Optimisation of an ICPMS equipped with collision/reaction cell system for the analysis of arsenic and selenium in natural water [Optimisation d'un ICPMS à cellule de collision/réaction pour l'analyse de l'arsenic et du sélénium dans les eaux naturelles]

More and more attention is given to the measurement of arsenic and selenium in natural waters. Total analysis is not sufficient to understand their environmental impact which has to be related to their chemical forms. Speciation analysis is thus necessary. ICPMS (inductively coupled plasma - mass spectrometry) is the detector of choice in environmental analysis, and its coupling with high performance liquid chromatography (HPLC) is easily achieved. However, the ICPMS detection of As and Se can suffer from argon based interferences. In the last past years, ICPMS equipped with collision/reaction cell system have been developped. The addition of one or several gas in the system permit the removal of these interferences. Flow rates of hydrogen and helium were carefully optimised to remove efficiently the interferences on m/z 75 and 80, while keeping maximum sensitivity. A mixture of 3.8 ml min -1 of H2 and 0.5 ml min-1 of He was selected as the most appropriate for As and Se determination. These conditions were found to be also suitable for speciation analysis. Analytical performances were calculated with this set of conditions for elemental and speciation analysis (separation of organic and inorganic Se on the one hand, and inorganic As and Se on the other hand). Detection limits are close to 30 ng(elt) l-1 for both elements in total analysis and in the range 100-200 ng(elt) l-1 for speciation analysis depending of the species. These results represent a 3 fold improvement in comparison with standard mode (C/RC switched off). Accuracy of the method was tested using a reference material (TMRain-95, National Water Research Institute, Canada) and different mineral waters were analysed.