Lipid specific molecular ion emission as a function of the primary ion characteristics in TOF-SIMS

In the present work, the emission characteristics of lipids as a function of the primary ion cluster size and energy were studied using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Characteristic fragmentation patterns for common lipids are described, and changes in secondary ion (SI) yields using various primary ion beams are reported. In particular, emission characteristics were studied for pairs of small polyatomic and nanoparticle primary ion beams (e.g., Bi₃⁺ versus Ar₁₀₀₀⁺ and Au₃⁺ versus Au₄₀₀⁺⁴) based on the secondary ion yield of characteristic fragment and intact molecular ions as a function of the lipid class. Detailed descriptions of the fragmentation patterns are shown for positive and negative mode TOF-SIMS. Results demonstrate that the lipid structure largely dictates the spectral presence of molecular and/or fragment ions in each ionization mode due to the localization of the charge carrier (head group or fatty acid chain). Our results suggest that the larger the energy per atom for small polyatomic projectiles (Bi₃⁺ and Au₃⁺), the larger the SI yield; in the case of nanoparticle projectiles, the SI increase with primary ion energy (200-500 keV range) for Au₄₀₀⁺⁴ and with the decrease of the energy per atom (10-40 eV/atom range) for Ar_n=500-2000⁺ clusters. The secondary ion yield of the molecular ion of lipids from a single standard or from a mixture of lipids does not significantly change with the primary ion identity in the positive ion mode TOF-SIMS and slightly decreases in the negative ion mode TOF-SIMS.