Artak E. Kostanyan

Multiple dual mode counter-current chromatography with variable duration of alternating phase elution steps.

The multiple dual mode (MDM) counter-current chromatography separation processes consist of a succession of two isocratic counter-current steps and are characterized by the shuttle (forward and back) transport of the sample in chromatographic columns. In this paper, the improved MDM method based on variable duration of alternating phase elution steps has been developed and validated. The MDM separation processes with variable duration of phase elution steps are analyzed. Basing on the cell model, analytical solutions are developed for impulse and non-impulse sample loading at the beginning of the column. Using the analytical solutions, a calculation program is presented to facilitate the simulation of MDM with variable duration of phase elution steps, which can be used to select optimal process conditions for the separation of a given feed mixture. Two options of the MDM separation are analyzed: 1 - with one-step solute elution: the separation is conducted so, that the sample is transferred forward and back with upper and lower phases inside the column until the desired separation of the components is reached, and then each individual component elutes entirely within one step; 2 - with multi-step solute elution, when the fractions of individual components are collected in over several steps. It is demonstrated that proper selection of the duration of individual cycles (phase flow times) can greatly increase the separation efficiency of CCC columns. Experiments were carried out using model mixtures of compounds from the GUESSmix with solvent systems hexane/ethyl acetate/methanol/water. The experimental results are compared to the predictions of the theory. A good agreement between theory and experiment has been demonstrated.

Controlled-cycle counter-current chromatography

The application of the technique of controlled-cycle operation to counter-current chromatography is suggested and evaluated. The operating cycle as applied to a chromatographic column would consist of two individually timed periods: (1) Flow period: a specified volume of mobile phase is fed into one end of the column, conveyed through it, and discharged out of the other end of the column. (2) Delay period: an equilibrium concentration distribution between the phases is allowed to be reached. Mathematical description of controlled-cycle chromatography, using an idealized model, is given and theoretical chromatograms of three-component samples for conventional continuous and controlled-cycle operations are compared. It is shown that for certain conditions the controlled-cycle operation could provide by an order of magnitude greater efficiencies (measured with number of theoretical plates) than conventional operation of a chromatographic column.

On influence of sample loading conditions on peak shape and separation efficiency in preparative isocratic counter-current chromatography.

The cell model theory of CCC is extended to develop analytical solutions for various sample loading conditions (non-impulse loading with separate feed stream and with the mobile phase flow, impulse and non-impulse periodic loading). Equations are presented allowing the simulation of the chromatograms for various sample loading conditions. These equations can help to predict the influence of sample loading conditions on the separation of a given feed mixture and select a suitable compromise between the productivity and the resolution in the isocratic preparative and production CCC separations. It is shown that: when the sample loading time is about 2% of the mean residence time, the effect of loading time can be neglected; the loading time about 20% of the mean residence time seems to be quite acceptable; proper selection of the conditions for the sample loading can allow increasing the productivity by an order of magnitude ensuring, a desirable separation.

Steady-state and non-steady state operation of counter-current chromatography devices☆

Different variants of separation processes based on steady-state (continuous sample loading) and non-steady state (batch) operating modes of CCC columns have been analyzed and compared. The analysis is carried out on the basis of the modified equilibrium cell model, which takes into account both mechanisms of band broadening - interphase mass transfer and axial mixing. A full theoretical treatment of the intermittent counter-current chromatography with short sample loading time is performed. Analytical expressions are presented allowing the simulation of the intermittent counter-current chromatography separations for various experimental conditions. Chromatographic and extraction separations have been compared and advantages and disadvantages of the two methods have been evaluated. Further technical development of the CCC machines to implement counter-current extraction separations is considered.

Controlled-cycle pulsed liquid-liquid chromatography. A modified version of Craig's counter-current distribution.

A new liquid-liquid chromatography technique developed from a combination of controlled-cycle operation and a pulsed-mixing technique is suggested and validated. The controlled-cycle pulsed liquid-liquid chromatography (CPLC) system operates without involving a centrifuge and consists, of a series of multistage units, and a method for imparting pulsation motion to the liquids inside the units (the pulsation cycle). This chromatography technique can be considered as an improved continuous form of Craigs counter-current distribution method, or, alternatively, as a form of droplet chromatography with the cycling mode of operation. The theoretical model has been designed to account for the effects of the basic parameters influencing the CPLC operation. The theoretical models suitability was proved by direct comparison between the experimental and model responses. The CPLC devices containing 1, 2, 4 and 5 multistage columns (each column was divided into 26 stages) have been designed, fabricated and tested; experiments were conducted to test the chromatographic behavior of organic (monocarboxylic) and mineral acids. The mass transfer rate in the stages depends on the nature of both--phase and sample systems: the highest values were achieved in experiments with acetic acid by using the octane/water biphasic system, where an equilibrium concentration distribution between stationary and mobile phases in the stages was attained. The results obtained demonstrated the potential of the new technique for preparative and industrial scale separations.

