A comparison of calibration strategies for quantitative laser ablation ICP-mass spectrometry (LA-ICP-MS) analysis of fused catalyst samples

Abstract

In the field of petrochemistry, the quantitative determination of trace elements in catalysts is crucial for optimizing various types of processes. Catalyst poisoning, resulting from the presence of contaminants, can lead to decreased performance and efficiency, even when these are present at trace level only. Inductively coupled plasma-mass spectrometry (ICP-MS) is a powerful technique for trace elemental analysis, but its application to catalysts is challenging due to their physicochemical characteristics challenging straightforward dissolution. Laser ablation (LA) coupled to ICP-MS (LA-ICP-MS) has emerged as a valuable approach for direct analysis of solid samples. However, developing an appropriate calibration strategy for reliable quantitative LA-ICP-MS analysis of catalyst samples remains a challenge. In this work, different calibration strategies for quantitative LA-ICP-MS analysis of fused catalyst samples were evaluated. The traditional strategy relied on external calibration against certified reference materials (CRMs) combined with internal standardization and was considered the reference approach. When using this approach, the relative bias with respect to the reference value was found to be <15%. Two novel calibration strategies were introduced and compared: a so-called multi-signal calibration approach and a solution-based calibration approach. The multi-signal calibration strategy involved varying the laser repetition rate (20, 30, 40 and 50 Hz) or laser beam diameter (10, 12, 15 and 20 μm), allowing a calibration curve to be constructed by comparing the analytical signal intensity for a single solid CRM with that for the sample, thus partially overcoming the shortage of CRMs for quantitative LA-ICP-MS analysis. The solution-based calibration approach was used for quantitative multi-element analysis without the need for any solid standard and required only minor hardware modifications to accommodate the introduction of aqueous standard solutions for calibration. Various glass certified reference materials were used for method development, calibration, and validation purposes. Furthermore, two fused alumina catalyst samples (used in the context of petroleum refining processes) were successfully analyzed as a proof-of-concept application. For both the multi-signal (matrix-matched conditions) and the solution-based calibration approaches, the average relative bias between the experimentally determined and certified/reference concentrations varied between −9% and +7%.