Crater formation and signal intensity in nano- and femto-second laser ablation inductively coupled plasma mass spectrometry

Summary form only given. Inductively coupled plasma-mass spectrometry (ICP-MS) is a widely used analytical technique and produces highly accurate results. One of the main disadvantages of the technique, however, is the necessity of solid sample preparation into a solution; this is remedied by the use of laser ablation (LA) for direct solid sampling. LA is the process of delivering energy to a sample via a laser and, consequently, removing part of the sample and forming a small crater on the surface of the sample. Currently there exist several issues in LA sample introduction to ICP-MS commonly called `elemental fractionation´. A better understanding of fundamental laser ablation mechanisms and particle generation during LA process are necessary in order to efficiently couple the laser beam into the sample, ablate a reproducible quantity of mass, minimize the plasma shielding and fractionation, and control and optimize ablated particle transport.We investigated the role of crater formation and ICP-MS signal intensity changes during nanosecond and femtosecond laser ablation sample introduction. Laser ablation was performed using pulses from an ultrafast femtosecond laser (800 mJ, 40 fs FWHM) and an Nd:YAG nanosecond laser (266 nm, 5 ns FWHM). Crater formation is largely dominated by laser characteristics such as spot size and pulse energy. It has been previously shown1 that nanosecond Gaussian beams produce cone-like craters under repetitive laser ablation. A change in pulse duration could change the shape and aspect ratio of the resultant crater. Femtosecond laser pulses result in reduced sample melting and can result in reduced fractionation due to lessening of thermal effects. Crater shape has an effect on the degree of fractionation due to sampling at different depths and positions in the sample. By choosing proper experimental conditions based on crater formation and laser characteristics, it is feasible to obtain results in the mass spectrometer that are more - omparable to the known composition of the sample. A comparison between crater size and shape of the two pulse durations is made, as well as an analysis of ICP-MS signal intensity and accuracy based on the crater profile. Optimized ablation conditions are described based on the experimental results.