A crosslinked, low pH-stable, mixed-mode cation-exchange like stationary phase made using the thiol-yne click reaction

Abstract

Mixed-mode cation-exchange stationary phases are useful for the separation of mixtures containing hydrophobic, acidic, and basic molecules. To ensure that weak organic bases are protonated and carboxylic acids are neutral low pH mobile phases are required. Mixed-mode stationary phases that are stable at pH < 3 are needed. We synthesized a crosslinked structure along the surface of thiol functionalized silica gel particles using the thiol-yne click reaction. The alkyne, 1,7-octadiyne, was added to the 3-mercaptopropyl silica gel, then crosslinked using 1,6-hexanedithiol. Elemental analysis showed low octadiyne ligand surface coverage, but, stoichiometrically, three sulfurs were added to each octadiyne ligand during the crosslinking step, indicating that crosslinking occurred. The effect of the crosslinking on the stability was tested with a 50:50 (v/v) pH 0.50 5% TFA aqueous:acetonitrile mobile phase at 70 °C for six days, over 35,000 column volumes. The stationary phase showed good stability with the retention of triphenylene decreasing only 20% during that time. The Tanaka test showed that the phase has a methylene selectivity of 1.20 ± 0.04, a high shape selectivity of 2.71 ± 0.03, and a 3.98 ± 0.05 cation-exchange factor at pH 2.70. The phase has a selectivity factor for nitrobenzene and benzene of 1.41 ± 0.01, indicating the electron donating charge transfer characteristic of the phase. The mixed-mode characteristics of the phase were investigated using a mixture of the monoamine neurotransmitters norepinephrine, dopamine, and serotonin. Baseline resolution of the monoamines could be obtained using a simple 20 mM potassium phosphate (pH 2.70)/methanol mobile phase. Altering both the methanol content and the potassium ion concentration altered the retention of the monoamines indicating mixed-mode cation exchange characteristic of the crosslinked stationary phase.