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This review article is a continuation of the previous reviews on the area of monolithic columns covering the progress made in the field over the last couple of years from the beginning of the second half of 2014 until the end of the first half of 2016. It summarizes and evaluates the evolvement of both polar and nonpolar organic monolithic columns and their use in hydrophilic interaction LC and CEC and reversed-phase chromatography and RP-CEC. The review article discusses the results reported in a total of 62 references.

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RESUMEN

In this last part of the series of investigations involving the postpolymerization modification of a hydroxy monolith (OHM) capillary column, the surface hydroxyl groups were first reacted with glycerol diglycidyl ether in the presence of boron trifluoride (BF3 ) to convert the surface into epoxy activated OHM. The column thus activated with epoxy functions was further functionalized with polar compounds such as glycerol and polyamines to yield polyol OHM and polyamine OHM columns, respectively, for use in hydrophilic interaction electrochromatography. In another postpolymerization functionalization of the epoxy-activated OHM, hydroxypropyl-ß-CD (HP-ß-CD) was grafted onto the epoxy OHM surface to produce HP-ß-CD OHM for enantioseparation by CEC (chiral CEC). The polyol OHM capillary column and polyamine OHM capillary columns exhibited the typical hydrophilic interactions columns vis-à-vis polar solutes such as phenolic compounds and nucleobases. The HP-ß-CD OHM column was able to resolve some racemic compounds as well as positional isomers. All the columns exhibited good reproducibility from run to run, day to day, and column to column.

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In this second part of the series of investigations involving the postpolymerization modification of a hydroxy monolith (OHM) capillary column, the surface hydroxyl groups were reacted with epoxy biphenyl thus yielding the so-called Biphenyl OHM capillary column. The modification involved the epoxy ring opening of the 2-biphenylyl glycidyl ether catalyzed by BF3 and its subsequent reaction with the hydroxyl groups on the OHM precursor surface. The Biphenyl OHM capillary column thus obtained exhibited the typical reversed phase behavior by primarily hydrophobic interactions vis-à-vis the homologous series of alkyl benzenes and in addition by π-π interactions toward nitroalkane homologous series via their π-electron rich nitro groups. This dual retention mechanism was very distinctly observed with a set of PAH solutes in the sense that the k values of the PAH solutes were comparable to those obtained on a more non polar stationary phase, namely the Epoxy OHM C-16 reported in the preceding article. Other aromatic solutes showed the dual retention mechanism on the Biphenyl OHM capillary including phenols, anilines derivatives, and phenoxy acid herbicides. The Biphenyl OHM capillary exhibited good reproducibility from run-to-run, day-to-day, and column-to-column.

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A novel precursor monolithic capillary column referred to as "hydroxy monolith" or OHM was prepared by the in situ copolymerization of hydroxyethylmethacrylate (HEMA) with pentaerythritol triacrylate (PETA) yielding the neutral poly(HEMA-co-PETA) monolith. The neutral precursor OHM capillary thus obtained was subjected to postpolymerization modifications of the hydroxyl functional groups present on its surface with 1,2-epoxyalkanes catalyzed by boron trifluoride (BF3 ) ultimately providing Epoxy OHM C-m capillary column at varying alkyl chain lengths where m = 8, 12, 14, and 16 for RP-CEC. Also, the same precursor OHM was grafted with octadecyl isocyanate yielding Isocyanato OHM C-18 column to provide an insight into the effect of the nature of the linkage to the surface hydroxyl groups of the OHM precursor. While the epoxide reaction leaves on the surface of the OHM precursor hydroxy-ether linkages, the isocyanato reaction leaves carbamate linkages on the same surface of the OHM precursor. This study revealed that changing the alkyl chain length resulted in changing the column phase ratio (ϕ) and also the solute distribution constant (K). While increasing the surface alkyl chain length increased steeply the solute hydrophobic selectivity, i.e. methylene group selectivity, the nature of the ligand linkage produced different retention for the same solutes and affected the selectivity of slightly polar solutes. The various monoliths proved very useful for RP-CEC of different small solutes at varying polarity over a wide range of mobile phase composition.

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