Volume Of Stationary Phase Research Articles

Multiple-open-tubular column enabling transverse diffusion. Part 3: Simulation of solute dispersion along a real three dimensional-printed column with quadratic channels

Multiple-open-tubular columns enabling transverse diffusion (MOTTD) consist of straight and parallel flow-through channels separated by a mesoporous stationary phase. In Part 1, a stochastic model of band broadening along MOTTD columns accounting for longitudinal diffusion, trans-channel velocity bias, and mass transfer resistance in the stationary phase was derived to demonstrate the intrinsic advantage of MOTTD columns over classical particulate columns. In Part 2, the model was refined for the critical contribution of the channel-to-channel polydispersity and applied to address the best trade-off between analysis speed and performance. In this Part 3, a MOTTD column with a square array of quadratic channels is fabricated by 3D-printing (combining polymer stereolithography with photolithography using photomasks) to deliver unprecedently small apparent channel diameters of 117.6 ± 5.0 μm. The colors in the microscopy photographs of the actual 3D-printed channels are binarized to delimitate the mobile phase volume from the stationary phase volume. The same numerical simulations as those in Part 2 are then performed for two MOTTD columns (external porosity ϵe=31.7%, same apparent channel diameter 117.6 μm): one containing 16 virtual perfect quadratic channels and the other 16 real 3D-printed channels. The reduced velocities (or Peclet numbers) are varied over a wide range from 0.2 to 5000 and the zone retention factors were fixed at k1=1.04, 5, and 25.The results demonstrate that smoothing the edges of the targeted quadratic channels by the 3D-printed technique is advantageous in terms of solute dispersion. It outperforms the negative effect of the channel-to-channel polydispersity which is mitigated by transverse diffusion of the analyte in the stationary phase. For Peclet numbers larger than 50, the HETP of the 3D-printed MOTTD column is found 7%, 15%, and 16% smaller than that of the MOTTD column consisting of a square array of perfect quadratic channels. This confirms the known effect of channel geometry on solute dispersion in microfluidic systems. Flow channels in fabricated MOTTD columns are preferred to be circular so that the distribution of transverse diffusion lengths across the open channels remains as tight as possible. Finally, the general theory of nonuniform columns of Giddings reveals that the polydispersity of the cross-sectional area (RSD 8.4%) along a single 3D-printed channel has no negative impact on solute dispersion in MOTTD columns. Overall, MOTTD columns could become a serious alternative technology to conventional particulate columns. This implies a novel fabrication process that delivers circular channel diameters smaller than 10 μm, cross-sectional area polydispersity no larger than 25%, external porosities in a range from 15% (high speed separations) to 75% (high performance separations), and conventional mesoporous silica as the stationary phase. It adresses new synthesis routes based on either organic fibers or tubular micelle templating agents in suspension with silica gel solutions.

Rare earth element separations by high-speed counter-current chromatography

Following the initial development of High-Speed Counter-Current Chromatography (HSCCC) in the 1960s, several studies have explored its applicability in the separation of rare earth elements (REEs). More recently, however, HSCCC publications have transitioned towards the separation of natural products or pharmaceuticals, leaving the application for REEs largely unexplored from a practical standpoint. Herein, we expand upon prior work in this field by evaluating the suitability of HSCCC to separation of a subset of non-radioactive REEs (Nd, Sm, Eu, Tb, and Y) at 10−4 mol levels using di-(2-ethylhexyl)phosphoric acid (HDEHP) in n-heptane as the stationary phase and hydrochloric acid as the mobile phase. First, the effect of flow rate on the stationary phase volume retention ratio and resolution of Nd/Sm/Eu subgroup was evaluated followed by optimization of step-gradient elution profiles resulting in additional recovery of Tb and Y within a seven-hour window. The five REEs were separated at the baseline resolution level or above. Elution profiles obtained from multiple runs across two independently operated columns and across independent runs were cross analyzed. Reproducibility in elution profiles point to future applications in radioelement separation chemistry, where both chemical and radiochemical purity are of importance.

A New Definition of the Stationary Phase Volume in Mixed-Mode Chromatographic Columns in Hydrophilic Liquid Chromatography.

