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.     

Visualization and quantification of the onset and the extent of viscous fingering in micro-pillar array columns

New experimental data of the viscous fingering (VF) process have been generated by studying the VF process in perfectly ordered pillar array columns instead of in the traditionally employed packed bed columns. A detailed quantitative analysis of the contribution of VF to the observed band broadening could be made by following the injected species bands using a fluorescence microscope equipped with a CCD-camera. For a viscosity contrast of 0.16 cP, a plate height increase of about 1 μm can be observed, while for a contrast of respectively 0.5 cP and 1 cP, additional plate height contributions of the order of 5–20 μm were observed. Citing these values is however futile without noting that they also depend extremely strongly on the injection volume of injected sample. It was found that, for a given viscosity contrast of 0.314 cP, the maximal plate height increase varied between 0.5 μm and 18 μm if the injection volume was varied between 3.0 nl and 32.7 nl. These values furthermore also strongly vary with the distance along the column axis.     

Visualization of viscous fingering in high-performance liquid chromatographic columns. Influence of the header design

Using an on-column visualization technique, band profiles of solutes migrating along an HPLC column were studied. The study showed that, under conditions where viscous fingering is prevalent, the design of the inlet header has little influence on the outcome of the viscous fingers. Two types of headers were studied. The first contained a small diameter inlet frit, which localized the majority of the sample in or near the central region of the column. The second header contained a wide frit and produced a more uniform radial distribution of the sample. In both cases, the extent of viscous fingering was essentially the same.     

Viscous fingering in packed chromatographic columns: non-linear dynamics

Viscous fingering (VF) is a hydrodynamic instability that occurs in a chromatographic column when a less viscous fluid displaces another more viscous one. This instability is detrimental to separation techniques as it leads to distorted peaks and peak broadening. Nonlinear interactions between developing fingers lead to complex dynamics investigated in the present study by means of numerical simulations based on a simple model for miscible VF of finite samples. We review the properties of nonlinear VF and discuss the quantitative measures that can be applied both on such numerical as well as on experimental data to gain insight into the influence of the parameters of the problem on the nonlinear properties of the fingers and on the broadening of output peaks.     

Viscous fingering in packed chromatographic columns: linear stability analysis

When a fluid is displaced by a less viscous one in a porous medium, a hydrodynamic instability appears leading to the formation of some kind of fingers of the upstream fluid invading the downstream one, hence the name "viscous fingering" (VF) given to this instability. In a LC column, such an instability is likely to appear at that of the two interfaces between the sample and the eluent which exhibits an unfavorable viscosity contrast. It leads to distorted peak shapes and contributes to peak broadening. This phenomenon has been observed for long in SEC and more recently in RPLC on elution peak shapes as well as with various methods of in-column visualization. A simplistic LC column model is described to explain the origin of the VF instability and its characteristics. The general principles for analyzing hydrodynamic instabilities are described and the results of the linear stability analysis performed by Tan and Homsy [C.T. Tan, G.M. Homsy, Phys. Fluids 29 (1986) 3549 [1]], at the onset of the VF phenomenon for a step interface between two fluids are here applied to typical operating conditions encountered in analytical LC. The most probable growth rate and wavelength (linked to the finger width) of the instability are given in terms of particle size and solute diffusion coefficient, with particular emphasis on the role of the carrier velocity. Previously published qualitative observations about VF in chromatography are examined and interpreted at the light of this theory. The role of the column geometry on the development of the instability, the possible sources of noise or fluctuations triggering the instability, and the various experimental situations in which a significant viscosity contrast is encountered in LC are discussed.     

Prediction of the influence of the heat generated by viscous friction on the efficiency of chromatography columns

The combination of the heat balance in a chromatographic column percolated by a stream of mobile phase and of the model of band migration under linear conditions along such a column permits the calculation of the axial and radial temperature distributions in the column, of the elution band profiles, and of the column efficiency under different sets of experimental conditions. The calculated results are always consistent with the experimental results published by different groups and often in good quantitative agreement. Minor discrepancies arise from difficulties in deriving precise estimates of the heat transfers from the column due to the massive endfittings of the column and to uncontrolled heat transfer from the column tube to ambient air.     

Viscous fingering induced flow instability in multidimensional liquid chromatography

Viscous fingering is a flow instability phenomenon that results in the destabilisation of the interface between two fluids of differing viscosities. The destabilised interface results in a complex mixing of the two fluids in a pattern that resembles fingers. The conditions that enhance this type of flow instability can be found in coupled chromatographic separation systems, even when the solvents used in each of the separation stages have seemingly similar chemical and physical properties (other than viscosity). For example, the viscosities of acetonitrile and methanol are sufficiently different that instability at the interface between these two solvents can be established and viscous fingering results. In coupled chromatographic systems, the volume of solvent transported from one separation dimension to the second often exceeds the injection volume by two or more orders of magnitude. As a consequence, viscous fingering may occur, when otherwise following the injection of normal analytical size injection plugs viscous fingering would not occur. The findings in this study illustrate the onset of viscous fingering in emulated coupled chromatographic systems and show the importance of correct solvent selection for optimum separation performance.     

Supercritical-fluid chromatography: Open columns vs packed columns

Supercritical-fluid chromatography (SFC) may be performed either in open (capillary) columns or in packed columns. Both approaches have been demonstrated numerous times in the literature. In this contribution it will be attempted to discuss some aspects of columns for SFC. Some advantages of both types of columns will be identified. Attention is paid to the stationary phase film thickness, the speed of analysis, and to the maximum number of theoretical plates (effect of column pressure drop). In this brief contribution many questions will be left unanswered and many significant aspects will be left undiscussed, illustrating that much research remains to be done in this area.     

