The effect of filtration on analyses of surface water samples. A study from the Salars of Coipasa and Uyuni, Bolivian Altiplano

Analyses by inductively coupled plasma atomic emission spectrometry (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS) and cold vapour atomic absorption (for Hg, CVAA) of a wide range of elements in some 300 surface water samples from the Salar de Uyuni and Salar de Coipasa catchments of the Bolivian Altiplano have been undertaken. Comparison of analyses of acidified aliquots of unfiltered sample water with water filtered at 0.45 μm reveals that the following elements are not affected significantly by filtration in this high-pH environment: B, Ca, Li, Mg, K, Si, Na, Sr, S. The following elements appear to experience significantly elevated concentrations in unfiltered samples, relative to filtered: Al, (As to a minor extent), Ba, Be, Cd, Cr, Co, Cu, Fe, Pb, Mn, Hg, Ni, P, Ag, Tl, Ti, V. The effect appears to be related to the presence, and subsequent dissolution in acid preservative, of Fe-, Al-or Mn-oxyhydroxide flocs (or coatings on silicate particles) in unfiltered samples, and their retention or precipitation on filters.