A study of the efficiency of monolithic silica gel capillary columns for gas chromatography

The efficiency and dynamic characteristics of seven silica-gel-based monolithic capillary columns were analyzed by separating on them a mixture of five light hydrocarbons. For helium carrier gas flowing at an optimum velocity, the height equivalent to a theoretical plate was found to be 0.15–0.20 mm, values comparable to those typical of packed capillary columns. An analysis of the Van Deemter curves for the columns under study demonstrated that the main contribution to the smearing of the chromatographic zone comes from the diffusional processes in the mobile phase while the mass transfer between the mobile and stationary phases plays only a minor role. At the same time, the parameter A in the Van Deemter equation, which characterizes the degree of column packing uniformity, was found to be negative. This result contradicts the classical theory of chromatography and calls for further studies of monolithic capillary columns.     

Effect of the carrier gas pressure on the dynamic and separation characteristics of divinylbenzene-based monolithic capillary columns for gas chromatography

The efficiency and dynamic characteristics of divinylbenzene-based monolithic capillary columns for gas chromatography were analyzed using a test mixture composed of five light hydrocarbons. The chromatographic properties of these columns were evaluated within the framework of two varieties of the van Deemter equation, the classical one and that proposed by Giddings (with consideration given to the pressure drop across the column). An analysis of the van Deemter curves demonstrated that the main contribution to peak smearing comes from the diffusion processes in the mobile phase. The contribution from the resistance to mass transfer between the mobile and stationary phases is less important. Negative values obtained for A in the van Deemter equation and for C _s in the Giddings model, parameters that characterize the stationary phase structure and mass transfer kinetics in the stationary phase, have no physical meaning, a result calling for further studies of this type of monolithic capillary columns since the classical theory supposed these parameters to be strictly positive. Under optimal conditions, the HETP of the monolithic columns was found to be 3 to 4 times smaller than that typical of open capillary columns of the same diameter.     

The influence of carrier gas pressure on the retention of sorbates on monolithic capillary columns in gas chromatography

The influence of carrier gas pressure on the retention factor k′ of light hydrocarbons C₁–C₄ in a monolithic capillary column based on divinylbenzene was studied. It was shown that, for monolithic columns and nonideal carrier gases, the pressure dependence of lnk′ was nonlinear over a wide pressure range and could be described by the classic Everett equation. It was concluded that the competitive adsorption model failed to describe the experimental data correctly, especially for strongly retained sorbates and/or heavy carrier gases. Original Russian Text ? A.A. Korolev, V.E. Shiryaeva, T.P. Popova, A.V. Kozin, A.A. Kurganov, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 4, pp. 776–783.     

Fast separation of light hydrocarbons by gas chromatography on monolithic capillary columns based on silica gel

Monolithic capillary columns based on silica gel were tested in the course of high-speed gas-chromatographic separations of a five-component mixture of C₁–C₄ hydrocarbons. It was found that short-length monolithic columns could be used because of their high specific efficiency; this allowed us to shorten the column dead time and the duration of analysis. The column performance of about 1000 theoretical plates per second was reached. The test sorbate mixture was completely separated on a 58.5-cm column with an efficiency of about 18 700 theoretical plates in a time shorter than 17 s. It was noted that CO₂ and N₂O should be predominantly used as carrier gases.     

Effect of the nature of the carrier gas on the chromatographic characteristics of monolithic silica capillary columns

Chromatographing a model mixture of hydrocarbons with various carrier gases (helium, hydrogen, nitrogen, carbon dioxide, and nitrous oxide) was used to study the separation ability of monolithic silica capillary columns. It was revealed that the nature of the carrier gas strongly affects the retention time of the sorbates and the height equivalent to a theoretical plate (HETP) of the column, with the values of both these parameters decreasing in the series He > H₂ > N₂ > CO₂ ～ N₂O. This effect was found to be more pronounced for normal hydrocarbons than for their isomers. For chromatographing with CO₂ or N₂O under optimum conditions, the HETP was 25–30 μm, a value indicative of a higher specific efficiency of monolithic capillary columns. Theoretical correlations between the HETP and the properties of the mobile phase were considered. As a result, it was concluded that elevated pressures of the carrier gas, which are required to ensure the optimum operation of monolithic capillary columns, may affect the properties of the chromatographic system.     

