E. N. Viktorova

Porosity of monolithic macroporous sorbents: Inverse hydrodynamic and size-exclusion chromatography study

Macroporous monolithic sorbents based on divinylbenzene are prepared in capillary tubes with the use of poor porogens of various sizes and molecular masses. The porosity of sorbents is studied via gravimetry and inverse hydrodynamic and size-exclusion chromatography methods. It is shown that all sorbents possess the biporous structure. Macropores with sizes on the order of a micrometer form an interconnected structure through which a solvent flows (flow-through pores). The diameter of impermeable pores is 30–40-molecular-mass solutes solely. As the molecular mass of the poor porogen decreases, the sizes of flow-through pores and the permeability of the monolith decline. At the same time, the fraction of free volume within the monolith accessible for the separation of polymer molecules is the highest for the sorbent prepared with the use of nonanol as a poor porogen.

Effect of the nature of a porogen on the porous structure of monolithic polydivinylbenzene sorbents

In quartz capillaries, macroporous monolithic sorbents based on divinylbenzene are synthesized and their porous structure is studied via inverse size-exclusion-hydrodynamic chromatography. Either a single-component porogen (a higher alcohol) or a two-component porogen (a mixture of a higher alcohol and mesitylene) is used for the synthesis of monoliths. The removal of a solvent good for a polymer from a porogen results in an increase in the size of flow-through channels and a decrease in the free-space volume inside the monolith; this space is used for the separation of polymer sorbates (the working volume of a column). At the same time, the volume of micro- and mesopores in the monolith structure is practically independent of the content of the good solvent in the porogen. It is inferred that the good porogen plays an active role in formation of the macroporous structure of monoliths. The structure of monoliths obtained on the basis of the two-component porogen with the use of nonanol and mesitylene or toluene is optimum for the molecular-mass analysis of polymers.

Conformational transitions in ultrahigh-molecular-mass polymers and their manifestation in chromatography on monolithic columns

For a monolithic capillary column based on polydivinylbenzene, a change in the elution profile of polystyrene standards with variation in the eluent-flow rate is studied. It is shown that, for polymers with molecular masses up to 3 × 106, the elution profile does not depend on the flow rate. For higher molecular mass polymers at low flow rates, there is a single almost Gaussian peak that splits into two peaks that move to different sides from the initial peak with an increase in the flow rate. A peak with a smaller retention time (peak I) rapidly attains the limiting elution time, and later on, its retention is independent of the eluent-flow rate. In contrast, the retention time of the other peak (peak II) continuously increases with an increase in the flow rate of the mobile phase, so that, at high flow rates, this peak is retained for a longer time than the low-molecular-mass marker. The intensity of peak I decreases, while the intensity of peak II increases with an increase in the eluent-flow rate; the ratio of their intensities tends toward a certain limiting value (≤1). The observed profile of elution of ultrahigh-molecular-mass polymers assumes the presence of a dynamic equilibrium similar to that existing in the case of first-order reversible reactions. For the assumed equilibrium, the rate constants of direct and back reactions are determined. It is found that these constants are close to the inverse maximum relaxation time of a polymer molecule. The character of the transformations of the polymer molecule in the chromatographic column is discussed.

Separation of polystyrenes by means of open tubular capillary chromatography.

Simulating polymer separation in flow-through channels of monolithic columns, separation of a mixture of polystyrene standards was investigated using open tubular capillary column of 2 μm inner diameter. High column efficiency was observed for polymers of molar mass ranged from few tens to few hundred kDas. Column efficiency significantly decreased for polymers with molar mass larger than 500 kDa nevertheless preserving value of few tens of thousands theoretical plates. Calibration curve observed for open capillary column is rather steep and can be well described by simple equation without quadratic term. In spite of low selectivity, capillary columns were able in separating wide range of polystyrene standards due to column high efficiency and in such a way supported an idea of hydrodynamic mechanism of polymer separation in flow-through channel of monolithic packings.

Separating ultra-high molecular weight polymers on monolithic capillary columns

The chromatographic behavior of a polystyrene sample with a molecular weight of 20 × 106 Da and narrow molecular weight distribution is studied on a monolithic capillary column based on polydivinylbenzene. It is found that either one symmetrical peak or two peaks that cannot be completely separated over the range of flow rates for the mobile phase can be observed in the chromatogram, depending on the flow rate of the mobile phase. The observed changes in the elution profile of polystyrene standards are attributed to the presence of dynamic equilibrium in the system (reaction chromatography), for which the apparent constants of forward and reverse reactions and the apparent equilibrium constant are obtained. It is noted that the apparent constant of direct reaction increases linearly with a rising eluent flow rate, while saturation and even a drop in the apparent rate constant of the reverse reaction is observed when the flow rate of the eluent rises. It is suggested that the observed changes are related to conformational transitions in the macromolecules of polystyrene, but a detailed explanation of the observed effect requires further investigation.

