Performance characteristics of ultra-violet femtosecond laser ablation inductively coupled plasma mass spectrometry at ∼265 and ∼200 nm

Abstract

The analytical figures of merit of ultra-violet femtosecond laser ablation inductively coupled plasma mass spectrometry (UV-fs-LA-ICP-MS) using the 3rd and 4th harmonics of Ti:Sapphire (∼265 and ∼200 nm, respectively) were explored. For this purpose, elemental ratios of aerosols produced by LA of silicate glass (SRM NIST 610) were studied under varying fluence conditions ranging from moderate values of 2 J cm⁻² up to 30 J cm⁻², taking into account e.g. laser-induced (⁶⁶Zn/⁶⁵Cu) and particle size-related (²³⁸U/²³²Th) phenomena. It could, for instance, be shown that signal ratios were less dependent on the wavelength or laser repetition rate chosen. Furthermore, fractionation indices defined using the temporal drift of elemental ratios over two equal parts of the acquired signal were subject to systematic changes for threshold-close fluences. As a consequence, corresponding ⁴²Ca-normalized values were found to deviate by more than 20% from unity. In contrast, LA at higher fluences resulted in less pronounced discrepancies, falling below 5% even for the most critical elements such as ⁶⁶Zn, ¹¹¹Cd, and ²⁰⁸Pb. The complete suppression of particle size-related fractionation quantified on the basis of the ²³⁸U/²³²Th-system turned out to be highly consistent with the absence of μ-sized particles which were measured by optical particle counting (OPC). The relative fraction of particles >0.5 μm was determined to be less than 5%, independent on the wavelength, fluence, or laser repetition rate chosen. Moreover, our results indicate the occurrence of ICP-induced elemental fractionation during analysis due to an increased mass loading of the plasma source if medium or high fluences are applied. Using the ⁶⁶Zn/⁶⁵Cu-ratio as a thermometric probe, the change in plasma ionization temperature among low and high mass loading conditions was estimated to be −900 K. Nevertheless, UV-fs-LA-ICP-MS analysis of different matrices (silicate glass SRM NIST 610 and brass (Zn ∼20%)) performed within the high fluence range was found to enhance the accuracy for non-matrix-matched calibration. Evidence is given that the enhancement observed mainly depends upon the suppression of laser- and/or transport-induced fractionation.