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14 August / September 2016


stronger to a diol bonded silica than MeOH adsorbed to a reversed phase C18 column infers that the peak distortion effects due to a strong additive (co-solvent) should be a much more common problem SFC, this is not confirmed yet and these early results must be systematically examined in future studies.


Conclusions


Systematic investigations - using both systematic experiments and numerical modelling – were performed to find the origin of the peak distortions due to the injection solvent effects in SFC using the mixed-stream injection mode. The first set of experiments (see Figure 2a and b, above) proved that the injection volume and the elution strength of the sample solution had a major impact on the shapes of the eluted peaks. Secondly, the sample band elution profile was numerically modelled on a theoretical basis assuming both un-retained and retained co-solvent injection plugs, respectively. These calculations quantitatively confirmed our first set of experiments but also pointed out that there is also an additional significant effect. Viscous fingering experiments were performed using viscosity contrast conditions imitating those encountered in SFC. These experiments clearly proved that viscous fingering effects play a significant role.


A third, but no less serious, source for peak deformation in SFC was just discovered as a logical consequence of the latter recent adsorption studies done of MeOH on common SFC phases [12,13]. We could also suggest a third more serious reason for peak distortions probably more important than the sample solvent – mobile phase viscosity contrast mismatch. This is a special type of deformation of overloaded band profiles that has so far only been demonstrated in LC before and here due to unusually strongly adsorbing additives, not to the modifier in LC. More particular when this strong additive in LC adsorbs stronger than the solute to the stationary phase when combined with such high additive levels in the eluent that its perturbation peak elutes before the solute peak then a very unusual phenomena takes place; namely that the Langmuir shaped (sharp front, diffuse rear) overloaded solute profile turns from Langmuir shape to anti- Langmuir shape (diffuse front, sharp rear). This is exactly what happens for the solute valerophenone; its overloaded band shape converts from a normal ‘Langmuirian’ band shape to an ‘anti-Langmuirian’ shape when changing from neat (pure) carbon dioxide


(CO2 ) to an eluent containing co-solvent


(cf. Figure 4). This new explanation for peak deformation in SFC is very premature and will be more systematically investigated and presented at our SPICA 2016 lecture [16].


Acknowledgement


This work, mostly the product of a collaboration between Emelie Glenne (PhD-student), Jörgen Samuelsson, PhD and Professor Torgny Fornstedt at the Fundamental Separation Science Group (www.separationscience.se) at Karlstad University and Hanna Leek, PhD, Kristina Öhlén, PhD and Director Magnus Klarqvist at AstraZeneca R&D, Molndal, Sweden, was supported by (i) the Swedish Knowledge Foundation for the KK HÖG 2014 project ‘SOMI: Studies of Molecular Interactions for Quality Assurance, Bio-Specific Measurement & Reliable Supercritical Purification’ (grant number 20140179) and by (ii) the Swedish Research Council (VR) in the project ‘Fundamental Studies on Molecular Interactions aimed at Preparative Separations and Biospecific Measurements’ (grant number 2015-04627).


References


[1] G, Guiochon, A. Tarafder, Fundamental challenges and opportunities for preparative supercritical fluid Chromatography, in Journal of Chromatography A 1218 (2011) 1037–1114.


[2] A. Tarafder, P. Vajda, G. Guiochon, Accurate on-line mass flow measurements in supercritical fluid chromatography, in Journal of Chromatography A 1320 (2013) 130–137.


[3] D. Åsberg, M. Enmark, J. Samuelsson, T. Fornstedt, Evaluation of co-solvent fraction, pressure and temperature effects in analytical and preparative supercritical fluid chromatography in Journal of Chromatography A, 1374 (2014) 254–260


[4] M. Enmark, P. Forssén, J. Samuelsson, T. Fornstedt, Determination of adsorption isotherms in supercritical fluid chromatography in Journal of Chromatography A (2013) 1312, 124-133 .


[5] M. Enmark, J. Samuelsson, E. Forss, P. Forssén, T. Fornstedt , Investigation of plateau methods for adsorption isotherm determination in supercritical fluid chromatography in Journal of Chromatography A. 1354 (2014) 129–138.


[6] M. Enmark, D. Åsberg, H. Leek, K. Öhlén, M. Klarqvist, J. Samuelsson, T. Fornstedt, Evaluation of scale-up from analytical to preparative supercritical fluid chromatography in Journal of Chromatography. A 1425 (2015) 280-286.


[7] J. Fairchild, J. Hill, P. Iraneta, Influence of sample influence of sample solvent composition for SFC Separations, in LC GC N. AM. 31:4 (2013) 326–333.


[8] D. Yun, G. Li, A. Rajendran, Peak distortions arising from large- volume injections in supercritical fluid chromatography, in Journal of Chromatography A 1392 (2015) 91–99.


[9] L Miller, I. Sebastian, Evaluation of injection conditions for preparative supercritical fluid chromatography. in Journal of Chromatography A 1250 (2012) 256-263.


[10] M. Enmark, D. Åsberg, A. Shalliker, J. Samuelsson, T. Fornstedt. A closer study of peak distortions in supercritical fluid chromatography as generated by the injection. In Journal of Chromatography A 1400 (2015) 131-139


[11] R.A. Shalliker, H.J. Catchpoole, G.R. Dennis, G. Guiochon, Visualising viscous fin-gering in chromatography columns: high viscosity solute plug, In Journal of Chromatography A 1142 (2007) 48–55.


[12] P. Vajda, G. Guiochon, Surface excess isotherms of organic solvent mixtures ina system made of liquid carbon dioxide and a silicagel surface, , In Journal of Chromatography A 1308 (2013) 139–143.


[13] E. Glenne, K. Öhlén, H. Leek, M. Klarqvist, J. Samuelsson, T, Fornstedt, A closer study of MeOH adsorption and its impact on solute retentions in supercritical fluid chromatography in Journal of Chromatography A, 1442 (2016) 129–139.


[14] T. Fornstedt, G. Guiochon, Comparison between Experimental and Theoretical Profiles of High Concentration Elution Bands and Large System Peaks in Nonlinear Chromatography, in Anal. Chem. 66 (1994) 2686–2693. doi:10.1021/ac00089a015.


[15] T. Fornstedt, P. Forssén, D. Westerlund, System peaks and their impact in liquid chromatography in TrAC Trends in Analytical Chemistry Juli Vol. 81 (2016) 42-50. DOI: 10.1016/j.trac.2016.01.008.


[16] T. Fornstedt et al at 16th International Symposium on Preparative and Industrial Chromatography and Allied Techniques, OC33 - Tuning of Peak Deformations, Due to Co-Solvent Adsorption, in Preparative Supercritical Fluid Chromatography.


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