Use of 3-(1,8-naphthalimido)propyl-modified silyl silica gel as a stationary phase for the high-performance liquid chromatographic separation of purine derivatives

Capillary electrochromatography of caffeine and its metabolites in rat brain microdialysate

A capillary electrochromatography (CEC) method has been developed for the separation of caffeine and its two metabolites 1-methylxanthine (1-MX) and 1,7-dimethylxanthine (1,7-DX). The stationary phase was 3-(1,8-naphthalimido) propyl-modified silyl silica gel (NAIP) and the best separations were achieved with 4.0 mM citrate buffer (pH 5.0) containing 80% methanol at an applied voltage of 25 kV. The compounds were completely separated in less than 3.5 min with good repeatability, which was approximately 3-times less than that in high-performance liquid chromatography (HPLC) with NAIP. The proposed method coupled with microdialysis was successfully applied to the monitoring of caffeine concentration in rat brain with detection limits of 1.11 microg/mL.

Investigations on the chromatographic behavior of hybrid reversed-phase materials containing electron donor–acceptor systems

The chromatographic behavior of three naphthalimide-type stationary phases were elucidated in terms of hydrophobic, silanophilic and pi-pi interaction properties, employing besides common chromatographic column test methods from Engelhardt and Tanaka, also new test mixtures of geometrical and functional aromatic isomers. It was found that the presence of electron donor/acceptor moieties within a reversed phase system did not only increase the overall retention times for aromatic solutes, but also lead to an enhanced shape selectivity of the hybrid stationary phase. In this context, shape discrimination is primarily based on the number of accessible pi-electrons for pi-pi interaction with the embedded electron deficient ligand moieties. The most outstanding results were obtained for the 1,4,5,8-naphthalenediimidic selector with its horizontal arrangement on the silica surface, which enables a direct face-to-face pi-pi interaction with aromatic solutes, with only little hydrophobic contribution.

Investigation of the novel mixed-mode stationary phase for capillary electrochromatography

A novel packing material, 3-(4-sulfo-1,8-naphthalimido)propyl-modified silyl silica gel (SNAIP), was prepared for the use as a stationary phase of capillary electrochromatography (CEC). The sulfonic acid groups on SNAIP stationary phase contributed to the generation of electroosmotic flow (EOF) at low pH and served as a strong cation-exchanger. In CEC with SNAIP, a mixed-mode separation was predicted, comprising hydrophobic and electrostatic interactions as well as electrophoretic migration process. In order to understand the retention mechanism on SNAIP, effects of buffer pH, concentration, and mobile phase composition on EOF mobility and the retention factors of barbiturates and benzodiazepines were systematically investigated. Moreover, the retention behavior of barbiturates on SNAIP was investigated and compared with those on octadecyl silica (ODS), phenyl-bonded silica, and 3-(1,8-naphthalimido)propyl-modified silyl silica gel to confirm the presence of pi-pi interaction on its retention mechanism. It was observed that a column efficiency was more than 85,000 N/m for retained compounds and the relative standard deviations for the retention times of EOF marker, thiourea, and five barbiturates were below 2.5% (n = 4). Under an applied voltage of 20 kV and a mobile phase consisted of 5 mM phosphate (pH 3.8) and 40% methanol, the baseline separation of five barbiturates was achieved within 3 min.