Biomedical subjects

Gary R Dennis

Publications and source records attributed to Gary R Dennis.

18 recordsLinked to original sources

PubMed2006

Visualising viscous fingering in chromatography columns: high viscosity solute plug.

The interface between two fluids that have different viscosities and are percolating through a porous bed is unstable. Sooner or later, a flow instability termed viscous fingering (VF) develops. This phenomenon is important in chromatography because the solute plug does not have the same viscosity as the mobile phase. Because the sample is often much more viscous than the mobile phase, it is the interface at the rear of the sample band that is usually unstable. This situation is frequent in many modes of chromatography, e.g., in preparative and in multidimensional chromatography, in size exclusion chromatography, in frontal analysis, and in other physicochemical measurements (e.g., determination of adsorption isotherms and of mass transfer parameters). When the solute plug is more viscous than the mobile phase, we observed that the solute band compressed. When the viscosity contrast increased up to 0.30 cP, fingers appeared to trail behind the solute plug. The development of fingers then became more substantial as the viscosity contrast increased. To avoid effects associated with VF, the mobile phase and the solute plug should have nearly the same viscosity.

Adsorption↗

PubMed2006

Interactions of poly(tert-butyl acrylate)-poly(styrene) diblock copolymers with lipids at the air-water interface.

Diblock copolymers with hydrophilic poly(tert-butyl acrylate) (PtBA) and hydrophobic poly(styrene) (PS) blocks were synthesized with a view to use them as a surfactant in tear film for increasing the ocular comfort in dry eye syndrome. Interactions of six PtBA-PS copolymers with four important lipids found in the tear film, namely cholesterol, cholesteryl palmitate, dipalmitoyl phosphatidylcholine, and phosphatidylinositol, were studied at the air-water interface using a Langmuir trough. Thermodynamics of mixing of the copolymers and the lipids in the mixed monolayers was determined by calculating excess free energy of mixing. The diblock copolymers showed repulsive interactions with cholesteol and cholesteryl palmitate, near neutral interactions with dipalmitoyl phosphatidylcholine, and attractive interactions with phosphatidylinositol. The lipids interacted with the PS component of the copolymer. The results indicate that a copolymer with a small hydrophilic group and a big hydrophobic group can be a likely candidate for forming stable interactions with the lipids present in the tear film and hence increase the ocular comfort.

Acrylates↗

PubMed2006

An assessment of the retention behaviour of polycyclic aromatic hydrocarbons on reversed phase stationary phases: selectivity and retention on C18 and phenyl-type surfaces.

In this manuscript the retention and selectivity of a set of linear and non-linear PAHs were evaluated on five different reversed-phase columns. These phases included C18 and C18 Aqua stationary phases, as well as three phenyl phases: Propyl-phenyl, Synergi polar-RP and Cosmosil 5PBB phase. Overall, the results revealed that the phenyl-type columns offered better separation performance for the linear PAHs, while the separation of the structural isomer PAHs was enhanced on the C18 columns. The Propyl-phenyl column was found to have the highest molecular-stationary phase interactions, as evidenced by the greatest rate of change in 'S' (0.71) as a function of the molecular weight in the PAH homologous series, despite having the lowest surface coverage (3% carbon load) (where S is the slope of a plot of logk versus the solvent composition). In contrast, the C18 Aqua column, having the highest surface coverage (15% carbon load) was found to have the second lowest molecular-stationary phase interactions (rate of change in S=0.61). Interestingly, the Synergi polar-RP column, which also is a phenyl stationary phase behaved more 'C18-like' than 'phenyl-like' in many of the tests undertaken. This is probably not unexpected since all five phases were reversed phase.

Benzene Derivatives↗

PubMed2006

Visualising the onset of viscous fingering in chromatography columns.

Viscous fingering is an important fluid transport phenomenon that manifests itself when two fluids having different viscosities move in the same direction. Their interface is unstable and a complex fingering pattern may arise. This phenomenon is important in chromatography because it may lead to a decrease or even a failure in separations. The onset of viscous fingering was visually observed by packing a glass column with particles having the same refractive index as the mobile phase and injecting plugs of dye solutions having viscosities different from that of the mobile phase. Severe fingering effects are observed if the viscosity difference exceeds 0.17 cP. However, for smaller viscosity differences, band distortions are observed that may affect retention data, band efficiency, and band resolution. Careful attention should be paid to matching the mobile phase viscosity and that of the injection plug when accurate chromatographic information is required.

