Phi trift 5 nanotof instrument

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) experiments were performed using a PHI TRIFT V nanoTOF instrument (Physical Electronics Inc., Chanhassen, MN, USA) equipped with a pulsed liquid metal ⁷⁹⁺Au primary ion gun (LMIG), operating at 30 keV energy. Dual charge neutralization was provided by an electron flood gun and 10 eV Ar⁺ ions. Spatial resolution was optimized for the collection of images by utilizing “unbunched” Au₁ instrument settings. Raw data were collected in positive SIMS mode at a number of locations, typically using a 50 × 50 micron raster area, with 4 minute acquisitions (24 (link)).
Analysis of colistin and rifampicin standards identified characteristic peak fragments for use in mapping the components in the combination powders. Peaks corresponding to the protonated molecular ion signal for each antibiotic were of low intensity; hence higher intensity characteristic fragments were used instead. For colistin, the sum of peaks at m/z ~ 30 atomic mass unit (amu) and ~86 amu were selected, corresponding to CH₄N⁺ and C₅H₁₂N⁺ fragments respectively. For azithromycin, the desosamine fragment at m/z ~ 158 amu (C₈H₁₆NO₂⁺) and further fragment at m/z ~ 98 amu (C₆H₁₂N⁺) were selected. Sample spectra and images were processed using WincadenceN software (Physical Electronics Inc., Chanhassen, MN, USA).