Biomedical subjects

Philip J Marriott

Publications and source records attributed to Philip J Marriott.

28 records · Page 2Linked to original sources

PubMed2004

Molecular structure retention relationships in comprehensive two-dimensional gas chromatography.

Comprehensive two-dimensional gas chromatography (GC x GC) offers new opportunities to develop relationships between molecular structure and retentions in the two dimensional (2D) separation space defined by the GC x GC retention in each dimension. Whereas single dimension GC provides only one retention property for a solute, and hence the specific relationship between retention and chemical property is not readily apparent or derivable, the 2D presentation of compounds in GC x GC provides a subtle and exquisite correlation of chemical property and retention unlike any other GC experiment. The 'orthogonality' of the two separation dimensions is intimately related to the manner in which different separation mechanisms, available through use of two dissimilar phases, are accessible to the different chemical compounds or classes in a sample mixture, and indeed the specific chemical classes present in the sample. The GC x GC experiment now permits various processes such as chemical decompositions, molecular interconversions, various non-linear chromatography effects, and processes such as slow reversible interactions that may arise with stationary phases or in the injector or column couplings, to be identified and further investigated. Here, we briefly review implementation of the GC x GC method, consider the molecular selectivity of GC x GC, and highlight a selection of molecular processes that can be probed by using GC x GC.

Chromatography, Gas↗

PubMed2003

Methods for generating second dimension retention index data in comprehensive two-dimensional gas chromatography.

Two methods of generating transportable second dimension retention data are outlined for comprehensive two-dimensional gas chromatography (GC x GC). They are both refinements of a previously outlined procedure, which adapted 'isovolatile' curves to retention prediction maps developed in our laboratory, extended to a more polar homologous series, the linear primary alcohols. The earlier work investigated maps based on alkanes and methyl ketones and methyl esters; here the method of data collection to generate the retention map has also changed, extending the retention base range. The resulting retention map permits a retention basis to be used for GC x GC data, however, a more polar retention set than alkanes is required. The calculation of 'retention indices' is aided by the generation of 'fractional reference compound' curves, by either direct manipulation of data local to the solute, or generation of a discrete curve coincident with the retention co-ordinates of the target compound.

Alcohols↗

PubMed2003

Application of comprehensive two-dimensional gas chromatography to the quantification of overlapping faecal sterols.

Standard solutions containing a mixture of seven sterols and 5alpha-cholestane as internal standard, and sample mixtures that comprised varying ratios of sterol and stanols from green lip mussel tissue and dried cow faeces were analysed by using comprehensive two-dimensional gas chromatography (GC x GC). Quantitative results were compared with single-column GC analysis. The latter samples included sterols of interest, but which cannot be readily obtained elsewhere. It may also mimic potential environmental samples where dairy production and aquaculture (oyster, mussel cultivation) share the same catchment; environmental sterol signatures may exhibit characteristics of both sample types comprising this mixture. Whereas single-column GC-flame ionisation detection was unable to reliably quantitate target sterols, the GC x GC experiment permitted small amounts of sterols and stanols to be detected and separated. Likewise GC-MS analysis was unable to detect some of the minor sterols which coeluted on a single column. The GC x GC mode allows complete separation of several important sterols and stanols, such as 24-ethylcoprostanol, campesterol and 24-methylenecholesterol, demonstrating the enhanced resolving power of the GC x GC system. Separation of 24-ethyl-epi-coprostanol from several algal-derived interfering components was achieved, leading to higher degree of confidence in the quantitative analysis of faecal sterols. The effects of a number of operating variables--column length, carrier flow-rate and elution temperature--on component resolution and presentation of data in the two-column analysis are described.

Animals↗

PubMed2003

Large-volume stacking with polarity switching and sweeping for chlorophenols and chlorophenoxy acids in capillary electrophoresis.

This paper describes approaches for stacking large volumes of sample solutions containing a mixture of chlorophenols and chlorophenoxyacetic acids as their anions in capillary zone electrophoresis, and compares results to standard capillary electrophoresis (CE) and normal stacking modes. In order to increase the amount of sample injected beyond the optimal conditions and maintain high resolution, the sample introduction buffer must be removed after the stacking process is completed. This is achieved by pumping the sample buffer out of the column using polarity switching. Large sample volumes are loaded by hydrodynamic injection, then stacked at the injection buffer/run electrolyte interface, followed by the removal of the large plug of low-conductivity sample matrix from the capillary column using polarity switching and finally the separation of the stacked anions in a basic buffer (pH 8.65). Around 10- and 40-fold improvement of sensitivity was achieved by normal stacking and large-volume stacking with polarity switching, respectively, when compared to the standard CE analysis. Sweeping-micellar electrokinetic capillary chromatography (MEKC) was also investigated for the purpose of comparison to the stacking technique. The method should be suitable for the analysis of these chemical compound classes in industrial chlorophenoxyacetic acid manufacture.

Buffers↗

PubMed2003

A review of environmental toxicant analysis by using multidimensional gas chromatography and comprehensive GC.

The purpose of this review is to highlight the developments in coupled-column gas chromatography methods for qualitative analysis of selected environmental toxicants such as dioxin, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), etc. In particular, the new technique of GC x GC will be introduced, and its role, and the promise it offers to this field is discussed. The benefits of enhanced separation to quantitative analysis will be considered. In order to perform an accurate risk assessment, both the dose and biological effects of environmental toxicants have to be determined with a high degree of certainty. This is most often achieved by using chromatographic methods. Given the complexity of most environmental sample extracts, single-column gas chromatography is unable to fully resolve all the components of interest frequently leading to a positive bias in the reported concentrations. Advanced separation tools, such as multidimensional gas chromatography (MDGC), were investigated quite early and demonstrated improvements in separation. However, limitations in the number of target analytes that could be analyzed in a single run as well as insufficient robustness lead to a continued interest in alternative solutions. The parallel development of mass spectrometric identification and quantification strategies proved useful in many cases, although it frequently failed to provide positive identification of chromatographically unresolved isomeric compounds. More recently, comprehensive two-dimensional gas chromatography (GC x GC) may offer a solution to that problem, especially because it offers enhanced resolution for complex mixtures containing trace level environmental toxicants.