Multiple dual mode counter-current chromatography with periodic sample injection: Steady-state and non-steady-state operation

The multiple dual mode (MDM) counter-current chromatography (CCC) separation consists of a succession of two isocratic counter-current steps and is carried out in series alternating between normal phase and reversed phase operation. The performance of the MDM technique can be improved by the sample re-injection between each of the dual-mode steps. The objective of this work was to develop analytical expressions to describe the MDM CCC with periodic sample injection, which can be used to simulate these processes and select optimal operating conditions for the separation of a given feed mixture. Two possible methods of the MDM separation with periodic sample injection are considered: 1 - steady-state separation: the duration of the flow periods of the phases is kept constant for all the cycles and the steady-state regime is achieved after a certain number of cycles. 2 - non-steady-state separation with variable duration of phase elution steps. It is shown that proper selection of the duration of phase flow times allows to reach complete separation of solutes in continuous steady-state operation mode even in a low efficiency column. This mode of operation provides both high productivity and high resolution. The non-steady-state method with variable duration of phase elution steps offers several options to split a mixture into groups of compounds and/or concentrate target compounds.

Intermittent counter-current extraction—Equilibrium cell model, scaling and an improved bobbin design

This paper describes an equilibrium cell model for intermittent counter-current extraction that is analytically solved for the first time for continuous sample injection between a pair of columns. The model is compared with practice for injections of a model mixture of compounds on a standard high-performance counter-current chromatography instrument giving good agreement for compound elution order and the times to maximum concentration for the eluted components. An improved design of end fittings for the counter-current chromatography bobbins is described which permits on-column switching of the mobile and stationary phases. This on-column switching successfully eliminates the displaced stationary phase seen in fractions when operating ICcE with standard flying leads and gives a 6% reduction in the retention time of compounds and improved resolution due to the elimination of the time delay required to pump the previous mobile phase from standard flying leads.

Analysis of cyclic liquid chromatography

A new method for the chromatographic separation of liquid mixtures is considered which is distinguished by a discrete (cyclic) supply of the mobile phase to a chromatographic apparatus. Theoretical analysis of the cyclic mode of chromatography is performed taking into account the effect of the rate of interphase mass transfer.

Support-free pulsed liquid-liquid chromatography

A simple technique of support-free liquid-liquid chromatography is suggested that operates without incorporation of a centrifuge. The pulsed chromatography apparatus consists of a stationary coiled tube and a pulsation device to produce reciprocating motion of liquid phases within each individual coil segment. This reciprocating motion generates a centrifugal force field varying in intensity and direction that leads to an improved mixing of the two liquid phases and retains the stationary phase in the coiled tubing. The intensity of the back and forth motion of liquid phases within each coil unit can be varied by varying the frequency and/or the amplitude of the pulsations generated by the pulsation device. As the magnitude of the stationary phase retention is of paramount importance for success of the technique, the retention of the stationary phase in the pulsed coil column was experimentally studied. A few experiments were conducted to test the chromatographic behavior of valeric (n-pentanoic) and caproic (n-hexanoic) acids. The results obtained demonstrate the potential of the new separation method for preparative purposes.

Steady state preparative multiple dual mode counter-current chromatography: Productivity and selectivity. Theory and experimental verification.

In the steady state (SS) multiple dual mode (MDM) counter-current chromatography (CCC), at the beginning of the first step of every cycle the sample dissolved in one of the phases is continuously fed into a CCC device over a constant time, not exceeding the run time of the first step. After a certain number of cycles, the steady state regime is achieved, where concentrations vary over time during each cycle, however, the concentration profiles of solutes eluted with both phases remain constant in all subsequent cycles. The objective of this work was to develop analytical expressions to describe the SS MDM CCC separation processes, which can be helpful to simulate and design these processes and select a suitable compromise between the productivity and the selectivity in the preparative and production CCC separations. Experiments carried out using model mixtures of compounds from the GUESSmix with solvent system hexane/ethyl acetate/methanol/water demonstrated a reasonable agreement between the predictions of the theory and the experimental results.