15
shown that basic stationary phases give α (Tb/Tp) <1; neutral phases give α (Tb/Tp)= 1 and acidic phases give α (Tb/Tp)>1. Based on these observations, the materials under
current investigation were classified accordingly, as reported in Table 6. The acidic phases comprise the silica and the amide materials. Amide materials are supposedly neutral in terms of the nature of their functionality [13], but Experimental
HILIC demonstrated a high α (Tb/Tp) value and it could therefore be expected to show an acidic nature in terms of retentions. The zwitterionic material, Syncronis HILIC proved to be neutral. Interestingly, Ikegami and his group found that some zwitterionic phases (i.e. ZIC-HILIC) were acidic, whereas others (i.e. Nucleodur HILIC) were neutral [13]. Irgum et al. confirmed these findings and suggested that ligand loading could be responsible for this dual nature of zwitterionic materials, since ZIC-HILIC columns are polymerically functionalised, whereas Nucleodur HILIC columns are monomerically functionalised and therefore have a lower ligand loading [14]. Syncronis HILIC, being monomerically functionalised and neutral, confirms Irgum’s suggestion.
Chromatographic parameters investigation This study highlighted that in HILIC the level of organic solvent in the mobile phase has a large influence on retention. HILIC behaviour
of decreasing retention with decreasing acetonitrile content was generally observed, indicating that partitioning is the main retention mechanism in these separations, regardless of the radically different chemistries.
The exception to the HILIC behaviour was demonstrated by Hypersil GOLD HILIC (weak anion exchanger), which exhibited secondary interactions when analysing acid compounds.
The presence of secondary interactions was also confirmed by the thermodynamic data, where the high differences in the enthalpy values would indicate the existence of strong specific interactions between the analytes and the functional groups of the stationary phases.
Salt content proved to be fundamental for the full resolution of both acids and bases. Its effect was particularly significant on Syncronis HILIC; the general increase in retention as the salt concentration was increased provided indirect evidence for the hydrophilic partitioning model.
References
[1] P. Hemstrom, K. Irgum, J. Sep. Sci. 29 (2006) 1784.
[2] A.J. Alpert, J. Chromatogr., 499 (1990) 17.
[3] D.V. McCalley, U.D. Neue, J. Chromatogr. A 1192 (2008) 225. [4] D.V. McCalley, J. Chromatogr. A 1217
(2010) 3408.
[5] G. Jin, Z. Guo, F. Zhang, X. Xue, Y. Jin, X. Liang, Talanta 76 (2008) 522.
[6] Y. Guo, S. Gaiki, J. Chromatogr. A 1074 (2005) 71.
[7] R.-I. Chirita, C. West, A.-L. Finaru, C. Elfakir, J. Chromatogr. A 1217 (2010) 3091.
[8] E. Pontén, LCGC Europe October 2012 Supplement.
[9] Y. Guo, S. Gaiki, J. Chromatogr. A, 1218 (2011) 5920.
[10] M. Lämmerhofer, M. Ritcher, J. Wu, R. Nogueira, W. Bicker, W. Lindner, J.
Sep.Sci. 31 (2008) 2572.
[11] B. Chauve, D. Guillarme, P. Cléon, J.-L. Veuthey, J. Sep. Sci. 33 (2010) 752.
[12] G. Marrubini, B.E.C. Mendoza, G. Massolini, J. Sep. Sci. 33 (2010) 803.
[13] Y. Kawachi, T. Ikegami, H. Takubo, Y. Ikegami, M. Miyamoto, N. Tanaka,
J.Chromatogr. A 1218 (2011) 5903.
[14] N.P. Dinh, T. Jonsson, K. Irgum,
J.Chromatogr. A 1218 (2011) 5880.
[15] A.J. Alpert, Anal. Chem. 80 (2008) 62.
[16] K. Kimata, K. Iawguchi, S. Onishi, K. Jinno, R. Eksteen, K. Hosoya, M. Araki, N. Tanaka, Journal of Chromatographic Science, 27 (1989) 721.
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