Effects of acid treatment on the trace metal content of chromatographic silica: bulk analysis, surface analysis and chromatographic performance of bonded phases

A series of studies has been carried out on the effect of refluxing silica chromatography particles for 0.5 h and 18 h in water, dilute hydrochloric a cid and dilute hydrofluoric acid. The bulk and surface trace metal concentr ations were measured by inductively-coupled plasma atomic emission spectros copy, static secondary ion mass spectrometry (SSIMS) and X-ray photoelectro n spectroscopy. Diffuse reflectance Fourier transform infrared spectroscopy was used to determine changes in "isolated" and "bonded" silanol groups. T he chromatographic behaviour of a series of weakly basic analytes was inves tigated on C-8 and C-18 bonded phases manufactured from the acid-treated si licas. The different reflux treatments all resulted in a reduction in the n umbers of isolated silanols compared with the untreated silica and SSIMS an alysis suggested that the HF-treated silicas had undergone a more efficient surface rehydroxylation. Bulk trace metals were removed most effectively b y the HF treatment, with the multivalent elements (Ti and Al) being the mos t difficult to remove. Surface specific analysis suggested that trace metal s were removed more rapidly from the surface of the silica compared to the bulk matrix and that the acid treatments resulted in halide contamination o f the silica surface. Evidence is presented to suggest that the bulk metal content of the silica is not representative of the concentration of metals at the chromatographic surface. The chromatographic investigations showed t hat the HF-treated silica gave substantially better performance towards wea k bases than the HCl-treated silicas. (C) 2001 Elsevier Science B.V. All ri ghts reserved.