New stationary phases for normal-phase HPLC based on silica gel modified with cobalt, nickel, zinc, and cadmium salts

Abstract

In recent years, data has been reported on the modification of silica gel with magnesium and calcium oxides, which leads to a decrease in both acidity and polarity of the surface of silica gel and, as a consequence, to an increase in the retention times of some organic compounds and higher efficiency and selectivity [1][2][3]. However, only few works have dealt with the modification of silica gels with transition metals.Silica gel modified with silver ions was prepared in [4] (the amount of adsorbed silver was 420-4500 µ g). The chromatographic behavior of this sorbent drastically differs from the behavior of unmodified silica gel. It was demonstrated that the retention of unsaturated fatty acids on silica gel modified with silver ions depends on the stability of the complexes formed by silver(I) and π electrons of the carbon-carbon double bond and on the number and geometry of carbon-carbon double bonds. This sorbent modified with silver ions exhibits higher selectivity than silica gel with bonded amino groups in the separation of phenols and phthalates. Examples of the use of metal-modified silica gels as stationary phases in normal-phase liquid chromatography reported in the literature allow a conclusion that the materials of this kind are highly promising. On the whole, however, the reasons for the change in the chromatographic properties of silica gels modified with metal salts are insufficiently studied.The aim of this work was to study the chromatographic behavior of silica gel modified with transition metal oxides.EXPERIMENTAL Instruments and materials. Stock 0.2 M modifying solutions were prepared by dissolving weighed portions of dry cobalt, nickel, zinc, and cadmium nitrates (all salts of analytical grade) in distilled water. Silasorb SPH silica gel (Lachema, Brno, Czech Republic) with the specific surface area 300 m 2 /g and the particle size 5 µ m was selected as the initial matrix. A mixture (70 : 22 : 8) of hexane, chloroform, and isopropanol (all of analytical grade) was used for the preparation of the eluent. Isopropanol, glycerol (analytical grade), and distilled water in the ratio 15 : 1 : 1 were used for the preparation of a solution for packing columns. m-Nitroaniline, p-nitroaniline, o -phenylenediamine, m -phenylenediamine, and 4-nitrophenylhydrazine (all of analytical grade) were selected as model compounds for studying the chromatographic behavior of different sorbents.Sorbents were dried and calcinated in a PM-9 muffle furnace (Russia). A column was filled with a sorbent on a B-110v high-pressure liquid pump for column packing (Beckman, Germany). Steel columns (4 × 50 mm) were used in this work. The retention of compounds was studied on a Staier liquid chromatograph (Aquilon, Russia) equipped with a plunger pump and a six-way valve with a loop volume of 20 µ L. The signal was detected using an Avp10 spectrophotometric detector (Shimadzu, Japan) at 254 nm. The IR spectra of samples were recorded in the region 400-4000 cm -1 on an IFS-113v Fourier-transform spectrometer (Bruker, USA).Exper...