On-line Dilution for ICP-MS With a Flow Injection Recirculating Loop Manifold

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A FI manifold consisting of a recirculating loop has been coupled to a plasma source mass spectrometer to provide successive on-line dilutions. Part of the loop is injected into the carrier stream followed by dilution, within the loop, of the remaining part with the carrier solution. For a manifold with a calculated dilution factor of 2.02 (based on the volume ratio), 10 successive injections gave a mean of 1.99 and a 95% confidence interval of ±0.065 for the ratio of successive peak heights. The between-run precision for a particular peak height ranged from 1.7 to 3.2% relative standard deviation (RSD). In a study of the decay of the concentration oscillations in the recirculating loop, it was found that the reciprocal of the time to achieve uniform concentrations decreased linearly with increasing flow rate and decreasing loop volume. The dilution behaviors of 19 elements were studied. Of these, nine (Ag, Ba, Cr, Cu, Ni, Pb, Sb, Tl and U) could be diluted from 100 ppb by three orders of magnitude with a precision of 5% RSD or better, six (As, Cd, Co, Th, V, Zn) could be diluted over the same range with precisions between 5 and 10% RSD, and four elements (Be, Mo, Se and Hg) displayed a systematic decrease in the dilution factor which was interpreted as retention of these elements within the loop. The influence of a wine matrix on the determination of Ce was removed by five successive dilutions with a factor of 3.04 per injection for a total dilution factor of 260. For a total loop volume of 1–2 ml, rapid damping of the concentration oscillations could be produced by the destructive interference produced by a two-line network (split and confluence) with tube lengths of 50 and 25 cm.