Polar columns used in the HILIC (Hydrophilic Interaction Liquid Chromatography) systems take up water from the mixed aqueous–organic mobile phases in excess of the water concentration in the bulk mobile phase. The adsorbed water forms a diffuse layer, which becomes a part of the HILIC stationary phase and plays dominant role in the retention of polar compounds. It is difficult to fix the exact boundary between the diffuse stationary and the bulk mobile phase, hence determining the column hold-up volume is subject to errors. Adopting a convention that presumes that the volume of the adsorbed water can be understood as the column stationary phase volume enables unambiguous determination of the volumes of the stationary and of the mobile phases in the column, which is necessary for obtaining thermodynamically correct chromatographic data in HILIC systems. The volume of the aqueous stationary phase, Vex, can be determined experimentally by frontal analysis combined with Karl Fischer titration method, yielding isotherms of water adsorbed on polar columns, which allow direct prediction of the effects of the composition of aqueous–organic mobile phase on the retention in HILIC systems, and more accurate determination of phase volumes in columns and consistent retention data for any mobile phase composition. The n phase volume ratios of 18 columns calculated according to the new phase convention strongly depend on the type of the polar column. Zwitterionic and TSK gel amide and amine columns show especially strong water adsorption.

HILIC characterization: Estimation of phase volumes and composition for a zwitterionic column

A methodology for the estimation of the different phase volumes in HILIC is presented. For a ZIC-HILIC column the mobile phase volume (hold-up volume) is determined in several acetonitrile- and methanol-water compositions by a Linear Free Energy Relationships (LFER) homologous series approach involving n-alkyl-benzenes, -phenones, and -ketones. We demonstrate that the column works as a HILIC column when the mobile phase contains high and medium proportions of methanol or acetonitrile. However, for acetonitrile contents below 20%, or 40% for methanol, same column works in RPLC. In between, a mixed HILIC-RPLC behavior is observed, and solutes of low molecular volume are retained as in HILIC mode, but the largest ones show RPLC retention. From the homologous series retention data and pycnometric measurements involving the pure organic solvents and their mixtures with water, the mean solvent composition of the water-rich transition layers between column functionalization and the bulk mobile phase, which act as stationary phase, is estimated. Finally, the phase ratio between stationary and mobile phases is also estimated for each eluent composition, allowing the calculation of the corresponding stationary phase volumes. All volumes are strongly dependent on the water content in the eluent, especially when acetonitrile is selected as mobile phase constituent. In HILIC mode, when the water content in the hydroorganic mobile phase increases, the volumes of mobile phase decrease, but the volumes of stationary phase (mainly the water layer adsorbed onto the bonded-phase and the water-enriched interface) increase. However, at high water concentrations, where the column works in RPLC mode, the mobile phase volume increases and the stationary phase (which is now the bonded zwitterion) volume decreases when increasing the water percentage in the mobile phase.

Microgeometry-independent equation for measuring infinite dilution activity coefficients using gas-liquid chromatography with static-wall-coated open-tubular columns

Gas-liquid chromatography is an effective method to determine infinite dilution activity coefficients (γ∞). Wall-coated open-tubular (WCOT) column which offers more advantages over packed column should be a preferable column type; however, the small carrier gas flow rate and stationary phase amount in WCOT columns limit its application in the determination of γ∞. Mathematical strategy made some progress to avoid the quantification problem in the determination of γ∞ by static-wall-coated open-tubular (SWCOT) columns. However, the previously reported strategy was based on the assumption that SWCOT column was geometrically an ideal hollow cylinder, which indeed deviates from the reality. In this study, without that assumption, we derived a new microgeometry-independent equation by using the relationship between the hold-up volume (VM) and the volume of stationary phase (VL), and used it to measure the γ∞ of various organic solutes in two ionic liquids (ILs) 1‑butyl‑3-methylimidazolium dicyanamide and 1,3-dibutyronitrile-imidazolium bis((trifluoromethyl)sulfonyl)imide, both of which contain double cyano groups in the anion or cation. Phase loading study was adopted to eliminate the influence of interfacial adsorption to partition. The infinite dilution partial molar excess enthalpy, selectivity and capacity were directly calculated from the experimental γ∞ values, and the linear solvation energy relationship (LSER) model was used to characterize the specific properties of both ILs. This new established equation will promote the application of SWCOT columns in thermodynamic measurement and benefit the fast screening of novel solvents for chemical separation processes.

