Study of pre-evaporation and matrix effects on multi-elemental analysis by ICP-TOFMS
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The ultimate goal of this project was to improve the sensitivity and detection limits of inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) and then apply it to the speciation analysis of As. To this end, two approaches were taken: the use of organic modifiers and that of a pre-evaporation interface between the spray chamber and the plasma torch. The radial profiles of some background ions and analytes in three different matrices (1% HNO3, 1% HNO3 with 2% v/v methanol, 1% HNO3 with 0.2% m/v sodium dodecylsulfate (SDS)) were investigated in ICP-TOFMS. Although these concentrations of methanol and SDS induced the same increase (37%) in sample introduction efficiency, the change in analyte signal as a function of m/z followed opposite trends in these two matrices. The results show that matrix effects arising from different organic modifiers affected the distribution of ions in the plasma differently as a result of changes induced in the predominant ionisation mechanisms in the plasma. The effect of a pre-evaporation interface on the distribution of ions in ICP-TOFMS was also investigated by spatial profiling, which showed that the optimal axial position of all elements shifted closer to the load coil. Furthermore, the radial profiles became significantly narrower and Gaussian. The decrease of droplet size thus improved the sensitivity and detection limits for multi-elemental analysis. A further investigation of the effect of the pre-evaporation interface revealed that, for cationic analytes, the signal enhancement had an inverse dependency on analyte mass upon heating the interface. In the case of As, a signal enhancement upon heating the interface only resulted when concomitant ionic analytes were present or with a 0.01 M NaOH matrix. All these observations could be rationalised by changes in the number of Coulomb fission events occurring during pre-evaporation, which depend on the size and charge of droplets, as well as the identity and concentration of the matrix. Finally, the baseline separation of four As species by ion exchange liquid chromatography with detection by ICP-TOFMS and application of the pre-evaporation interface tube to As speciation were achieved using a mobile phase of NH4NO3, following an investigation of its matrix effects.