R. Tijssen

Quantitative Aspects of Comprehensive Two‐Dimensional Gas Chromatography (GC×GC)

A software program was developed to enable the quantification of the complex 3D-data sets as produced by GC×GC. Using this software, it was demonstrated that the detectability limit of GC×GC in our study is 18 times better than that of ‘normal’ capillary gas chromatography (CGC). This enhancement is due to the signal increase produced by the thermal modulation effect. The relative standard deviation of 0.9% as measured on a test mixture was excellent. Furthermore, a comparison was made for the group-type separation of heavy gas oils between the hyphenation of LC and GC (LC-GC) and GC×GC. Although these separations are different in nature, the agreement of the results of both methods was very good. The results of GC×GC may even be more accurate, since, different from CGC, even in the most complex chromatograms the baseline in the second dimension chromatograms is always present.

Prediction of comprehensive two-dimensional gas chromatographic separations: A theoretical and practical exercise

Two-dimensional gas chromatography offers an unsurpassed and ordered separation combined with a very high peak capacity. Especially when applied to the separation and quantitative characterisation of complex mixtures it constitutes a leap forward with respect to state-of-the-art capillary GC. This is realised by the two orthogonal separations to which an entire sample is subjected. However, the selection of the proper combination of stationary phases and the temperature program is rather complicated and time consuming. A model is developed to predict which combination of columns is the most appropriate for a specified separation problem. The model is based on calculating retention times and peak widths of the compounds to be separated, on both columns, thus predicting the eventual chromatogram. It starts from estimating the retention factors (k) of the compounds at their elution temperatures. This is performed with the help of the (calculated) vapour pressures and the enthalpic contribution to the activity coefficient as obtained from the Kovats retention indices. Some examples illustrate the usefulness of the model.

Packed-column hydrodynamic chromatography using 1-μm non-porous silica particles

Abstract 150×3 mm I.D. columns, packed with 1-μm non-porous spherical silica particles, were used to separate soluble synthetic polymers by hydrodynamic chromatography. The columns exhibited a plate height of about 1.4 μm allowing very fast and efficient separations of polymers in the molecular mass range 103−2·106 g/mol. The migration behaviour of polymers could be well described by a simple theoretical model. The applicability of packed bed HDC for the fast separation of polymers was illustrated with separations of polystyrene and poly(methyl methacrylate) mixtures.

Design and fabrication of a hydrodynamic chromatography chip

A chromatography chip is presented in which the analysis is performed by polymer separation entirely based on geometry. Design and fabrication are discussed for a chip incorporating an injection structure and a separation channel. The injection is characterized by fluorescence measurements. Chromatographic performance was shown by separation of fluorescein and 26 nm fluorescent particles.

Comparison of methods for the determination of diffusion coefficients of polymers in dilute solutions : The influence of polydispersity

A comparison between various methods to determine diffusion coefficients of polymers in dilute solutions has been made. It is shown that Taylor dispersion analysis (TDA), dynamic light scattering (DLS), hydrodynamic chromatography (HDC), and size exclusion chromatography (SEC) can all be used to accurately determine diffusion coefficients when the polymer samples have low polydispersities. By the analysis of a series of practically representative styrene acrylonitrile copolymer (SAN) samples, it is shown that polydispersity of the samples and the presence of low-molecular-mass material cause considerable differences between the methods. It was found that TDA is mostly disturbed by the presence of low-molecular-mass material, whereas DLS is more sensitive to the polydispersity of the polymer. With broad samples, DLS gives the Z-average diffusion coefficient. SEC can be used to obtain a diffusion coefficient distribution as well as an average diffusion coefficient of a polydisperse sample. Although, the same was expected for HDC, it was found that this method could only be successfully used for polymer samples having low polydispersities. Deviations between SEC, HDC, and TDA found for narrow samples were not related to the chemical composition of the samples.

Pore flow effects in electrically driven size-exclusion chromatography

The applicability of electrically driven (ED) size-exclusion chromatography for the separation of synthetic polymers was investigated in capillary columns packed with 5 μm particles with different pore sizes using dimethylformamide as the mobile phase. It was found that under ED conditions a significant intraparticle pore flow was generated. The relative intraparticle velocity with respect to the average interparticle velocity increased with the pore size and ionic strength. It was also observed that with increasing pore flow the plate height of polymers decreased considerably. On the other hand, the intraparticle velocity impaired the selectivity of the separation. In some cases the loss in selectivity was so large that hardly any separation of polymers was achieved. The effect of pore flow on the retention of polymers in porous particles could be described well with a simple theory.