A direct comparison between membrane adsorber and packed column chromatography performance

The purpose of this work was to compare side by side the performance of packed bed and membrane chromatography adsorption processes for protein purification. The comparison was performed using anion exchange media with the same ligand immobilized on the adsorbing surface, namely the strong Q quaternary ammonium group, R-CH2-N+-(CH3)3, and bovine serum albumin (BSA) as a model protein. In addition, the stationary phase volume was held constant for each geometry (3 mL) and runs were executed using the same mobile phase superficial velocity. As expected, the packed bed column showed higher equilibrium binding of BSA at 66.9 mg/mL versus 43.04 mg/mL for the membrane adsorber. Dynamic binding capacities were also higher in the packed bed; for example, at 97.5 cm/h, a capacity of 62.8 mg/mL was measured for the packed bed versus 20.7 mg/mL for the membrane adsorber. The higher equilibrium and dynamic capacities of the packed bed are likely due to the higher surface area per unit volume of the resin. However, the maximum productivity for the membrane adsorber was 111 mg/(mL h), a value that was 3.3 times higher than the one of the packed column. The bed utilization - defined as the ratio of the dynamic binding capacity at 10% breakthrough to the saturation binding capacity - was also higher for the packed column at long residence times and lower at short residence times confirming the better performance of membrane chromatography at high flow rates.

Simultaneous determination of eight phthalate ester plasticizers in sports beverages using supercritical fluid chromatography

Phthalate esters (PAEs) are a group of serious environmental pollutants that are carcinogenic or tumorigenic to humans. In this work, a green, rapid, and effective analytical method based on supercritical fluid chromatography (SFC) has been proposed for the simultaneous determination of the eight PAE plasticizers in sports beverage samples. They are dimethyl phthalate (DMP), diethyl phthalate (DEP), dipropyl phthalate (DPRP), dibutyl phthalate (DBP), dipentyl phthalate (DPP), benzyl butyl phthalate (BBP), di-2-ethylhexyl phthalate (DEHP), and di-n-octyl phthalate (DNOP). Liquid phase extraction (LPE) was employed to extract the PAE plasticizers before testing using SFC with ultraviolet (UV) detection. SFC parameters, including stationary phase screening, modifier composition and volume percentage, column temperature, flow rate, and backpressure, were optimized. Under the optimum conditions, all the eight PAE plasticizers could be determined simultaneously by SFC within 6 min at 225 nm. The optimized eluent was supercritical CO2 with 3% (v/v) methanol. The performance of the developed method was also evaluated. The eight PAE plasticizers exhibited satisfactory linearities with correlation coefficients (r) of 0.9991-0.9997 in the range of 0.05-25 mg/L. The limits of detection ranged from 7.5 to 15 μg/L (S/N=3, n=3). Recoveries of all the PAE plasticizers for the spiked samples ranged from 91.7%-100.2% with relative standard deviations of no more than 6.5% (n=3). This method is green, time-saving, simple, selective, robust, and convenient for the analysis of the eight PAE plasticizers in real sports beverage samples.

Desarrollo y validación de un método ambientalmente amigable para la determinación de carbofurano en suelos

<p>En este trabajo se desarrolló y validó un método analítico ambientalmente amigable para la extracción y cuantificación del N-metilcarbamato carbofurano, en muestras de suelos cultivados con papa criolla. La técnica usada para la extracción fue dispersión de matriz en fase sólida (mspd). La cuantificación de los residuos de carbofurano se llevó a cabo por cromatografía de gases con detector de microcaptura de electrones (μ-ecd), con el método del estándar externo. Este método fue comparado con la técnica de extracción Soxhlet para muestras contaminadas con Furadan® 3SC. Se evaluaron diferentes parámetros que afectan el proceso de extracción mspd, como la fase estacionaria, el solvente y el volumen de elución en muestras de suelos dopadas con Furadan® 3SC, con una concentración final de 50 mg/kg de ingrediente activo (carbofurano); se tuvo como variable de respuesta el porcentaje de recuperación. Se encontró que las mejores condiciones de extracción de carbofurano para estas matrices se dieron con 0,2 g de muestra macerados con 0,8 g de sílica gel como adsorbente en una relación 1:4 muestra/adsorbente. Las muestras fueron extraídas en un cartucho con 5 ml de hexano-acetona en relación 8:2 como solvente de elución. La metodología propuesta permitió determinar niveles de trazas de carbofurano en muestras de suelo con porcentajes de recuperación del 96,58 %.</p>