Chromatography, High Pressure Liquid↗

PubMed2006

Multidimensional liquid chromatography for sample characterisation.

While LC finds enormously widespread use in almost all areas of chemical science, the technique is limited as a means of identification because compounds do not elute with unique retention times. This limitation spurred the growth of hyphenated instrumental methods of analysis, such as LC-MS/MS, which because of the MS/ MS detection became a method of identification. However, techniques like LC-MS/ MS are specialised and require high initial purchase and running costs, inhibiting the more widespread growth of the technique. In an attempt to increase the separation power of LC, multi-dimensional LC was developed. This expanded the separation space and subsequently has allowed the development of methods with fingerprinting ability due to the lower probability of component overlap. The work in this study illustrates the application of 2-D LC as a means of chemical fingerprinting. We employed a sample base of various low molecular weight oligostyrenes and their diastereomers that represent a population of compounds whose selectivities in a one-dimensional separation are almost unity and hence essentially impossible to separate. Yet in a 2-D domain almost all individual components occupy unique 2-D retention times.

Journal Article↗

PubMed2006

The surface activity of purified ocular mucin at the air-liquid interface and interactions with meibomian lipids.

PURPOSE: Ocular mucins are thought to contribute to the stability of the tear film by reducing surface tension. The purpose of this study was to compare the effect of different mucins and hyaluronic acid (HA) alone and mixed with meibomian lipids on the surface pressure at an air-liquid interface. METHODS: A Langmuir trough and Wilhelmy balance were used to measure and compare the surface activity of bovine submaxillary gland mucin (BSM), purified BSM, purified bovine ocular mucin and HA, and mixtures of these with meibomian lipids, phosphatidylcholine, and phosphatidylglycerol. Their appearance at the surface of an air-buffer interface was examined using epifluorescence microscopy. RESULTS: Purified ocular mucin had no surface activity even at concentrations that were 100 times more than normally occur in tears. By contrast, commercial BSM caused changes to surface pressure that were concentration dependent. The surface pressure-area profiles showed surface activity with maximum surface pressures of 12.3-22.5 mN/m depending on the concentration. Purified BSM showed no surface activity at low concentrations, whereas higher concentrations reached a maximum surface pressure of 25 mN/m. HA showed no surface activity, at low or high concentrations. Epifluorescence showed that the mucins were located at the air-buffer interface and changed the appearance of lipid films. CONCLUSION: Purified bovine ocular mucin and HA have no surface activity. However, despite having no surface activity in their own right, ocular mucins are likely to be present at the surface of the tear film, where they cause an increase in surface pressure by causing a compression of the lipids (a reorganization of the lipids) and alter the viscoelastic properties at the surface.

Air↗

PubMed2005

Interactions of sodium montmorillonite with poly(acrylic acid).

The chemical-structural modifications of the natural clay sodium montmorillonite during interaction with poly(acrylic acid) were studied mainly by X-ray photoemission spectroscopy. Samples of modified montmorillonite were prepared from the reaction of sodium montmorillonite ( approximately 0.5 g) and an aqueous solution of poly(acrylic acid) (pH approximately 1.8, 50 g) at varying temperatures. X-ray diffraction indicated that the montmorillonite interlayer space ( approximately 13 A), formed by regular stacking of the silicate layers (dimension approximately 1x1000 nm), expanded to approximately 16 A as the reaction was carried out at room temperature and at 30 degrees C. At 60 degrees C, the interlayer space further expanded to approximately 20 A. The results of X-ray photoemission spectroscopy indicated that poly(acrylic acid) molecules exchange sodium ions on the surface of the silicate layers. These combined results allowed development of a reaction model that explains the dependency of the interlayer expansion with temperature. Information concerning the surface chemical reactions and systematic increases in the interlayer distances is particularly useful if montmorillonite and poly(acrylic acid) are to be used for formation of nanocomposite materials.

Acrylic Resins↗

PubMed2005

Molecular differences between deuterated and protonated polystyrenes using reversed-phase high-performance liquid chromatography.

