Non-matrix-matched calibration in bulk multi-element laser ablation - Inductively coupled plasma - Mass spectrometry analysis of diverse materials

Laser ablation inductively coupled plasma - mass spectrometry (LA-ICP-MS) is a frequently used microanalytical technique in elemental analysis of solid samples. In most instances the use of matrix-matched calibration standards is necessary for the accurate determination of elemental concentrations. However, the main drawback of this approach is the limited availability of certified reference materials. Here, we present a novel conceptual framework in LA-ICP-MS quantification without the use of matrix-matched calibration standards but instead employment of an ablation volume-normalization method (via measurement of post-ablation line scan volumes by optical profilometry) in combination with a matrix-adapted fluence (slightly above the ablation threshold). This method was validated by cross-matrix quantification of reference materials typically investigated by LA-ICP-MS, including geological and biological materials. This allows for more accurate and precise multi-element quantification, and enables quantification of previously unquantifiable elements/materials.

Utilizing ablation volume for calibration in LA-ICP-MS mapping to address variations in ablation rates within and between matrices

Quantification in 2D LA-ICP-MS mapping generally requires matrix-matched standards to minimize issues related to elemental fractionation. In addition, internal standardization is commonly applied to correct for instrumental drift and fluctuation, whereas also differences in ablated mass can be rectified for samples that cannot be sectioned and subjected to total ablation. However, it is crucial that the internal standard element is homogeneously distributed in the sample and that the laser light absorptivity is uniform over the surface. As in practice these requirements are often not met, this work will focus on correction of ablation rate differences within/between samples and standards by normalizing the element maps using the associated ablation volume per pixel as measured by optical profilometry. Due to the volume correction approach the element concentrations are no longer defined as mass per mass concentrations (in μg g-1) but by mass per volume concentrations (in μg cm-3), which can be interconverted in case matrix densities are known. The findings show that ablation volume-aided calibration yields more accurate element concentrations in 2D LA-ICP-MS maps for a decorative glass with highly varying elemental concentrations (murrina). This research presents a warning that if there are variations in ablation rates between samples and standards within and across matrices, even when their sensitivities are the same, generic LA-ICP-MS calibration protocols may not accurately depict the actual element concentrations.

Evaluation of two-phase sample transport upon ablation of gelatin as a proxy for soft biological matrices using nanosecond laser ablation - inductively coupled plasma - mass spectrometry

BACKGROUND

Recent papers on LA-ICP-MS have reported that certain elements are transported in particulate form, others in gaseous form and still others in a combination of both upon ablation of C-based materials. These two phases display different transport behaviour characteristics, potentially causing smearing in elemental maps, and could be processed differently in the ICP which raises concerns as to accuracy of quantification and emphasizes the need for matrix-matching of external standards. This work aims at a better understanding of two-phase sample transport by evaluating the peak profile changes observed upon varying parameters such as laser energy density and wavelength.

RESULTS

It is demonstrated that upon ablation of gelatin, elements are transported predominantly in particulate phase, but already at low laser energy density, a significant fraction of some elements is transported in the gaseous phase, which is even more expressed at higher energy density. This behaviour is element-specific since the ratio of the signal intensity for the analyte element transported in gas phase to the total signal intensity was 0 % for 23Na, 43 % for 66Zn and as high as 95 % for 13C using a 193 nm laser. The results also suggest an effect of the laser wavelength, as all elements show either the same or higher amount of gas phase formation upon ablating with 213 nm versus 193 nm. It was even established that elements that fully occur in particulate form upon ablation using 193 nm laser radiation are partly converted into gaseous phase when using 213 nm.

SIGNIFICANCE

This work provides a thorough investigation of the underexposed phenomenon of two-phase sample transport upon ablation of biological samples upon via LA-ICP-MS. It is shown that for some elements a fraction of the ablated material is converted and transported in the gas phase, which can lead to significant smearing effects. As such, it is important to evaluate element-specific peak profiles on beforehand and, if necessary, adapt instrument settings and slow down data acquisition.

Exploring the Benefits of Ablation Grid Adaptation in 2D/3D Laser Ablation Inductively Coupled Plasma Mass Spectrometry Mapping through Geometrical Modeling

This study aims to investigate the potential benefits of adapting the ablating grid in two-dimensional (2D) and three-dimensional (3D) laser ablation inductively coupled plasma mass spectrometry in a single pulse mapping mode. The goals include enhancing the accuracy of surface sampling of element distributions, improving the control of depth-related sampling, smoothing the post-ablation surface for layer-by-layer sampling, and increasing the image quality. To emulate the capabilities of currently unavailable laser ablation stages, a computational approach using geometrical modeling was employed to compound square or round experimentally obtained 3D crater profiles on variable orthogonal or hexagonal ablation grids. These grids were optimized by minimizing surface roughness as a function of average ablation depth, followed by simulating the post-ablation surface and related image quality. An online application (https://laicpms-apps.ki.si/webapps/home/) is available for users to virtually experiment with contracting/expanding orthogonal and hexagonal ablation grids for generic 3D super-Gaussian laser crater profiles, allowing for exploration of the resulting post-ablation surface layer roughness and depth.

Quantification anomalies in single pulse LA-ICP-MS analysis associated with laser fluence and beam size

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has undergone major improvements in recent years which have led to reduction of the analysis time, higher spatial resolution, and better sensitivity.