Comparison of the peak resolution and the stationary phase retention between the satellite and the planetary motions using the coil satellite centrifuge with counter-current chromatographic separation of 4-methylumbelliferyl sugar derivatives

Coil satellite centrifuge (CSC) produces the complex satellite motion consisting of the triplicate rotation of the coiled column around three axes including the sun axis (the angular velocity, ω1), the planet axis (ω2) and the satellite axis (the central axis of the column) (ω3) according to the following formula: ω1=ω2+ω3. Improved peak resolution in the separation of 4-methylumbelliferyl sugar derivatives was achieved using the conventional multilayer coiled columns with ethyl acetate/1-butanol/water (3: 2: 5, v/v) for the lower mobile phase at the combination of the rotation speeds (ω1, ω2, ω3)=(300, 150, 150rpm), and (1:4:5, v/v) for the upper mobile phase at (300:100:200rpm).The effect of the satellite motion on the peak resolution and the stationary phase retention was evaluated by each CSC separation with the different rotation speeds of ω2 and ω3 under the constant revolution speed at ω1=300rpm. With the lower mobile phase, almost constant peak resolution and stationary phase retention were yielded regardless of the change of ω2 and ω3, while with the upper mobile phase these two values were sensitively varied according to the different combination of ω2 and ω3. For example, when ω2=147 or 200rpm is used, no stationary phase was retained in the coiled column while ω2=150rpm could retain enough volume of stationary phase for separation. On the other hand, the combined rotation speeds at (ω1, ω2, ω3)=(300, 300, 0rpm) or (300, 0, 300rpm) produced insufficient peak resolution regardless of the choice of the mobile phase apparently due to the lack of rotation speed except at (300, 0, 300rpm) with the upper mobile phase. At lower rotation speed of ω1=300rpm, better peak resolution and stationary phase retention were obtained by the satellite motion (ω3) than by the planetary motion (ω2), or ω3>ω2.The effect of the hydrophobicity of the two-phase solvent systems on the stationary phase retention was further examined using the n-hexane/ethyl acetate/1-butanol/methanol/water system at different volume ratios. In the satellite motion at (ω1, ω2, ω3)=(300, 150, 150rpm), almost constant stationary phase retention was obtained with the lower mobile phase regardless of the hydrophobicity of the solvent system whereas the stationary phase retention varied according to the volume ratio of the two-phase solvent system for the upper mobile phase. However, stable stationary phase retention was observed with either phase used as the mobile phase.In order to analyze the acceleration acting on the coiled column, an acceleration sensor was set on the column holder by displacing the multilayer column. The combination of the rotation speeds at (300, 100, 200rpm) showed double loops in the acceleration track, whereas (300, 150, 150rpm) showed a single loop, and all other combinations showed, complex tracks.The overall results indicate that the satellite motion is seriously affected by the combination of rotation speeds and the hydrophobicity of the two-phase solvent system when the upper phase was used as the mobile phase for separation.

Analytical approach to determining human biogenic amines and their metabolites using eVol microextraction in packed syringe coupled to liquid chromatography mass spectrometry method with hydrophilic interaction chromatography column

Analysis of biogenic amines (BAs) in different human samples provides insight into the mechanisms of various biological processes, including pathological conditions, and thus may be very important in diagnosing and monitoring several neurological disorders and cancerous tumors. In this work, we developed a simple and fast procedure using a digitally controlled microextraction in packed syringe (MEPS) coupled to liquid chromatography mass spectrometry (LC–MS) method for simultaneous determination of biogenic amines, their precursors and metabolites in human plasma and urine samples. The separation of 12 low molecular weight and hydrophilic molecules with a wide range of polarities was achieved with hydrophilic interaction chromatography (HILIC) column without derivatization step in 12min. MEPS was implemented using the APS sorbent in semi-automated analytical syringe (eVol®) and small volume of urine and plasma samples, 50µL and 100μL, respectively. We evaluated important parameters influencing MEPS efficiency, including stationary phase selection, sample pH and volume, number of extraction cycles, and washing and elution volumes. In optimized MEPS conditions, the analytes were eluted by 3×50μL of methanol with 0.1% formic acid. The chromatographic separation of analytes was performed on XBridge Amide™ BEH analytical column (3.0mm×100mm, 3.5µm) using gradient elution with mobile phase consisting of phase A: 10mM ammonium formate buffer in water pH 3.0 and phase B: 10mM ammonium formate buffer in acetonitrile pH 3.0. The LC–HILIC–MS method was validated and, in optimum conditions, presented good linearity in concentration range within 10–2000ng/mL for all the analytes with a determination coefficient (r2) higher than 0.999 for plasma and urine samples. Method recovery ranged within 87.6–104.3% for plasma samples and 84.2–98.6% for urine samples. The developed method utilizing polar APS sorbent along with polar HILIC column was applied for simultaneous bioanalysis of trace amounts of polar endogenous biogenic amines in real human urine and plasma samples.

