Quantification of laser-induced aerosols by microdroplet calibration and investigation of matrix effects using LA-ICP-TOFMS†

Abstract

The application of monodisperse microdroplets for non-matrix-matched quantification in LA-ICP-TOFMS was investigated for inorganic and organic matrices. Suppression behavior in droplet signals caused by addition of typical major elements of geological samples (Al, Si, Ca, Ti, and Fe) in the μg g⁻¹ range was studied using microdroplets introduced via a falling tube and compared to solution nebulization. Signal suppression patterns observed for microdroplets could be attributed to neither mass load effects nor in-plasma oxide formation, nor reproduced via solution nebulization, suggesting a fundamentally different behavior of microdroplets in the plasma. Radial diffusion profiles were acquired to assess in-plasma behavior of droplets and laser-induced aerosol from NIST SRM 610 (glass). Diffusion profiles overlapped and showed similar full width at half maxima (FWHM) for microdroplets and the laser-induced aerosol, with minor spatial shifts in intensity maxima, likely due to not complete on-axis droplet introduction into the plasma. Quantification based on microdroplet calibration yielded relative deviations from reference values below ±20% across certified reference materials and an in-house prepared gelatine standard. Quantification of gelatine samples using NIST SRM 610 (glass) as an external standard resulted in larger deviations compared to droplet-based calibration, which yielded values in agreement with digestion data. These results demonstrate the suitability of monodisperse microdroplets for non-matrix-matched calibration in LA-ICP-TOFMS, particularly for elements non-certified or uncommonly reported in reference materials used in LA-ICP-MS.