Simultaneous separation of common mono- and divalent cations on an acid-treated silica gel column by ion chromatography with indirect photometric detection and tyramine–oxalic acid, containing 18-crown-6 as eluent

The application of unmodified silica gel (Super Micro Bead Silica Gel B-5, SMBSG B-5) as cation-exchange stationary phase in ion chromatography with indirect photometric detection for common mono- and divalent cations (Li+, Na+, NH4+, K+, Mg2+ and Ca2+) was carried out using 0.75 mM tyramine [4-(2-aminoethyl)phenol]–0.25 mM oxalic acid at pH 5.0 as the eluent. Although complete group separation between these mono- and divalent cations was achieved on the SMBSG B-5 column (150×4.6 mm I.D.) in 12 min, peak shapes of NH4+ and K+ were strongly tailed. Hence, the application of SMBSG B-5 silica gel treated with conc. hydrochloric acid at reflux-temperature for 12 h for the simultaneous separation of these cations was carried out. Although the retention volumes of these cations decreased on the acid-treated SMBSG B-5 silica gel column, the peak shapes of NH4+ and K+ were quite improved. Excellent simultaneous separation and highly sensitive detection at 275 nm [detection limits (signal-to-noise ratio of 3 and injection volume of 20 μl), 0.34 μM for Li+, 0.47 μM for Na+, 0.39 μM for NH4+, 0.59 μM for K+, 0.24 μM for Mg2+ and 0.28 μM for Ca2+] were achieved in 15 min on the acid-treated SMBSG B-5 column using 0.5 mM tyramine–0.2 mM oxalic acid–10 mM 18-crown-6 (1,4,7,10,13,15-hexaoxacyclooctadecane) at pH 5.5 as the eluent.