Hydrophilic interaction chromatography: A promising alternative to reversed-phase liquid chromatography systems for the purification of small protonated bases.

The overloaded band profiles of the protonated species of propranolol and amitriptyline were recorded under acidic conditions on four classes of stationary phases including a conventional silica/organic hybrid material in reversed-phase liquid chromatography mode (BEH-C18), an electrostatic repulsion reversed-phase liquid chromatography C18 column (BEH-C18+), a poly(styrene-divinylbenzene) monolithic column, and a hydrophilic interaction chromatography stationary phase (underivatized BEH). The same amounts of protonated bases per unit volume of stationary phase were injected in each column (16, 47, and 141 μg/cm(3)). The performance of the propranolol/amitriptyline purification was assessed on the basis of the asymmetry of the recorded band profiles and on the selectivity factor achieved. The results show that the separation performed under reversed-phase liquid chromatography like conditions (with BEH-C18, BEH-C18+, and polymer monolith materials) provide the largest selectivity factors due to the difference in the hydrophobic character of the two compounds. However, they also provide the most distorted overloaded band profiles due to a too small loading capacity. Remarkably, symmetric band profiles were observed with the hydrophilic interaction chromatography column. The larger loading capacity of the hydrophilic interaction chromatography column is due to the accumulation of the protonated bases into the diffuse water layer formed at the surface of the polar adsorbent. This work encourages purifying ionizable compounds on hydrophilic interaction chromatography columns rather than on reversed-phase liquid chromatography columns.

Coating properties of a novel water stationary phase in capillary supercritical fluid chromatography.

The coating properties of a novel water stationary phase used in capillary supercritical fluid chromatography were investigated. The findings confirm that increasing the length or internal diameter of the type 316 stainless-steel column used provides a linear increase in the volume of stationary phase present. Under normal operating conditions, results indicate that about 4.9 ± 0.5 μL/m of water phase is deposited uniformly inside of a typical 250 μm internal diameter 316 stainless-steel column, which translates to an area coverage of about 6.3 ± 0.5 nL/mm(2) regardless of dimension. Efforts to increase the stationary phase volume present showed that etching the stainless-steel capillary wall using hydrofluoric acid was very effective for this. For instance, after five etching cycles, this volume doubled inside of both the type 304 and the type 316 stainless-steel columns examined. This in turn doubled analyte retention, while maintaining good peak shape and column efficiency. Overall, 316 stainless-steel columns were more resistant to etching than 304 stainless-steel columns. Results indicate that this approach could be useful to employ as a means of controlling the volume of water stationary phase that can be established inside of the stainless-steel columns used with this supercritical fluid chromatography technique.

Methodology for optimally sized centrifugal partition chromatography columns

Centrifugal Partition Chromatography (CPC) is a separation process based on the partitioning of solutes between two partially miscible liquid phases. There is no solid support for the stationary phase. The centrifugal acceleration is responsible for both stationary phase retention and mobile phase dispersion. CPC is thus a process based on liquid–liquid mass transfer. The separation efficiency is mainly influenced by the hydrodynamics of the phases in each cell of the column. Thanks to a visualization system, called “Visual CPC”, it was observed that the mobile phase can flow through the stationary phase as a sheet, or a spray. Hydrodynamics, which directly governs the instrument efficiency, is directly affected during scale changes, and non-linear phenomena prevent the successful achievement of mastered geometrical scale changes. In this work, a methodology for CPC column sizing is proposed, based on the characterization of the efficiency of advanced cell shapes, taking into account the hydrodynamics. Knowledge about relationship between stationary phase volume, cell efficiency and separation resolution in CPC allowed calculating the optimum cell number for laboratory and industrial scale CPC application. The methodology is highlighted with results on five different geometries from 25 to 5000mL, for two applications: the separation of alkylbenzene by partitioning with heptane/methanol/water biphasic system; and the separation of peptides by partitioning with n-butanol/acetic acid/water (4/1/5) biphasic system. With this approach, it is possible to predict the optimal CPC column length leading to highest productivity.