Isotopic substitution is a technique used to highlight particular bonds within a molecule for kinetic, spectroscopic and structure analysis. It is presumed that although some properties such as stretching frequencies will not be the same for substituted analogues, the chemical interactions will not vary appreciably as a function of labelling. Reversed-phase liquid chromatography has been used to demonstrate that there are significant differences between the chromatographic behaviour of a sequence of deuterated and protonated oligomeric polystyrenes. Two-dimensional reversed-phase liquid chromatography was used to show that even the diasteromers of the oligomers (n = 5) have retention mechanisms that are dependent on the subtle changes to the molecular conformation and electronic structure, which are a consequence of deuteration.

Chromatography, High Pressure Liquid↗

PubMed2005

Utilising retention correlation for the separation of oligostyrenes by coupled-column liquid chromatography.

The separation of n = 2-5 n-butyloligostyrenes has been illustrated by reversed-phase reversed-phase (RP-RP) coupled-column liquid chromatography. The coupled-column separation has been achieved by use of a C18 column with methanol as the mobile phase followed by a DiamondBond C18 column with acetonitrile (ACN) mobile phase. The DiamondBond C18 is a hybrid carbon clad zirconia (CCZ)-C18 stationary phase. Unlike a C18-carbon clad zirconia two-dimensional liquid chromatographic system, which is orthogonal, the C18 and DiamondBond C18 columns combination exhibit correlations based upon the molecular weight of n-butyloligostyrenes. Using an alternative strategy to two-dimensional liquid chromatography, the molecular weight dependence displayed by both the C18 column and DiamondBond C18 has been used to increase throughput or decrease analysis time in the analysis of the n-butyloligostyrenes. However, this is at the expense of a portion of the two-dimensional peak capacity displayed by the C18-carbon clad zirconia system.

Chromatography, Liquid↗

PubMed2005

The surface pressure dynamics and appearance of mixed monolayers of cholesterol and different sized polystyrenes at an air-water interface.

Synthetic polymers are increasingly being used in situations where they are designed to interact with biological systems. As a result, it is important to investigate the interactions of the polymers with biochemicals. We have used cholesterol, as an example of an important biological surfactant component, to study its interactions with polystyrene. Mixed monolayers of cholesterol and one of two different molecular weight polystyrenes were formed at an air-water interface to investigate their interactions and to determine whether the size of the polystyrene affected the interaction. The pressure-area (pi-A) isocycles of mixed monolayers of cholesterol and polystyrene MW 2700 or polystyrene MW32700 showed that strongest attractive interactions occur at high surface pressures and in polystyrene rich films. The excess area and excess free energy of mixing were most negative at high surface pressures and at high mole fraction of polystyrene. The most stable mixed monolayers were formed with X(PS2700) = 0.9 and X(PS32700) = 0.09. Microscopic observation of the mixed monolayers of cholesterol and polystyrene showed the formation of stable islands in the cholesterol/polystyrene mixtures. These observations, the nature of the inflection points in the isocycles, and the anomalous changes in free energy lead us to conclude that there is a stable rearrangement of polystyrene into compact islands when it is mixed with cholesterol. Any excess cholesterol is excluded from these islands and remains as a separate film surrounding the islands.

Air↗

PubMed2005

Surface pressure measurements of human tears and individual tear film components indicate that proteins are major contributors to the surface pressure.

PURPOSE: Tear film stability has been associated with a low surface tension (high surface pressure), which has been attributed to a variety of tear film components. In this study, we examined the contribution of various tear proteins, mucin, and meibomian lipids to the surface pressure of human tears. METHODS: A Langmuir trough was used to measure and compare the surface activities of albumin, lipocalin, beta-lactoglobulin, lactoferrin, lysozyme, secretory IgA, mucin, meibomian lipid, and tears. RESULTS: All proteins exhibited surface activity. The surface pressure-area (Pi-A) profiles of most protein films at equilibrium surface pressure (Pieq) were sigmoidal and showed hysteresis between the expansion and compression phases of the cycle. Pieq of most proteins took 4-9 hours to occur. By contrast, the Pi-A profiles for meibomian lipid films were hyperbolic rather than sigmoidal and had little hysteresis, and Pieq was attained within 1 hour. The Pi-A profiles of mucin films showed mostly hyperbolic characteristics with small hysteresis. The Pi-A profiles of films of tears were sigmoidal, showed strong hysteresis, and reached Pieq at about 5 hours. Partitioning of the proteins and whole tears into the subphase also occurred. CONCLUSION: Comparison between the dynamic Pi-A profiles of tears and those of individual tear film components shows that tear film proteins not only are capable of surface activity but also are major contributors to the surface activity of the tear film.