Significant viscosity dependent deviations from classical van Deemter theory in liquid chromatography with porous silica monolithic columns

Significant deviations from classical van Deemter behaviour, indicative of turbulent flow liquid chromatography, has been recorded for mobile phases of varying viscosity on porous silica monolithic columns at elevated mobile phase flow rates.     

Preparation of monolithic silica columns for high-performance liquid chromatography

Preparation methods of monolithic silica columns for HPLC including the surface modification were reviewed. Chemical modification methods recently reported to obtain stationary phases for reversed-phase (RP), chiral, ion-exchange, and hydrophilic interaction chromatography (HILIC) separations were discussed. Recent results related to preparation methods of monolithic silica were also covered. The characteristics and properties of silica monoliths and some applications of monolithic silica columns for different analytical and bioanalytical fields will be commented.     

Thermodynamic parameters of the sorption of light hydrocarbons on monolithic capillary columns in gas chromatography

A silica monolithic capillary column was used to measure the differential enthalpy and entropy of sorption of light hydrocarbons eluted with four different carrier gases. It was demonstrated that a linear correlation between the enthalpy and entropy of sorption (compensation effect) is also typical of monolithic capillary columns, with the slope of this correlation being strongly dependent on the nature of the carrier gas. The largest slope was observed for the helium carrier gas, while for carbon dioxide it is close to zero, and, in addition, the entropy of sorption is virtually independent of the number of carbon atoms in the sorbate molecule.     

Applications of monolithic columns in gas chromatography and supercritical fluid chromatography

Porous monoliths are well‐known stationary phases in high‐performance liquid chromatography and capillary electrochromatography. Contrastingly, their use in other types of separation methods such as gas or supercritical fluid chromatography is limited and scarce. In particular, very few studies address the use of monolithic columns in supercritical fluid chromatography. These are limited to silica‐based monoliths and will be covered in this review together with an underlying reason for this trend. The application of monoliths in gas chromatography has received much more attention and is well documented in two reviews by Svec and Kurganov published in 2008 and 2013, respectively. The most recent studies, covered in this review, build on the previous findings and on further understanding of the influence of preparation conditions on porous properties and chromatographic performance of poly(styrene‐co‐divinylbenzene), polymethacrylate, and silica‐based monolithic columns while expanding to polymer‐based monoliths with incorporated metal organic frameworks and to vinylized hybrid silica monoliths. In addition, the potential application of porous layer open tubular monolithic columns in low‐pressure gas chromatography will be addressed.     

Efficiency of monolithic capillary columns in high-pressure gas chromatography

Monolithic sorbents for gas chromatography obtained in quartz capillaries are analyzed by means of kinetic curves (Poppe plots). The values for the time of a theoretical plate and the maximum number of plates are found to be strongly dependendent on the parameters of monolith synthesis, i.e., the relative amount of a monomer in the initial mixture, the temperature, and the polymerization time. Synthesis conditions are established using the kinetic curves, allowing the preparation of sorbents suitable for both performing high-rate analysis and achieving the most effective separation. It was shown that plotting kinetic curves on the basis of van Deemter curve data requires we allow for the compressibility of mobile phase.     

Loading capacities of monolithic capillary columns in gas chromatography

The loading capacities of monolithic capillary columns based on silica gel and divinylbenzene are studied for two carrier gases, CO₂ and N₂. It is shown that the efficiency of the column is more sensitive to the overload of the column than the retention time of the sorbate is, especially for the CO₂ carrier gas. It is established that the loading capacity of a monolithic column based on silica gel decreases significantly in going from N₂ to CO₂. For columns based on divinylbenzene, the loading capacity is found to be virtually the same for both carrier gases. For monolithic columns, the loading capacity per one meter of column length is found to be 10 and more times higher than that for a standard open capillary column.     

