Laser ablation of a sample in liquid-LASIL

This technical note describes the development of Laser Ablation of a Sample In Liquid (LASIL), a technique where the ablation occurs at a solid sample surface submerged in a liquid. LASIL can be performed in a 25 µl isolated, freestanding droplet that acts as a micro-laser cavity, to produce a suspended particulate that can be analysed either directly, or following in-droplet chemistry, by calibration against aqueous standards. The technique is robust and easy to implement being carried out in air, offline to the detection apparatus. The analytical characteristics of LASIL are its ease of quantification, containment of particles, the ease of generating suspended solids in solution from insoluble materials and the control over dissolution and dilution to generate measurable concentrations. NIST 611 (trace elements in glass) was employed as a test sample as it is a commonly used reference material in conventional Laser Ablation (LA) studies. Droplet LASIL allowed the quantification of trace elements in NIST 611 and also investigation of the particle sizes and shapes generated by the ablation process. Particle sizes were found to vary with laser fluence, with higher fluences producing a wider particle size distribution with greater variation in shape. The types of particles found were: jagged particles of 1–2 µm in diameter most probably created by micro-jet impingement, spherical nanometre sized particles from vapour condensation and melt ejection, and thin, string-like particles from particle agglomeration or liquid jet fragmentation. At lower fluences the particle morphology tended towards spherical shapes and formed agglomerates. At this small particle size (below 250 nm), Brownian motion ensures a very slow settling rate in the liquid medium yielding solutions that are stable for analysis over several days. Alternatively, as demonstrated here, post-ablation chemistry can be carried out in the droplet, e.g. acid dissolution, or clean up using micro-extraction techniques. The liquid droplet was analysed by inductively coupled plasma-mass spectrometry (ICP-MS) with calibration against aqueous standards. The ablation yield from the sample was normalised using the found versus known concentration of uranium in the sample and ratioing measured elemental concentrations to this factor. LASIL on a sample immersed in liquid facilitated the study of the effect of the solution composition on the LASIL process.