Effect of trace metal concentration on chromatographic performance of ODS bonded silicas with modified metal content

Three batches of 5 mum spherical silica particles with differing added amou nts of iron were manufactured and each of these was bonded with n-dimethylo ctadecylsilyl (ODS) followed by end-capping with trimethylsilyl (TMS) group s. The concentration of twelve trace metals in the untreated silicas and in the bonded silicas tvas measured by ICP-AES. Iron was confirmed to be pres ent at different concentrations in each of the silicas: 300 ppm ('high Fe') , 191 ppm ('medium' Fe), and 132 ppm ('low Fe') in the base silicas, and th e same pattern was also observed in the bonded silicas. The addition of iro n during manufacture displaced other metals from the silicas, notably titan ium aluminium. Following refluxing of the silicas in dilute hydrochloric ac id a substantial decrease in trace metal content was observed for most meta ls apart from aluminium and titanium, which showed very little reduction (< 5%). A range of neutral, acidic, basic, and metal chelating test solutes was use d to examine the chromatographic performance of each bonded phase, and thes e results were then correlated with the trace metal contents of the bulk st ationary phases. Neutral solutes were unaffected by trace metal content. Me tal chelating solutes also showed no correlation with the iron content of t he silicas, probably due to the total metal content between the phases bein g similar. However, the chromatographic retention and peak shape of weak ba ses and weak acids did correlate with iron, aluminum and titanium concentra tions in the ODS bonded silicas. This study confirmed that individual rathe r than total metal concentration can affect the chromatographic behaviour o f an ODS silica phase.