Albumins↗

PubMed2004

Comprehensive two-dimensional separations of complex mixtures using reversed-phase reversed-phase liquid chromatography.

A comprehensive two-dimensional reversed-phase reversed-phase liquid chromatographic system for the separation of a complex mixture of oligostyrenes was developed using results from a previous theoretical assessment of the informational similarity, percent synentropy, orthogonality and peak capacity of hypothetically coupled systems. The degree of sample attribute order in the first separation dimension was also used in the development of the experimental two-dimensional system. A C18(methanol)/CCZ(acetonitrile) two-dimensional system was chosen for the comprehensive analysis of the oligostyrene mixtures because this system had the lowest solute crowding, highest orthogonality and was observed to have order with respect to a sample attribute in the first separation dimension. The separations achieved were in full agreement with the results from information theory and (a geometric approach to) factor analysis assessments. High sampling rates in the first liquid chromatographic dimension were shown to be impossible or inefficient when the peak capacity and separation time of the second dimension was high or when the aim of the exercise was to isolate individual sample constituents in high yield.

Chromatography, Liquid↗

PubMed2004

Separation of oligostyrene isomers in a complex mixture using two-dimensional heart-cutting reversed-phased liquid chromatography.

The development of a two-dimensional liquid chromatographic system requires a process of assessment that can yield an optimum performing system with minimal experimental evaluation. Information Theory and a geometric approach to Factor Analysis are two techniques that when used in combination, provide important information on the expected two-dimensional performance. In the present study, we compare the predicted separation performance of two-dimensional systems that have been subjected to analysis by Information Theory and Factor Analysis to that of actual chromatographic separation performance. Our test separation comprised a mixture of 32 oligostyrene structural isomers and stereoisomers. The optimal combination as determined by Information Theory and Factor Analysis consisted of a C18 column with a methanol mobile phase in the first dimension and a carbon clad zirconia column with an acetonitrile mobile phase in the second dimension. This system was also shown to be the most successful practical system when a heart-cutting approach was employed. The practical results were in total agreement with the results from Information Theory and Factor Analysis. The number of isomers resolved using this system was 27. A second system, namely one comprising of a C18 column and methanol mobile phase in the first dimension and a carbon clad zirconia column with a methanol mobile phase in the second dimension was also predicted to be a system with high separation potential. However, practical assessment of this system did not realise the theoretical predictions, largely due to the long separation times required in the second dimension. Furthermore, all combinations that employed a C18 column with an acetonitrile mobile phase in the first dimension failed to realise the theoretical separation potential due to high solute crowding, low orthogonality and a disordered arrangement of bands along the first separation axis. This was also predicted by the theoretical assessment.

Chromatography, High Pressure Liquid↗

PubMed2003

Evaluation of the two-dimensional reversed-phase-reversed-phase separations of low-molecular mass polystyrenes.

The resolving power of four reversed-phase-reversed-phase two-dimensional (2D) chromatographic systems was evaluated using information theory (IT) and a geometric approach to factor analysis. The first separation dimension employed a C18 column, while the second separation dimension employed a carbon clad zirconia column. Mobile phases in each dimension were either methanol or acetonitrile. The sample matrix that was employed in this study contained a mixture of 58 components, comprised of stereoisomers and structural isomers of a systematic variation in molecular weight. Each of the components were oligostyrenes, with between two and five configurational repeating units having either n-butyl, sec-butyl or tert-butyl end groups. In the two-dimensional systems employed in this study, between 46 and 49 of the 58 components could be separated, depending on the mobile phase combinations--with apparent resolution that could not possibly be achieved in a single one-dimensional separation. The results from this study indicate that in order to fully evaluate the resolving power of a 2D system multiple methods of analysis that evaluate the separation potential are most appropriate. This becomes increasingly important when the sample contains components that are very closely related and the retention of solutes displays a high degree of solute crowding.

