Tracking mass removal of portable laser ablation sampling by its acoustic response

Abstract

If the sample cannot be brought to the lab for sampling, then the sampling needs to be done at the sample location. The portable laser ablation (pLA) system is one of the candidates to address this challenge, as it is a powerful tool for field deployable sampling of solids with the benefits of laser based analysis methods. However, sampling in the field is not always as convenient as in a defined laboratory setting and requires a neat prior method development. In the field, the evaluation of the sampling performance is not straightforward with current hardware. During mass removal by (p)LA, an acoustic pressure wave is formed for every individual LA event. In order to track these signals, a directional microphone was integrated into the handheld pLA-module. This allowed the monitoring of the acoustic signal close to the ablation site to count and quantify LA events from each individual pulse. In order to monitor mass removal and acoustic response quasi-simultaneously, the pLA sampling device was coupled to an ICPMS via a gas-exchange device (GED). Performance was investigated using various samples. Discrepancies were observed between samples with low and high ablation thresholds. For samples with low ablation thresholds, e.g. metals, the acoustic signal was mainly influenced by the laser fluence at the ablation site and not by the amount of mass removal. Comparatively, when ablating samples with high ablation thresholds, such as glasses and minerals, these variations were less pronounced, resulting in the observation of a linear relationship. Consequently, this allows the use of the acoustic energy as a measure of the amount of mass removal. In summary, results show that this approach enables a straightforward way to track all actual ablation events during on-site sampling and allows an instant evaluation of the ablation performance, thereby reducing unsuccessful sampling sessions.