High-resolution single pulse LA-ICP-MS mapping via 2D sub-pixel oversampling on orthogonal and hexagonal ablation grids - A computational assessment

Dino Metarapi; Johannes T van Elteren

doi:10.1016/j.talanta.2023.124699

High-resolution single pulse LA-ICP-MS mapping via 2D sub-pixel oversampling on orthogonal and hexagonal ablation grids - A computational assessment

Talanta. 2023 Oct 1:263:124699. doi: 10.1016/j.talanta.2023.124699. Epub 2023 May 25.

Authors

Dino Metarapi¹, Johannes T van Elteren²

Affiliations

¹ Department of Analytical Chemistry, National Institute of Chemistry, Hajdrihova 19, 1000, Ljubljana, Slovenia.
² Department of Analytical Chemistry, National Institute of Chemistry, Hajdrihova 19, 1000, Ljubljana, Slovenia. Electronic address: elteren@ki.si.

PMID: 37267882
DOI: 10.1016/j.talanta.2023.124699

Abstract

Laser beam profiles in analytical laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) instruments are in general homogenized to produce a flat-top beam profile. However, in practice, they are mostly super-Gaussian in nature, and for small laser beam sizes (<5 μm) they even approach a Gaussian profile. This implies that the amount of surface material sampled by the laser (=ablation volume) directly depends on the beam profile and ablation grid. By contraction of the ablation grid (=sub-pixel mapping) not only more accurate surface sampling is realized, but also a higher pixel density, an improved spatial resolution, and a better signal-to-noise ratio. Although LA sampling is predominantly performed on an orthogonal grid, hexagonal or staggered/interleaved sampling may further improve the image quality as regular hexagons are more compact than squares (=lower perimeter/area) and suffer less from orientation bias (=lower anisotropy). Due to the current limitations of LA stages in executing precise hexagonal sampling with small beam sizes, computational protocols were employed to simulate LA-ICP-MS mapping. Simulation was performed by discrete convolution using the crater profile as the kernel, followed by the application/addition of Poisson/Flicker noise related to the local concentration and instrumental sensitivity/noise. A freely accessible online app was developed (https://laicpms-apps.ki.si/webapps/home/) to study the effect of sampling grid contraction (orthogonal and hexagonal) on the image map quality (spatial resolution and signal-to-noise ratio) by virtual ablation of phantoms. Comparison of experimental LA-ICP-MS maps obtained through orthogonal and hexagonal sampling methods could only be performed using a beam size of 150 μm and a macroscale inkjet-printed resolution target. This was due to the unavailability of precise hexagonal sampling stages and microscale resolution targets, which prevented the use of smaller beam sizes.

Keywords: Chirp resolution target; Contrast-detail diagram; Hexagonal lattice; Hexagons; Simulation; Visual acuity.