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In 2002, we wrote an Analytical Chemistry feature article describing the Physics of Laser Ablation in Microchemical Analysis. In line with the theme of the 2002 article, this manuscript discusses current issues in fundamental research, applications based on detecting photons at the ablation site (LIBS and LAMIS) and by collecting particles for excitation in a secondary source (ICP), and directions for the technology.

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As laser ablation becomes more ubiquitous for direct solid sampling with inductively coupled plasma mass spectrometry (ICP-MS), the need to understand and mitigate fractionation (non-stoichiometric generation of vapor species) becomes critical. The in¯uence of laser-beam wavelength on fractionation is not well established; in general, it is believed that fractionation is reduced as the wavelength becomes shorter. This manuscript presents an investigation of fractionation during ablation of NIST glasses and calcite using three UV wavelengths (157 nm, 213 nm and 266 nm). Fractionation can be observed for all wavelengths, depending in each case on the laser-beam irradiance and the number of laser pulses at each sample-surface location. The transparency of the sample in¯uences the amount of sample ablated (removed) at each wavelength, and the extent of fractionation. Pb/Ca and Pb/U ratios are used as examples to demonstrate the degree of fractionation at the different wavelengths.

The craters resulting from high-irradiance (1×109–1×1011 W/cm2) single-pulse laser ablation of single-crystal silicon show a dramatic increase in volume at a threshold irradiance of 2.2×1010 W/CM2. Time-resolved shadowgraph images show ejection of large particulates from the sample above this threshold irradiance, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of a superheated layer near the critical state. Considering the transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency), the calculated thickness of the superheated layer at a delay time of 200–300 ns agreed with the measured crater depths. This agreement suggests that induced transparency promotes the formation of a deep superheated layer, and explosive boiling within this layer leads to particulate ejection from the sample.

We compared the analytical performance of ultraviolet femtosecond and nanosecond laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The benefit of ultrafast lasers was evaluated regarding thermal-induced chemical fractionation, that is otherwise well known to limit LA-ICPMS. Both lasers had a Gaussian beam energy profile and were tested using the same ablation system and ICPMS analyzer. Resulting crater morphologies and analytical signals showed more straightforward femtosecond laser ablation processes, with minimal thermal effects. Despite a less stable energy output, the ultrafast laser yielded elemental (Pb/U, Pb/Th) and Pb isotopic ratios that were more precise, repeatable, and accurate, even when compared to the best analytical conditions for the nanosecond laser. Measurements on NIST glasses, monazites, and zircon also showed that femtosecond LA-ICPMS calibration was less matrix-matched dependent and therefore more versatile.