Determination of sub-parts-per-trillion levels of rare earth elements in natural waters by inductively coupled plasma time-of-flight mass spectrometry after flow injection on-line sorption preconcentration in a knotted reactor

A flow injection (FI) on-line sorption system, for the preconcentration and separation in a knotted reactor (KR), is coupled with inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) for the fast, selective and sensitive determination or ultra-trace concentrations of rare earth elements (REEs) in environmental waters. The on-line preconcentration and separation of the REEs is achieved by sorption of the REE complexes formed in the pH range 3.7-4.6 on the inner walls of a (300 cm x 0.5 mm) PTFE KR precoated with the chelating reagent 1-phenyl-3-methyl-4-benzoylpyrazol-5-one (PMBP) at a pH of 9.6. The analytes are eluted and transported to the ICP-TOFMS system with 2% (v/v) HNO3 containing 0.5 mug l(-1) of Rh and In as internal standards, using ultrasonic nebulisation. The REEs are effectively preconcentrated and separated from the major matrix constituents-alkali and alkaline earth elements. Of particular significance is their separation from barium, which produces considerable isobaric interferences on some of the analytes. Using a preconcentration time of 30 s and a sample flow rate of 4.4 ml min(-1), enhancement factors of 15 to 22 for the different REES are achieved. The detection limits (3 sigma) range from 3 to 40 pg l(-1). The precision for 9 replicate determinations of 0.1 mug l(-1) of each analyte is better than 5% (RSD). The accuracy of the method is demonstrated by the analysis of environmental waters and by recovery measurements on spiked samples.