Modelling the temporal intensity distribution in laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) using scanning and drilling mode

Signal equations basing on dispersion functions describing the measured temporal intensity distribution for laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) used in scanning and drilling mode are developed. Variable ablation rates due to either varying focussing conditions typical for drilling mode and due to the changes of physical and chemical properties in inhomogeneous samples as typically investigated in scanning mode are considered for. The model accounts for intermixing of the sample aerosol within the sampling chamber, the influence of transport in a cylindrical transport channel and the fact that normally not the entire vapour generated and transported to the ICP can be observed. The absolute signal response is influenced by the actually ablated, transported and observed analyte mass. The dispersion functions describing the relative signal response depend on sample chamber volume, the volume of the transport channel, the laser shot frequency, the carrier gas flow rate and the part of observable cross-section at the MS interface compared to the entire cross-section filled by the vapour. All these parameters depend on the experimental set-up and the selected operating conditions only. Using the signal equation the influence of all mentioned parameters on signal course is shown both theoretically and experimentally. The signal equation can be used for calculation of optimal experimental conditions. On this basis, an algorithm is proposed providing the relative temporal distribution of any analyte with significantly higher temporal resolution than the measured temporal intensity distribution itself. Furthermore, usage of dispersion functions for investigation of a given transport system, for explanation of typical signal deviations, for the proof of homogeneous regions in a heterogeneous sample, for examination of changes in ablation rate and for investigation of fractionation effects is shown.