Benefits of NaCl addition for time-of-flight secondary ion mass spectrometry analysis including the discrimination of diacylglyceride and triacylglyceride ions

Rationale: Diacylglycerides (DAGs) and triacylglycerides (TAGs) are two important lipid classes present in all mammalian cells that share similar chemical structures but differ in biological function in cells and tissues. Differentiation of these two species during time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis is therefore important, but has been difficult due to the formation of DAG-like ions during the ionization process of TAGs. Methods: We investigated the use of salt adduct formation as a quick and simple method to determine the origin of the DAG-like ions in ToF-SIMS spectra. NaCl was added to lipid standards of a DAG and a TAG and differences in fragmentation patterns were identified. The salt was then applied to prepared tissue samples by spraying with a saturated solution of NaCl in methanol and samples were analysed with ToF-SIMS using a 40 keV (CO₂)_6k⁺ primary ion beam. Results: A 40 Da peak shift was observed in the DAG spectrum that was not observed in the TAG spectrum ([M + H − H₂O]⁺ to [M + Na]⁺) while the isobaric [M − RCOO]⁺ peak did not shift allowing differentiation between the two species. Spraying NaCl on to tissue sections indicated that the DAG-like ions originated from TAGs. Conclusions: With the method described in this paper, simple addition of salt by spraying on the sample leads to better interpretation of complex mass spectra from biological tissue samples, discriminating DAG and TAG fragment peaks.