ABLATIVE AND TRANSPORT FRACTIONATION OF TRACE-ELEMENTS DURING LASER SAMPLING OF GLASS AND COPPER

The fractionation of trace elements due to ablation and transport proc esses was quantified during Q-switched infrared laser sampling of glas s and copper reference materials. Filter-trapping of the ablated produ ct at different points in the sample introduction system showed ablati on and transport sometimes caused opposing fractionation effects, lead ing to a confounded measure of overall (ablative + transport) fraction ation. An unexpected result was the greater ablative fractionation of some elements (Au, Ag, Bi, Te in glass and Au, Be, Bi, Ni, Te in coppe r) at a higher laser fluence of 1.35 x 10(4) W cm(-2) than at 0.62 x 1 0(4) W cm(-2), which contradicted predictions from modelling studies o f ablation processes. With glass, there was an inverse logarithmic rel ationship between the extent of ablative and overall fractionation and element oxide melting point (OMPs), with elements with OMPs < 1000 de grees C exhibiting overall concentration increases of 20-1340%. Fracti onation during transport was quantitatively important for most certifi ed elements in copper, and for the most volatile elements (Au, Ag, Bi, Te) in glass. Elements common to both matrices showed 50-100% higher ablative fractionation in copper, possibly because of greater heat con ductance away from the ablation site causing increased element volatil isation or zone refinement. These differences between matrices indicat e that non-matrix-matched standardisation is likely to provide inaccur ate calibration of laser ablation inductively coupled plasma-mass spec trometry analyses of at least some elements. (C) 1997 Elsevier Science B.V.