Three column intermittent simulated moving bed chromatography: 1. Process description and comparative assessment

The three column intermittent simulated moving bed (3C-ISMB) process is a new type of multi-column chromatographic process for binary separations and can be regarded as a modification of the I-SMB process commercialized by Nippon Rensui Corporation. In contrast to conventional I-SMB, this enables the use of only three instead of four columns without compromising product purity and throughput. The novel mode of operation is characterized by intermittent feeding and product withdrawal as well as by partial recycling of the weakly retained component from section III to section I.Due to the smaller number of columns with respect to conventional I-SMB, higher internal flow rates can be applied without violating pressure drop constraints. Therefore, the application of 3C-ISMB allows for a higher throughput whilst using a smaller number of columns. As a result, we expect that the productivity given in terms of throughput per unit time and unit volume of stationary phase can be significantly increased.In this contribution, we describe the new process concept in detail and analyze its cyclic steady state behavior through an extensive simulation study. The latter shows that 3C-ISMB can be easily designed by Triangle Theory even under highly non-linear conditions. The simple process design is an important advantage to other advanced SMB-like processes. Moreover, the simulation study demonstrates the superior performance of 3C-ISMB, namely productivity increases by roughly 60% with respect to conventional I-SMB without significantly sacrificing solvent consumption.

Thermodynamic studies of a zwitterionic stationary phase in hydrophilic interaction liquid chromatography

A zwitterionic 3-P,P-diphenylphosphonium-propylsulfate stationary phase called HZI was synthesized for hydrophilic interaction liquid chromatography (HILIC). A set of 15 solutes including apolar aromatic compounds, positively charged β-blockers, polar nucleic acid bases and negatively charged compounds was used to prepare van’t Hoff plots between 10 and 70°C with HILIC mobile phases containing between 10 and 20% v/v 20mM or 50mM ammonium acetate buffer (pH 4.1) in acetonitrile. The unique Π–Π interaction capability of the HZI phase is evidenced by the low but significant retention of the apolar aromatic compounds in the HILIC mode as well as by the strong retention of the aromatic containing β-blockers. The linear van’t Hoff plots gave the ΔH° and ΔS° thermodynamic changes of the solute transfer from the mobile to the stationary phase. Plotting the ΔH° values versus the corresponding ΔS° ones produced a line excluding the three apolar aromatic solutes and a negatively charged compound. The excluded compounds corresponded to curved van’t Hoff plots. If the non-linearity of van’t Hoff plots is classically explained by changes in solute transfer interactions, we showed that it could also be explained by a temperature induced change in the hydrated stationary phase volume seen by the solutes.

Overload behavior and apparent efficiencies in chromatography

Over the last 50 years, many analysts have attempted to analyze column overloading by measuring the drop of the column HETP with increasing sample size and to predict elution band profiles from overloaded columns by combining the influences of the thermodynamic overloading and of axial dispersion, using a simplistic perturbation model. This approach violates the principle of mass conservation. The results of the predictions do not agree with those of exact calculations made with the equilibrium-dispersive (ED) model of chromatography for constant axial dispersion. The plots of the reduced apparent column efficiency N/Nkin, versus the injected concentration, logC0, or the injected mass, logm0, may provide useful information only regarding the onset of overloading of any particular column but they are meaningless to compare the overloading behavior of columns packed with different packing materials, unless the columns used satisfy impractical requirements (same efficiency, same retention factors, and sample sizes used proportional to the volume of stationary phase in the column).

Modelling of elution–extrusion counter-current chromatography using perfect replacement approach

Basing on the perfect replacement approach the equilibrium cell model is developed to describe the separation process in elution–extrusion counter-current chromatography (EECCC). As is known, EECCC consists of three steps: classical elution, sweeping elution, and extrusion. The perfect replacement approach means that during sweeping elution step, the mobile phase contained in the column moves and interacts with the “old” stationary phase in the same mode as during the classical elution step; the “new” and “old” stationary phases do not mix; and after the contacting with the mobile phase the concentration of solutes in the “old” stationary phase remains constant and this stationary phase volume is pushed ahead to the exit of the column. Equations are presented allowing the simulation of the chromatogram of solutes eluted from the column with the mobile phase during the elution period and the chromatogram of solutes pushed out of the column with the stationary phase during the extrusion period of EECCC. These equations can help to choose the optimal conditions for conducting elution–extrusion counter-current chromatography.

