Selectivity differences of coordination compound stationary phases for polyaromatic hydrocarbons and polar analytes in gas and liquid phases

- Cd(II)-based coordination compounds were synthesised in different diameter vessels.
- The resulting materials were characterised by SEM, EDX and XRD.
- Materials applied in GC and LEC, showed different size selective behaviours.
- LSER revealed stronger dispersion and H-bonding with acid analytes in LEC.
- H-bonding with basic analytes slightly increased retention in GC.

A precursor solution to the metal-organic framework (MOF), Cd(1,4,7,10-tetrakis(4-carboxybenzyl)-1,4,7,10-tetraazacyclododecane), was aged in a surface-functionalised 250 μm-diameter capillary, to generate a film containing the constituents of the bulk phase crystalline material. EDX analysis revealed a higher ratio of ligand elements (C, N and O) to Cd in the capillary coated phase compared to that achieved from bulk synthesis in a vial. Separation performance of the coated capillary for gas chromatography (GC) was compared to the performance of the original MOF crystals packed in tips for liquid elution chromatography (LEC) towards the separation of polycyclic aromatic hydrocarbons (PAHs) and small probe analytes with different properties. This study showed that different retention mechanisms were observed for the same set of analytes in gas and liquid phases. Independently of the trend of vapour pressure difference, the GC result showed less specific size selective retention, but with stronger retention towards larger PAHs. A linear solvation energy relationship correlation further revealed enhanced retention contributions (positive stationary phase descriptor values) by cavity formation/dispersion interactions and H-bonding with acid functionality of the probe analytes in both GC and LEC (though stronger in LEC), whilst H-bonding with basic functional groups slightly increased retention in GC; retention was strengthened by dipole-dipole/dipole-induced dipole interactions only in LEC.