Determining Si/Al atomic ratios in particulate matter via quadrupole single-particle ICP-MS with combined unit-mass and bandpass resolution modes: a proof-of-concept study on Antarctic snow.

Micrometer-scale mineral dust deposited in Antarctica represents a valuable archive for reconstructing atmospheric variability and for improving our understanding of dust-climate interactions. Among the various types of particle-specific information available, elemental ratios, such as the Si/Al atomic ratio, are especially useful as geochemical indicators of particle composition and provenance. However, the nature of these samples, characterized by coexisting particle populations and limited particle numbers, makes this determination particularly challenging. In this work, a novel quadrupole-based single-particle ICP-mass spectrometry (SP-ICP-QMS) approach to determine population-specific Si/Al atomic ratios containing chemically heterogeneous particle populations is presented. To overcome the sequential acquisition inherent to ICP-QMS instrumentation, Al and Si measurements at unit-mass resolution (m/z 27 and 28, respectively) were combined with bandpass acquisition centered at m/z 27 (2.47 ± 0.06 amu resolution at 10% peak height), enabling simultaneous collection of 27Al+ and 28Si + signals. This strategy was first used to identify the Al and Si distributions belonging to the same particle population and, consequently, to determine the corresponding atomic ratios in a suspension containing SiO2 microparticles (MPs) and two different types of synthesized zeolite MPs with different Si/Al atomic ratios. The reliability and accuracy of the method were confirmed by comparing the results with those obtained from individual Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX) and SP-ICP-MS characterization of the zeolite samples. As a proof-of-concept, the analysis of an Antarctic snow sample demonstrated the potential of the developed method to provide population-specific information - Si/Al atomic ratios - essential for environmental assessments based on high-resolution short-term records.