Elemental fractionation in the formation of particulates, as observed by simultaneous isotopes measurement using laser ablation ICP-oa-TOFMS

Abstract

Laser ablation sampling is now successfully used in ICP-MS for the detection of trace elements. Quantitative analysis of samples with laser ablation sampling is, however, often hindered by the fractionation of elements during the laser ablation process, transportation of ablated material, and during atomization–ionization in plasma, due to their different evaporation-condensation properties. The widely reported feature of fractionation in LA-ICP-MS analysis is a slow variation of the signal during treatment of the surface sample, which varies for different groups of elements. There are also short-term ion signal fluctuations after laser sampling. They are noticeably higher than those for solution analysis, due to the presence of large particles in the sample flow. These particles can have an elemental composition noticeably different from the sample. Isotope ratios are much less subjected to slow fractionation effects and to fractionation in particles and therefore can be measured with better precision if simultaneous monitoring of all isotopes is possible.

An orthogonal acceleration time-of-flight mass spectrometer can measure the whole mass spectrum simultaneously at a rate of 30 000 times per second. This makes it possible to detect the elemental composition of single particles of a sputtered sample, atomized in an ICP-torch. Consequently, the use of ICP-oa-TOF can reveal surprising features of elemental fractionation in some conditions of laser ablation sampling, and helps to make the method quantitatively reliable.