Monolithic capillary columns based on ethylene glycol dimethacrylate for separation of polymers by molecular mass

Polar monolithic capillary columns for the molecular-mass separation of polystyrene standards are synthesized on the basis of ethylene glycol dimethacrylate. The monolith structure is optimized through variation in the type of porogen because variation in other synthesis parameters (the time and temperature of polymerization, the amount of monomer in the initial feed) is inefficient. The separation of polymers on monolithic sorbents proceeds via a combined exclusion-hydrodynamic mechanism. In terms of a model that allows for contributions of both mechanisms to retention, calibration plots are drawn for the synthesized columns. Monolithic columns with the optimum monolith structure make it possible to use up to 60% of the column free volume for the efficient separation of polymers in a broad molecular-mass range.

Unusually high efficiency of the separation of polymers by hydrodynamic chromatography on hollow capillary columns

A series of polystyrene standards was separated by means of hydrodynamic chromatography on a open capillary with a length of 5 m (active zone is 4.5 m) and an inner diameter of 2 μm. Unusually high efficiency in the separation of polymers with masses of up to 500 kDa was noticed: 1.5 × 106 theoretical plates at a retention time of ∼70 min for a sample of polystyrene with a mass of 390 kDa. It was established that the column efficiency decreases rapidly for polymers with a mass above 500 kDa. A calibration curve for polystyrene standards was constructed. It was shown that, in contrast to the models described in the literature, it is described by a simple two-term expression without quadratic terms. It is concluded that hydrodynamic chromatography is a technique complementary to exclusion chromatography, especially in the separation of high-molecular polymers.

Investigating the structure of a monolithic capillary column by means of hydrodynamic and size-exclusion chromatography

AbstactThe porosity of a monolithic capillary column having a structure optimized for gas chromatographic analysis was investigated by means of hydrodynamic and size-exclusion chromatography. It was found that the total porosity of the column exceeded 90%, and the column had a bimodal pore structure with a micropore diameter of about 1.5 nm and a macropore diameter of about 1.2 μm. The column separated with good selectivity high molecular mass polystyrene standards with molecular masses higher than 100 kDa and low molecular mass solutes of up to 500 Da. The structure of monolithic column has to be optimized for application in hydrodynamic chromatography with an aim to provide selectivity on separation of polymers with molecular mass from 1 to 100 kDa.

Influence of the diameter of monolithic capillary columns on their gas chromatography characteristics

Divinylbenzene polymer monolithic capillary columns were prepared on the basis of capillaries 0.01 to 0.53 mm in diameter. Separation properties of the columns were investigated with the use of a test mixture of light hydrocarbons. The permeability and C parameter in the Van Deemter equation were determined for all the columns. For the most part, the columns had similar characteristics: permeability was in the range (2.2 ± 0.2) × 10−9 cm2, with parameter C in the range (0.7 ± 0.2) × 10−3 s (with n-butane as a sorbate). It was thus established that capillary diameter has only a slight effect on the efficiency of monolithic capillary columns (unlike packed capillary columns and microcolumns, whose properties, according to the literature data, depend strongly on the column diameter). The difference in properties between the narrowest monolithic column (capillary diameter 0.01 mm) and the others is explained by column overloading.

Molecular mass analysis of polymers on monolithic capillary columns based on divinylbenzene

Monolithic capillary columns based on polydivinylbenzene with monolith structure, optimized for the separation of high-molecular compounds, are synthesized. The structure of the monolith is optimized by varying the chemical composition of the porogene. It is shown that monolithic columns with the maximum amount of free volume can be obtained by using a porogene consisting of nonanol and toluene or mesitylene. On the basis of Van Deemter functions measured for the synthesized columns, it was concluded that for sorbates differing strongly in molecular weight and thus their diffusion coefficients, we cannot find a single optimum flow rate, and the optimum separation of polymeric sorbates requires that we reach a certain compromise. It was shown that in these circumstances, monolithic columns with the optimum structure of monolite allow almost completely separation of a mixture of 15 low molecular weight polymeric sorbates into individual components.