Counter-current chromatography: Simple process and confusing terminology

The origin of counter-current chromatography is briefly stated, followed by a description of the mechanism of elution of solutes, which illustrates the elegance and simplicity of the technique. The CCC retention equation can be mentally derived from three facts; that a substance with a distribution coefficient of 0 elutes at the mobile phase solvent front (one mobile phase volume); and one with a distribution coefficient of 1 elutes at the column volume of mobile phase; and solutes with higher distribution coefficients elute at additional multiples of the stationary phase volume. The pattern corresponds to the classical solute retention equation for chromatography, VR=VM+KCVS, KC not being limited to integer values. This allows the entire pattern of solute retention to be visualized on the chromatogram. The high volume fraction of stationary phase in CCC greatly enhances resolution. A survey of the names, symbols and definitions of several widely used chromatography and liquid–liquid distribution parameters in the IUPAC Gold Book and in a recent summary in LC–GC by Majors and Carr revealed numerous conflicts in both names and definitions. These will retard accurate dissemination of CCC research unless the discordance is resolved. It is proposed that the chromatography retention parameter, KC, be called the distribution coefficient and that a new biphasic distribution parameter, KΔ(A), be defined for CCC and be called the species partition ratio. The definition of VM should be clarified. VH is suggested to represent the holdup volume and VX is suggested for the extra-column volume. HV and HL are suggested to represent the volume and length of a theoretical plate in CCC. Definitions of the phase ratio, β, conflict and should be clarified.

Counter-current chromatography for oil analysis: Retention features and kinetic effects

Application of counter-current chromatography (CCC) for oil analysis has been suggested for the first time. CCC looks very promising as a tool for pre-concentration and isolation of trace elements from oil. Features of stationary phase retention of two-phase liquid systems (oil or oil products–aqueous nitric acid solutions) in CCC have been investigated. The influence of physicochemical properties of crude oil and oil products used as a mobile phase on the volume of stationary phase (acidic aqueous solutions) retained in CCC was studied. Chromatographic behavior of several oil samples was studied. It has been shown that physicochemical properties of test oil influence its chromatographic behavior. Optimal values of density and viscosity ( ρ < 0.85 g/cm 3, n < 7 cSt) of crude oil and oil products that could be analyzed using CCC were estimated. The influence of the column rotational speed and flow rate of mobile phase on the stationary phase retention was also investigated. It is known that kinetic aspects (mass transfer of elements between phases) can play a very important role in selecting an optimal composition of stationary phase for the pre-concentration of elements from oil. The influence of nitric acid concentration in the stationary phase on mass transfer was studied. Kinetic characteristic for trace element recovery has been investigated for the optimization of pre-concentration conditions of trace elements from crude oil and oil products. The extraction recoveries of Zn, Mn, Fe, Ni, V, Cu, Cd, Pb and Ba by CCC in dynamic mode are in the range of 75–95% while they are lower than 35% under batch conditions.

Binary concepts and standardization in counter-current separation technology

Counter-current separation (CS) technology is currently faced with the challenge of being fit for the purpose of omics analysis, which involves highly complex samples and digitized research environments. Resembling a network of binary decisions, CS requires standardization of operation parameters in order to be efficient. While recent CS engineering solutions uniformly involve centrifugal force designs to overcome the limitation of the earth's 1 × g force, factors of instrument design, operation, and graphical representation of the outcome are equally important targets for standardization. For example, chromatograms that emphasize the unique K-based nature of CS, such as reciprocal symmetry (ReS) plots, foster the fundamental understanding of CS operation. Because significant differences exist in underlying mechanism (e.g., stationary phase volume), outcome (e.g., construction of chromatograms), and scale (e.g., factors affecting overall method sensitivity) of solid–liquid vs. liquid–liquid chromatography technologies, standardization will enable the systematic exploration of the differential properties of the two LC technologies, and will be key to making CS fit for the digital omics age.