Chromatography, Gas↗

PubMed2002

Comprehensive two-dimensional gas chromatography with fast enantioseparation.

The development of fast chiral analysis for use in comprehensive two-dimensional gas chromatography in which a short second dimension enantioselective capillary column provides a route to precise measurement of chiral ratios of enantiomers is described. Retention times as short as 8 s are reported for (+/-)-limonene, with adequate enantioseparation maintained (Rs approximately 1.0) on a 1-m cyclodextrin derivative-coated capillary column. Sufficiently fast elution on the second column was achieved by using GC/ MS in which the subambient pressure (vacuum outlet) conditions promote increased diffusion coefficients and higher component volatility; a 4-fold reduction of second-dimension retention time was observed, as compared with ambient pressure outlet conditions. The enantiomeric distribution of several monoterpene compounds in bergamot essential oil is reported as a demonstration of the method. Total analysis time of the target components was approximately 8.5 min.

Journal Article↗

PubMed2002

Retention time reproducibility in comprehensive two-dimensional gas chromatography using cryogenic modulation an intralaboratory study.

A survey was conducted to determine the reproducibility of retention times in both the first (D1) and second dimension (D2) axes of the two-dimensional separation space, in the comprehensive two-dimensional gas chromatographic analysis of an essential oil sample using cryogenic modulation. The retention times in the two dimensions for a number of individual components comprising hydrocarbon, alcohol, ester and ketone chemical classes in a Melaleuca alternifolia essential oil were recorded from replicate analyses using four separate column sets and two identical gas chromatographs. Run-to-run, day-to-day, instrument-to-instrument, and column set-to-column set reproducibility were demonstrated from the experimental design. A total of 60 GC x GC analyses were conducted. The longitudinally modulated cryogenic system produced reproducible modulation start times and consistent modulation phase profiles for individual components in all experiments, and retention time variations in both dimensions were negligible. The average run-to-run reproducibility of 43 components for six replicate injections was found to be 0.12% RSD in the first dimension, and 0.74% RSD in the second dimension. Day-to-day reproducibility showed statistically "significant" difference (F-test), but this was partly ascribable to the excellent within-day reproducibility that led to apparent day-to-day differences. Confidence in absolute retention times (hence component positions) in the two-dimensional separation space is critical to component identification.

Chromatography, Gas↗

PubMed2002

Positional and geometric isomer separation of FAME by comprehensive 2-D GC.

The technique of comprehensive GC (GC x GC) was applied to the analysis of a standard mixture of FAME. The methodology involved the use of two directly coupled capillary GC columns providing different retention mechanisms, with a pulsing modulator located near their union. The first column was chosen to elute analytes based on b.p. variations, and the second column was based on polarity. Thus, the separation in the two dimensions was orthogonal, since solutes delivered simultaneously to the second column had similar b.p., and the second column separated these primarily on their differentiating mechanisms of polarity. Greater sensitivity of detection and narrower peak widths were obtained; here, peak response increases of about 20-fold were obtained, with pulsed peak widths of about 150 ms. Peaks were displayed in a 2-D contour plot to allow the complexity of the compounds to be seen and their b.p. and polarity properties to be readily recognized. Chromatographic separation of geometric and positional isomers of FAME in the 2-D space is possible. Since retention can be related to the degree and manner of unsaturation and isomerization, and as peak positions are highly reproducible in the 2-D retention map, this is a useful aid for component identification in the absence of appropriate standards. In this work, two column combinations were used to examine the effects of polarity changes on component separation. Improved quantitation based on FID area measurement was demonstrated. A sample of marine oil gave 49 resolved, identified peaks, with at least an additional 20 peaks resolved but not identified.

Animals↗

PubMed2002

Headspace solid-phase microextraction--comprehensive two-dimensional gas chromatography of wound induced plant volatile organic compound emissions.

Plant emissions of volatile organic compounds from mechanically wounded Agrostis stolonifera, Pennisetum clandestinum, Eucalyptus leucoxylon and Trifolium repens have been sampled by headspace-solid phase microextraction (HS-SPME) and analysed by using comprehensive two-dimensional gas chromatography (GCxGC) for measurement of the plant emissions. GCxGC produces a fingerprint of the volatile organic compounds in a 2D separation space that may be approximately interpreted as a boiling point-polarity space, and may then be presented as a two-dimensional contour plot. This allows identification of sample-dependent variations in component distributions in the 2D plot, which will contain information about plant differences and should therefore facilitate recognition of different plant materials and displays the gross differences in volatiles between each plant species.

Agrostis↗

PubMedDate not supplied

A review of basic concepts in comprehensive two-dimensional gas chromatography.

The technique of comprehensive two-dimensional gas chromatography (GCxGC) is reviewed. A description of technical aspects of the method illustrates how the GCxGC result is achieved through the use of dual-coupled columns and the modulation of capillary chromatographic peaks. This review presents an expanded section dealing with the relationship between the modulation phase and frequency and the resulting peak pulse profiles. Experimental results that support the appreciation and understanding of the effects that pulsing has on a chromatographic peak are provided. The main goals of GCxGC analysis are discussed with respect to analytical sensitivity and peak capacity arising from zone compression effects and fast analysis on the second column. A typical application of GCxGC is presented, along with a consideration of implementation of the GCxGC method.

Journal Article↗