Comparison of an ultrasonic nebulizer with a cross-flow nebulizer for selenium speciation by ion-chromatography and inductively coupled plasma mass spectrometry

The purpose of this work was to compare an ultrasonic nebulizer (USN) with a cross-flow nebulizer (CFN) for selenium speciation with inductively coupled plasma mass spectrometry (ICP-MS) detection. The influence of instrumental parameters as well as composition of the solvent on the selenium species, trimethylselenonium ion (TMSe), selenomethionine (SeMet), selenite and selenate, were studied. Using the CFN, there was no difference in sensitivity of the four selenium species and the species responded similarly to all changes. In the USN, sensitivity increased between 14 and 21 times for the species. Selenate and TMSe showed the largest increase in sensitivity while selenite and SeMet showed lower increases. The species behaved differently on changing the heat temperature of the USN and the pH of the solutions. When the carbon enhancement effect was exploited by adding 3% methanol, the increase in sensitivity with the USN was between 2.5 and 8.7 times. It was observed that yttrium could not be used as internal standard in the USN as the sensitivity of this element was dependent on pH as well as the presence of different selenium species, while rhodium was unaffected by these parameters. The USN was found inapplicable for normal sample uptake as memory effects were severe and low blank values, therefore, difficult to obtain. Thus, this nebulizer could only be used for transient signals. When coupling the nebulizers to an anion-exchange chromatographic system, peak widths of the chromatographic signals were comparable. The sensitivity with the USN was 10 times larger than with the CFN. Detection limits, however, were not decreased correspondingly. Detection limits calculated on the basis of Se-82 were between 0.15 and 0.24 mu g l(-1) Se with the USN, while they were between 0.43 and 1.6 mu g l(-1) Se with the CFN.