Deposition and element fractionation processes during atmospheric pressure laser sampling for analysis by ICP-MS

We have used an ArF excimer laser coupled to a quadrupole inductively coupled plasma mass spectrometry ICP-MS.for the measurement of a range of elements during excavation of a deepening ablation pit in a synthetic glass NIST 612.. Analyte behaviour shows progressive volatile element enrichment at shallow hole depths, with a change to refractory element enrichment as the ablation pit deepens further. Examination of ablation pit morphology and the surface condensate deposited around the ablation site reveals the importance of sequential condensation of refractory, then volatile phases from the cooling plasma plume after the end of the laser pulse. We interpret the observed element fractionation behaviour to reflect a change in ablation processes from photothermal dominated to plasma dominated mechanisms. The development of the surface deposit is greatly reduced by ablating in an ambient atmosphere of He instead of Ar and is accompanied by a two- to four-fold increase in ICP-MS sensitivity. q1998 Elsevier Science B.V