Time-of-flight secondary ion mass spectrometry (ToF-SIMS)

Surface composition of the composite formulations was characterized using Time-of-flight secondary ion mass spectrometry (nanoToF instrument, Physical Electronics Inc., Chanhassen, Minnesota, USA) as described elsewhere with slight modifications (Q. Zhou et al., 2011). Data were obtained from 4 areas (100 × 100 μm each) per sample. Characteristic peak fragments for azithromycin and L-leucine were identified. For colistin, the peaks at m/z ~30 atomic mass unit (amu) and ~86 amu, corresponding to [CH₄N⁺] and [C₅H₁₂N⁺] fragments, respectively, were selected. For meropenem, the fragment at m/z ~ 68 amu corresponding to [C₄H₆N⁺] was selected as the characteristic peak. Sample spectra were processed using the WincadenceN software (Physical Electronics Inc., Chanhassen, MN, USA) to construct high-resolution surface composition maps.