spiked with siloxanes and by analyzing biogas obtained from a local wastewater treatment facility. The model predictions agreed well with the experimental results. The SPB-Octyl×DB-1 stationary phase pair constrained the hydrocarbons to a narrow range of secondary retention times and fully isolated the siloxanes from the hydrocarbon band. The resulting GC×GC method allows siloxanes to be resolved from complex mixtures of hydrocarbons without requiring the use of a selective detector.