Enhanced sensitivity in inductively coupled plasma sector field mass spectrometry for direct solid analysis using laser ablation (LA-ICP-SFMS)

Abstract

Different hardware components in the interface region of an inductively coupled plasma sector field mass spectrometer (ICP-SFMS), like sample cone dimensions and material, the interface pressure and ion sampling position, were modified to study their contribution to ion extraction and transmission from a plasma operated under dry conditions as used for laser ablation. Analytical figures of merit, such as background levels, limits of detection and signal intensities, used to describe the effects in the interface were determined on two well-known standard reference materials, NIST SRM 612 and MPI-DING (GOR128-G). An aluminium cone with a sample cone orifice diameter of 0.8 mm and an aluminium skimmer with a cone diameter of 1.0 mm together with a second vacuum pump gave a vacuum pressure of 1.8 × 10⁻⁷ mbar. The blank level signals for elements in the lower mass region (e.g., Na, Si, Ca) decreased by a factor of 10, whereas the sensitivity for these elements increased 10 times. Thus, after incorporating such modifications, the limits of detection for elements suffering from elevated background intensities were improved by two orders of magnitude. For elements in the higher mass region (starting from mass 85), the limits of detection were in the low ng g⁻¹ range for a 30 µm diameter laser spot. Modification to the sampling cones resulted in the reduction of oxide formation, which was demonstrated by monitoring the ThO⁺/Th⁺ ratio (0.01%). These interface changes made it possible to, for example, decrease the laser spot size down to 16 µm when directly sampling the geological glass reference material, MPI-DING (GOR128-G), while retaining trace element capabilities in the concentration range of 100 ng g⁻¹. An agreement between the measured and the reported reference values of 5–10% was achieved, showing the potential for higher lateral resolution analyses when using sector field instruments for direct solid sampling.