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 ran ge of elements during excavation of a deepening ablation pit in a synt hetic glass (NIST 612). Analyte behaviour shows progressive volatile e lement 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 t he ablation site reveals the importance of sequential condensation of refractory. then volatile phases from the cooling plasma plume after t he end of the laser pulse. We interpret the observed element fractiona tion behaviour to reflect a change in ablation processes from photothe rmal dominated to plasma dominated mechanisms. The development of the surface deposit is greatly reduced by ablating in an ambient atmospher e of ne instead of Ar and is accompanied by a two- to four-fold increa se in ICP-MS sensitivity. (C) 1998 Elsevier Science B.V.