In this work a prototype of a continuous 2D-ThFFF instrument was constructed, a preliminary theoretical model of its operation was developed, and the experimental and theoretical results were compared. An analytical ThFFF method was used for the determination of polymer concentrations in each continuously collected fraction and determination of the retention factor . The samples were polystyrene polymer standards with different molar masses and narrow polydispersity. In the continuous 2D-ThFFF, cyclohexane and a binary solvent consisting of cyclohexane and ethylbenzene were used as carriers. Both are suitable for ThFFF experiments, and the instrument was inert to their physico-chemical properties. In the analytical ThFFF, however, tetrahydrofuran was used as the carrier, after first evaporating the collected fractions to dryness and then adding the THF solvent to dissolve the samples. This was done for practical reasons and to decrease the consumption of other carrier solvents.
In the first experiments two polystyrene samples of different molar masses were used for preliminary testing of the performance and experimental conditions of the continuous 2D-ThFFF method. A positive effect of the thermal gradient on the sample deflection was observed, although increased rotation rate resulted in broader deflected sample zones. In further study, a 50% decrease in the channel thickness led to significant reduction in the zone broadening with and without the thermal field. In addition, with decrease of both the radial and angular flow rates the deflection of the sample trajectories increased. In addition to the flow components and channel geometry, the use of a cyclohexane‒ethylbenzene binary carrier and samples of higher molar mass showed enhanced performance in the continuous fractionation. The systematic variation of the experimental parameters allowed determination of the conditions required for the continuous fractionation of polystyrene polymers according to their molar mass. As an example, almost baseline separation was achieved with two polystyrene samples of molar mass 51.000 and 1000.000 g/mol (Fig.
18).
48
Figure 18 Continuous fractionation of a mixture of two polystyrene samples: , PS 51k; ■, PS 1000k. The carrier was a binary solvent of cyclohexane and ethylbenzene and the run conditions were rotation speed 0.024 rpm, radial flow rate 9.6 l/min, channel thickness 125 m, collection time 90 min.
Sample introduction rate was 1.250 l/min (paper III).
A theoretical model was developed and applied for prediction of the sample trajectory and its angular displacement under various experimental conditions. The trends in the deflection angles without and with a thermal gradient (θ0 and θr, respectively) were qualitatively in agreement with predictions of the model, but significant quantitative differences between the experimental results and theoretical predictions were also found.
Some possible reasons for the discrepancies between theory and experiment were discussed. The stop-flow procedure used for sample relaxation in conventional FFF is not possible with the continuous separation system, and the sample is assumed to be relaxed already at the sample inlet point. The instrumental limitations, such as a rather low maximum thermal gradient, a limited number of collection ports, and the noncontinuous fraction collection, restrict the full-scale operation. However, none of the considered complications could provide an unambiguous explanation for the observed discrepancies.
The theoretical model will, however, provide guidelines for future interpretation and optimization of separations by the continuous 2D-ThFFF method. The theoretical model constitutes a first step in the modeling of the system, and there are structural design and construction issues that can be addressed in future instruments.
Since ThFFF separates samples according to their chemical composition, molar mass, and particle size, the applications of continuous 2D-ThFFF in science and industry can be extended from polymers to other macroscopic objects, such as particles and microgels.
Through use of some other fields than thermal force field, the continuous 2D method can be extended to biotechnology and the characterization of new materials. However, in the present form, the preparative separation of the continuous 2D-ThFFF is not very good in practice, because the separation resolution in FFF techniques is generally not very high,
49
and the much narrower molar mass distributions needed in analytical and industrial chemistry cannot easily be achieved by any elution-based separation methods. Therefore, much additional experimental and theoretical work will be needed to discover the benefits and drawbacks associated with the continuous 2D-FFF channel variants.
50
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