The influence of polyatomic primary ion chemistry on matrix effects in secondary ion mass spectrometry analysis

Rationale: The application of mass spectrometry imaging techniques to determine 2D and 3D chemical distribution ideally provides uniform, high sensitivity to multiple components and reliable quantification. These criteria are typically not met due to variations in sensitivity due to the chemistry of the analyte and surrounding surface chemistry. Here we explore the influence of projectile beam chemistry and sample chemistry in time-of-flight secondary ion mass spectrometry (ToF-SIMS). To the authors’ knowledge this is the first time the combined effects of projectile chemistry and sample environment on the quantitative determination of mixed samples have been systematically studied.
Methods: Secondary ion yields of lipid and amino acid mixtures were measured under 20 keV C60, Arn, and (H2O)n cluster ion bombardment (n=2000 or4000) using ToF-SIMS. Ion suppression/enhancement effects were studied in dry sample films and in trehalose and water ice matrices.
Results: The extent of the matrix effects as well as the secondary ion yield was found to depend on the chemistry of the primary ion beam and (for C60, Arn) on the nature of the sample matrix. Under (H2O)n bombardment the sample matrix had negligible effect on the analysis.
Conclusions: Compared to C60 and Arn, water-containing cluster projectiles enhanced the sensitivity of ToF-SIMS determination of the chosen analytes and reduced the effect of signal suppression/enhancement in multicomponent samples and in different sample matrices. One possible explanation for this is that the (H2O)4000 projectile initiates on impact a nanoscale matrix environment that is very similar to that in frozen hydrated samples in terms of the resulting ionisation effects. The competition between analytes for protons and the effect of the sample matrix is reduced with water-containing cluster projectiles. These chemically reactive projectile beams have improved characteristics for quantitative chemical imaging by ToF-SIMS compared to their non-reactive counterparts.