Sensitive analysis of trace caffeine in human serum by HPLC using tetraazacalix[2]arene[2]triazine-modified silica as SPE sorbent

In this paper, a rapid and effective SPE-HPLC method, using tetraazacalix[2]arene[2]triazine modified silica gel (NCSi) as solid-phase extraction (SPE) sorbent, was established to enrich and determine caffeine in human serum. All the main influence factors of SPE including amount of NCSi sorbent, sample flow rate, and volume and composition of washing solution were investigated and evaluated in the sample pretreatment step. The limits of detection (LODs) and quantitation (LOQs) were 1 ng mL(-1) and 5 ng mL(-1), respectively, whereas the overall recoveries ranged from 90.3% to 93.5%, the relative standard deviation (RSD) of the method was less than 3.4%. In addition, the retention mechanism of caffeine on NCSi was investigated by quantum chemistry calculations. The optimized supramolecular structures showed that pi-pi and CH-pi interactions exist between NCSi and caffeine. Thermodynamic results from quantum chemistry calculation (QCC) were consistent with the retention behaviors of caffeine on NCSi. The method of non-covalent interaction (NCI) 28 index developed by Yang and co-workers was used to distinguish strong interaction, van der Waals interaction and repulsive steric interactions. The QCC results further illustrated the stronger retention of caffeine on the tetraazacalix[2]arene[2]triazine-modified silica stationary phase. All of the results implied that NCSi was a promising SPE sorbent, and can be applied to the preconcentration and determination of trace caffeine in human serum.