Fractionation Effects in Laser Ablation Inductively Coupled Plasma Mass Spectrometry

Aspects of laser ablation sample introduction for inductively coupled plasma mass spectrometry (ICPMS) have been investigated. For some analytes, nonrepresentative subsampling or fractionation is the major cause of poor analytical accuracy. Fractionation is prevalent for ablation at low laser fluence and with multiple laser pulses incident on the same area of the sample surface. The fluence dependence is explained by the relative depths of the melt- and heat-affected zones. Volatile analyte elements that are segregated in the bulk, or become segregated as the ablation zone is heated, are most prone to fractionation. For metal alloys, the extent of fractionation can be qualitatively predicted from the binary-phase diagram of the corresponding analyte matrix. Analysis by Auger electron spectroscopy showed that miscible elements may also be segregated at the near surface, with the extent of segregation growing with multiple laser pulses. Such segregation results in increased fractionation.