Capabilities of laser ablation inductively coupled plasma time-of-flight mass spectrometry

Abstract

In this paper, we characterize an inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) instrument (icpTOF, TOFWERK AG, Thun, Switzerland) in combination with laser-ablation sample introduction. Three sample introduction approaches for LA-based ICP-TOFMS analysis are described: (1) steady-state LA with a conventional high-dispersion LA cell, (2) single-pulse analysis with large spot sizes (44 μm diameter) using a low-dispersion LA cell, and (3) pulse-resolved, high-speed, high-resolution (5 μm spot sizes) elemental imaging with the same low-dispersion LA cell. These sample-introduction schemes span the range of approaches most interesting for users of LA-ICP-TOFMS, from routine bulk quantification, to low-sample-consumption trace-element analysis, to demanding elemental imaging applications. From steady-state signal intensities, element concentrations in NIST SRM 612 and USGS BCR-2G were quantified within the uncertainty range of the preferred values when NIST SRM 610 was used as the external reference material. Relative deviations were less than 10% in most cases. When using a 44 μm diameter spot and a laser repetition rate of 10 Hz, limits of detection (LODs) were in the single digit ng g⁻¹ range for the most sensitive isotopes. Isotope-ratio precision was in the sub per mill regime and governed by counting statistics. Similar accuracies were also achieved in low-dispersion LA-ICP-TOFMS experiments, when NIST SRM 612 and USGS BCR-2G element concentrations were quantified using signal intensities from single 44 μm diameter laser pulses. LODs were in the tens of ng g⁻¹ range for most sensitive isotopes resulting in absolute LODs in the tens of attograms. Capabilities of the icpTOF for elemental imaging are demonstrated with pulse-resolved multi-elemental imaging of a multi-phase geological thin section. With the low-dispersion LA cell and a spot diameter of 5 μm, aerosol plumes were confined to less than 10 ms, which allowed elemental imaging at a laser repetition rate of 100 Hz with minimized pulse-to-pulse mixing and an adjacent-pixel dynamic range of greater than 10². Quantitative results for elements of major, minor and trace concentrations were in agreement with bulk composition of individual regions, which had been determined via petrographic microscopy and high-dispersion laser ablation inductively coupled plasma quadrupole mass spectrometry (LA-ICPQMS). LODs were in the single digit μg g⁻¹ range for most sensitive isotopes.