Pore structural characterization of monolithic silica columns by inverse size-exclusion chromatography

In this work, a parallel pore model (PPM) and a pore network model (PNM) are developed to provide a state-of-art method for the calculation of several characteristic pore structural parameters from inverse size-exclusion chromatography (ISEC) experiments. The proposed PPM and PNM could be applicable to both monoliths and columns packed with porous particles. The PPM and PNM proposed in this work are able to predict the existence of the second inflection point in the experimental exclusion curve that has been observed for monolithic materials by accounting for volume partitioning of the polymer standards in the macropores of the column. The appearance and prominence of the second inflection point in the exclusion curve is determined to depend strongly on the void fraction of the macropores (flow-through pores), (b) the nominal diameter of the macropores, and (c) the radius of gyration of the largest polymer standard employed in the determination of the experimental ISEC exclusion curve. The conditions that dictate the appearance and prominence of the second inflection point in the exclusion curve are presented. The proposed models are applied to experimentally measured ISEC exclusion curves of six silica monoliths having different macropore and mesopore diameters. The PPM and PNM proposed in this work are able to determine the void fractions of the macropores and silica skeleton, the pore connectivity of the mesopores, as well as the pore number distribution (PND) and pore volume distribution (PVD) of the mesopores. The results indicate that the mesoporous structure of all materials studied is well connected as evidenced by the similarities between the PVDs calculated with the PPM and the PNM, and by the high pore connectivity values obtained from the PNM. Due to the fact that the proposed models can predict the existence of the second inflection point in the exclusion curves, the proposed models could be more applicable than other models for ISEC characterization of chromatographic columns with small diameter macropores (interstitial pores) and/or large macropore (interstitial pore) void fractions. It should be noted that the PNM can always be applied without the use of the PPM, since the PPM is an idealization that considers an infinitely connected porous medium and for materials having a low (<6) pore connectivity the PPM would force the PVD to a lower average diameter and larger distribution width as opposed to properly accounting for the network effects present in the real porous medium.     

Narrow bore silica capillary columns for liquid-modified adsorption gas chromatography

Summary The preparation of narrow bore capillary columns for liquid-modified adsorption chromatography is described, and the advantages of this technique for rapid analysis of complex mixtures are discussed.Several applications to the analysis of complex mixtures are reported, and the possibility of on-column injection is demonstrated.     

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.     

Properties of monolithic silica columns for HPLC.

Monolithic silica columns and their use in high peak-capacity HPLC separations are reviewed. Monolithic silica columns can potentially provide higher overall performance than particle-packed columns based on the variable external porosity and variable through-pore size/skeleton size ratios. The high permeability of monolithic silica columns resulting from the high porosity is shown to be advantageous to generate large numbers of theoretical plates with long capillary columns. High permeability together with the high stability of the network structures of silica allows their use in high-speed separations required for a second-dimension column in two dimensional HPLC. Disadvantages of monolithic silica columns are also described.     

Flow of the carrier gas in open-tubular capillary columns in temperature-programmed gas chromatography

A second-order non-linear partial different equation was derived to describe the dependence of carrier gas pressure in the column on the column distance and the time under temperature programmed conditions. This equation was solved numerically by the modified finite difference method for various column parameters. Constant inlet and outlet pressures were used as boundary conditions. The retention times calculated on assumption of a constant pressure profile along the column. Significant differences between retention times of corresponding solutes calculated by the two methods were found, especially when relatively long columns(L>50m) with small internal diameter(d<0.3mm)and high temperature program rates (r>5°/min)are used.     

Investigation of monolithic capillary columns based on ethylene glycol dimethacrylate in gas chromatography

The influence of the conditions of the synthesis of polymer monoliths based on ethylene glycol dimethacrylate on the properties of monolithic capillary columns for gas chromatography was investigated. It was established that the time of polymerization, the temperature of synthesis, and the composition of the polymerization mixture have a strong effect on the dynamic and chromatographic properties of the columns. It was concluded that monolithic capillary columns synthesized under optimum conditions are characterized by a height equivalent to a theoretical plate of 30–40 μm, which is considerably less than that for traditional hollow capillary columns of the same diameter.