Electrically driven capillary size exclusion chromatography

SummaryElectrically driven size exclusion chromatography (ED-SEC) of polystyrenes in packed capillaries using dimethylformamide as solvent is demonstrated. The efficiency and retention behaviour of polystyrenes under pressure and electro drive were investigated. Under pressure drive the plate height (H) increases steadily with increasing linear velocity (u) whereas under electro drive the H-u curves largely coincide and are very flat. At higher velocities the plate heights are about 50% smaller with electro drive than with pressure drive. Calculations show that with increasing ionic strength, the flow through the particles may increase causing a clear diminishing of the elution window.

Determination of the compositional heterogeneity of polydisperse polymer samples by the coupling of size-exclusion chromatography and thermal field-flow fractionation

Abstract An off-line coupling of size-exclusion chromatography (SEC) and thermal field-flow fractionation (ThFFF) was used successfully to cross-fractionate copolymers and polymer blends. Various fractions of different molecular mass were obtained from polydisperse polymer samples by SEC. Because the molecular diffusion coefficients were known, the ThFFF analysis of these fractions yielded directly thermal diffusion coefficients. As thermal diffusion is strongly affected by the chemical nature of the polymer, the chemical composition of polymer samples can be studied as function of the molecular mass with this technique. This is illustrated with the SEC-ThFFF cross-fractionation of a polystyrene sample blended with a polybutadiene and a polytetrahydrofuran standard, and of butadiene- and styrene-methylmethacrylate copolymers.

Influence of the carrier composition on thermal field-flow fractionation for the characterisation of sub-micron polystyrene latex particles

A study on the influence of the carrier composition in a ThFFF system on the retention and thermal diffusion of sub-micron polystyrene latex particles has been carried out. Various factors that may influence retention were studied. These include: the type of electrolyte and surfactant, their respective concentrations, and the addition of an organic modifier. Particle retention is highly sensitive to small changes in the carrier composition. It is demonstrated that under the conditions applied, secondary effects, such as particle-wall and particle-particle interactions, are negligible. Addition of surfactants is required to minimise particle-wall interactions. Generally, retention increases at higher electrolyte concentration. Furthermore, the addition of acetonitrile (ACN) to an aqueous carrier leads also to an increased retention. The type of surfactant as well as its concentration is of influence on the retention time. The three surfactants that were studied, i.e., sodium dodecyl sulfate, Brij 35 and cetyltrimethylammonium bromide, showed significant differences in particle retention behaviour. The observed differences in retention in the carriers can be attributed to actual changes in thermal diffusion. D(T) appears to be mainly determined by the interaction between the particles surface and the carrier liquid, and is therefore highly sensitive to changes in the chemical composition of the particle surface and the carrier. Strong differences in size selectivity were found for different carrier compositions. This allows a relatively easy optimisation of the separation. On the other hand, it complicates the size and composition analysis of particles.

Characterization of thermal diffusion of polystyrene in binary mixtures of THF/dioxane and THF/cyclohexane

The thermal diffusion coefficient (Dτ) was determined for three polystyrene standards of different molecular masses in binary mixtures of tetrahydrofuran/dioxane and tetrahydrofuran/cyclohexane of various compositions. The Dτ values were obtained by combining retention data from thermal field-flow fractionation measurements with diffusion data from dynamic light scattering experiments. In agreement with earlier work of Schimpf and Giddings, the thermal diffusion coefficient was found to be virtually independent of the molecular mass of the polymers. In the binary mixtures of tetrahydrofuran and dioxane, both good solvents for polystyrene, the Dτ value was approximately equal to the average of the Dτ values in the pure solvents, weighted according to the mole fractions of the solvents in the mixture. However, for polystyrene in binary mixtures of tetrahydrofuran and cyclohexane this linear behavior of the thermal diffusion phenomenon was not observed. The addition of cyclohexane to tetrahydrofuran has initially only a minor effect on the molecular and thermal diffusion coefficients of the polystyrene standards. Because cyclohexane is a theta solvent for polystyrene, the preferential solvation of polystyrene by tetrahydrofuran could be an explanation for these results.