Deposition and element fractionation processes occurring during atmospheric pressure laser sampling for analysis by ICPMS

S.M. Eggins1, L.K. Kinsley and J.M. Shelley

1. GEMOC, ANU

Pulsed UV laser ablation is a rapidly emerging technique for microsampling and analysis by inductively coupled plasma mass spectrometry (ICPMS). The use of analytical techniques dependent on an atmospheric pressure Ar inductively coupled plasma (ICP) as an ion source requires that laser ablation sampling be performed in an atmospheric pressure ambient gas stream in order to transport the ablation products to the ICP. Under these conditions, the relaxation of the shock front produced by the expansion of the plasma plume can result in significant redeposition of ablated material on and around the target substrate. This and any other redeposition processes occurring within the laser ablation site itself or onto the ablation cell and transport system walls reduce the ablation yield and can fractionate relative element concentrations from those present in the target material. Minimising the interaction of ablation products with the surrounding environment is essential to achieve the accuracy and precision demanded by many promising analytical applications.

We have used an ArF excimer laser coupled to a quadrupole ICPMS (Fisons PQ2 XR) for the measurement of multiple elements, doped at levels of 40 µg/g in a synthetic calcium, sodium, aluminosilicate glass (NIST 612), during excavation of a deepening ablation pit. Analyte behaviour is characterised by decaying signal intensities as the ablation pit deepens, until a marked inflection point at ~10% signal maxiumum, where the signal becomes unstable and the decay rate reduces. Before the inflection point depth, volatile elements decay less rapidly than more refractory elements, resulting in progressive fractionation of volatile/refractory element ratios. A profound reversal to enrichment in refractory elements occurs at the inflection point . The ablation pit depth corresponding to this turnover point scales with the width (constant aspect ratio) but is also dependent on laser energy, such that for ablation at higher fluences, the fractionation turnover occurs at deeper aspect ratios. The morphology of ablation pits reveals a narrowing of the pit diameter with depth and a visible condensate on the pit walls. It also appears that the fractionation turnover point corresponds to the depth at which the ablation process ceases to produce a flat-bottomed pit. We interpret these observations to reflect a change in ablation processes from photoablation dominated to plasma dominated mechanisms.

Analysis of the material deposit surrounding the target site reveals it to be preferentially enriched in volatile elements, particularly Pb, and precludes the surface condensate from being the source of volatile/refractory element fractionation observed during pit ablation. The development of this deposit is reduced by ablating at lower fluences and changing the ambient gas to He. Experiments conducted with constant total Ar and He flows to the ICP but varying the ablation site ambient gas from Ar to He realise a 2 to 4-fold increase in analyte signal intensity. This takes place along with a corresponding decrease in the mass of deposited material surrounding the target site and no significant change to the mass yield from the ablation site itself. Ablation in He allows the plasma plume to propagate further from the sample surface prior to relaxation of the shock front, thereby increasing the transport efficency to >90% as compared to only 25-50% for ablation in Ar.

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