Monolithic columns in high-performance liquid chromatography

Monolithic media have been used for various niche applications in gas or liquid chromatography for a long time. Only recently did they acquire a major importance in high-performance column liquid chromatography (HPLC). The advent of monolithic silica standard- and narrow-bore columns and of several families of polymer-based monolithic columns has considerably changed the HPLC field, particularly in the area of narrow-bore columns. The origin of the concept, the differences between their characteristics and those of traditional packed columns, their advantages and drawbacks, the methods of preparation of monoliths of different forms, and the current status of the field are reviewed. The actual and potential performance of monolithic columns are compared with those of packed columns. Monolithic columns have considerable advantages, which makes them most useful in many applications of liquid chromatography. They are extremely permeable and offer a high efficiency that decreases slowly with increasing flow velocity.     

The dry-packing of columns in liquid chromatography

This contribution comprises two parts, the one covering some variables in the dry-packing of columns for liquid chromatography, the other the use of an automatic dry-packer in liquid chromatography. A high-pressure stainless steel column was dry-packed using a 20-44 μm size, irrregularly shaped, silica gel support. It was found that, when support was added in discrete amounts, lateral tapping produced a column of higher efficiency (HETP = 0.69 mm) than was the case for vertical tapping (HETP = 1.37 mm). Simultaneous vertical and lateral tapping produced an intermediate result. Bulk filling of the column followed by tapping was inferior to adding support in discrete amounts. An apparatus constructed to perform the vertical and lateral tapping operations automatically confirmed the above conclusions. It was concluded that commercially available dry-packing apparatus do not offer the best action for packing an efficient column since they involve predominantly vertical tapping.     

Capillary columns in liquid chromatography: between conventional columns and microchips

Liquid chromatography on columns with small internal diameters has been reviewed as the intermediate technique between conventional liquid chromatography and microchip separations. The development of micro column separations in the early years has been described, starting with the papers of Horváth and co-workers and Ishii and co-workers, continuing into the first part of the eighties, then making a leap in time to recent innovations with small-bore columns. Based on internal diameters a classification of the different analytical HPLC columns has been suggested. The advantages of small-bore columns have been discussed, with particular emphasis on the advantage of coupling to concentration sensitive detectors when the sample amount is limited. Open tubular columns are treated as a part of the historic background. The recent developments include a brief look into the current status of monolithic columns, the use of packed nano columns and micro columns with electrospray mass spectrometry, and the potential of two-dimensional comprehensive liquid chromatography. Finally, the coupling of sample preparation to analytical columns and the future applications of the novel technological improvements to the microchip separation methods have been discussed.     

Use of mordenite columns in ion-exclusion chromatography

The separation of nitrite ion by ion-exclusion chromatography has been carried out on a laboratory-made mordenite column. The results show that this cheap solid acid can replace, at least in this analysis, the commercial polysulphonated resins and that the separation between these two anions of environmental concern can be carried out very rapidly on such a column.     

Temperature gradients in HPLC columns due to viscous heat dissipation

Summary Temperature effects in HPLC columns due to viscous heat dissipation are examined. For the case when the thermostatted column wall and mobile phase at the column inlet are at the same temperature an explicit solution of the heat transport equation is given. The predicted temperature profile is parabolic at large distances from the column entrance; the magnitude of the effect is proportional to the square of the mobile phase velocity, and is of the order of a few degrees centigrade. At the upper end of the column a relaxation occurs over a length of a few centimers. Experimental results confirm the validity of the predictions made and indicate that the various assumptions and approximations are justified. Plate height curves obtained with two mobile phases with differing viscosities show a much smaller efficiency for the less viscous mobile phase. The curves show an upward curvature at high reduced velocities. Both phenomena can be related to thermal effects. It is concluded that viscous heat dissipation constitutes an obstacle to obtaining higher speed and efficiency in HPLC by the use of smaller particles. Possible remedies, such as the use of smaller bore columns or special thermostatting devices, look troublesome from the experimental point of view.     

Polymeric columns for liquid chromatography

Polymeric columns are becoming used more frequently in modern liquid chromatography applications as improvements in polymeric packing materials are realized and as more applications are developed for these materials. Modern polymers have overcome earlier problems associated with their use; higher rigidity now allows them to be used at normal eluent flow rates resulting in faster analyses, and improved synthesis techniques have resulted in efficiencies comparable to the best silica materials, sometimes exceeding 100,000 plates/m. In addition, polymers offer distinct advantages over silica packing for particular applications. Because silica packings are readily degraded by aqueous eluents, they are not always suitable for separations involving ionic species. Polymeric columns are particularly useful in determinations of amino acids, peptides, proteins, organic acids, carbohydrates, and inorganic cations and anions. Polymers are also characterized by exceptional lot-to-lot reproducibility. Reproducibility is often problematic with silica packings, particularly the bonded phases.     

Reproducible preparative liquid chromatography columns

The level of reproducibility for sets of preparative liquid chromatography (prep-LC) columns was studied using a self-packing axial compression system. Standard deviations of less than 5% of the mean of the column efficiency and less than 2% of the mean of the retention time are reported for three sets of packed, prep-LC columns.     

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.