ONLINE DILUTION FOR ICP-MS WITH A FLOW-INJECTION RECIRCULATING LOOP MANIFOLD

A FI manifold consisting Of a recirculating loop has been coupled to a plasma source mass spectrometer to provide successive on-line dilutio ns. Part of the loop is injected into the carrier stream followed by d ilution, within the loop, of the remaining part with the carrier solut ion. For a manifold with a calculated dilution factor of 2.02 (based o n 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 pea k heights. The between-run precision for a particular peak height rang ed from 1.7 to 3.2% relative standard deviation (RSD). In a study of t he decay of the concentration oscillations in the recirculating loop, it was found that the reciprocal of the time to achieve uniform concen trations decreased linearly with increasing flow rate and decreasing l oop volume. The dilution behaviors of 19 elements were studied. Of the se, nine (Ag, Ba, Cr, Cu,Ni, Pb, Sb, Tl and U) could be diluted from 1 00 ppb by three orders of magnitude with a precision of 5% RSD or bett er, six (As, Cd, Co, Th, V, Zn) could be diluted over the same range w ith precisions between 5 and 10% RSD, and four elements (Be, Mo, Se an d Hg) displayed a systematic decrease in the dilution factor which was interpreted as retention of these elements within the loop. The influ ence of a mine matrix on the determination of Ce was removed by live s uccessive dilutions with a factor of 3.04 per injection for a total di lution factor of 260. For a total loop volume of 1-2 ml, rapid damping of the concentration oscillations could be produced by the destructiv e interference produced by a two-line network (split and confluence) w ith tube lengths of 50 and 25 cm.