Molecular Weight↗

PubMed2003

Comprehensive coupled reversed-phase reversed-phase separations of a complex isomeric mixture.

Using predictions based on results obtained from Information Theory and Factor Analysis for the two-dimensional separation of a complex isomeric mixture, a practical experimental comprehensive coupled reversed phase-reversed phase chromatographic system was developed. In total four reversed phase-reversed phase systems were studied, each of which theory predicted would be able to resolve essentially equal numbers of components. However, in practice only one of these coupled systems realised the theoretical potential. This system employed as the first dimension, a C18 stationary phase with methanol as the mobile phase and as the second dimension, carbon clad zirconia as the stationary phase and acetonitrile as the mobile phase. In this system, 27 of the 32 isomers of a mixture of oligostyrenes were resolved. Failure of the remaining coupled systems to achieve the theoretical potential was attributed to high solute crowding, low efficiency of separation space utilisation and long analysis times in the second dimension.

Journal Article↗

PubMed2003

The effects of novel amphipathic block copolymers on stabilization of the rat tear film.

PURPOSE: To determine whether various novel amphipathic polymers could be used to stabilize the tear film of the rat. The rheologic properties of these polymers were examined to investigate whether particular structural or physical characteristics improve the stability of the tear film. METHODS: Amphipathic polymers or particular phospholipids were mixed with a test solution of tears and saline and applied to the clean, dry corneal surface of a rat. The specular reflection of the tear film was observed at high magnification and recorded. For each of the polymers or lipids, the effects on surface regularity and tear break-up time were compared. After the experiments, histologic sections of the tested eyes were prepared and examined for acute cytotoxic effects on the cornea and ocular conjunctiva. RESULTS: Tear film break-up time was markedly affected by differences in polymer structure. Copolymers consisting of separate hydrophobic and hydrophilic regions appeared to be the best stabilizers. No acute cytotoxic effects were observed in histologic sections of corneas to which the polymers had been applied. CONCLUSIONS: Amphipathic polymers can be designed to increase tear film stability. Increased tear film stability occurred more readily with copolymers, possibly through their interaction with both lipid and aqueous tear components.

Acrylic Resins↗

PubMed2002

Two dimensional reversed-phase-reversed-phase separations isomeric separations incorporating C18 and carbon clad zirconia stationary phases.

Informational theory and a geometric approach to factor analysis were employed to evaluate the degree of orthogonality of a two-dimensional reversed-phase-reversed-phase chromatographic system. The system incorporated a C18 column as one dimension and a carbon clad zirconia column as the second dimension. In order to study the resolving power of this system, the separation of a sample matrix containing an artificial mix of 32 isomers (structural and diastereoisomers) was evaluated. Using this system, between 25 and 28 of the 32 isomers could be separated, depending on the mobile phase combinations--with resolution that could not possibly be achieved in a single one dimensional separation. The results from this study indicate that in order to fully evaluate the resolving power of a 2D system multiple methods of analysis are most appropriate. This becomes increasingly important when the sample contains components that are very closely related and the retention of solutes is clustered in one quadrant of the 2D space. Ultimately, the usefulness of the 2D separation is determined by the goals of analyst.

Carbon↗

PubMed2002

Retention behaviour of polystyrene oligomers in reversed-phase liquid chromatography.

Reversed-phase liquid chromatography (RPLC) was employed to investigate the behaviour of low-molecular-mass polystyrene oligomers with three different end groups, n-butyl, sec-butyl, and tert.-butyl polystyrenes. Exothermodynamic retention studies on the polystyrene oligomers were carried out using a C18 stationary phase column and 100% methanol mobile phase over the temperature range 15 to 60 degrees C. The resulting van't Hoff plots were linear over the entire temperature range for all three end group polystyrenes. Enthalpy-entropy compensation (EEC) showed a linear compensation for the higher-order oligomers, but was non-linear for the lower-order oligomers, indicating a change in the mechanism of retention. Differences in the extent of retention for each of the three end groups were also apparent. The ramifications of these differences are discussed.

Chromatography, Liquid↗