The formation of trace element-enriched particulates during laser ablation of refractory materials

Scanning electron microscopy of the laser ablation products from two refractory materials (NIST Standard 610 Glass and mammal tooth) revealed the presence of particulates which were significantly (five- to > 500-fold) enriched in certain trace elements compared to the original sample. Their formation depended on the elemental composition and refractoriness of the sample: the products from NIST 610 Glass included Au, Ag, Pb, Bi, Cu and Zn particles, white a non-refractory target (a copper coin) did not produce any enriched particulates. Based on their size (up to 9 mu m diameter) and within-particle dement heterogeneity, the most likely mechanism for their formation is zone refinement (migration and segregation) of trace elements within the melted portion of the sample, followed by ejection of element-enriched molten droplets by laser shock. Transport and ionization of the particulates would account for the order of magnitude spikes observed in the signals of these elements. Using the frequency and magnitude of the spikes as an indication of enriched particulate formation, analysis of 17 elements during ablation of NIST 610 Glass suggested that in general there was an inverse exponential relationship between particulate formation and the melting point of the element oxide. Trace element-enriched particulates have several implications for laser ablation-inductively coupled plasma-mass spectroscopy analyses, including isotopic ratio determinations, analytical precision, the degree of non-representative sampling, and element fractionation during transport.