ToF-SIMS experiments were performed using a PHI TRIFT5 nano
ToF equipped with a gold (Au) liquid metal ion gun operated in
pulsed mode. The mineral samples were mounted on indium foil
and dried under high vacuum 4 × 10−7 Pa. An acceleration voltage
of 30 kV with “unbunched” beam setting was used to produce a
high spatial resolution with a spot size of ~150 nm.With the analysis
time of 2 min/frame, the primary ion dose was ≤1013 ions/cm2,
which ensured static conditions and minimal surface damage. Both
positive and negative ion spectra were acquired using a raster size
of either 200 × 200 μm or 250 × 250 μm depending on the number
of particles analysed per frame. Each particle was defined as a
region of interest (ROI). ROI boundaries were set within the contrast
edges of the regions. Positive ion mass spectra were calibrated to
CH3
+, C2H5
+, C3H7
+ and negative ion mass spectra were calibrated to
CH−, C2H− and Cl− peaks. Twenty four particles (ROI) were investigated
at each flotation conditions to obtain statistically significant
results [20]. Prior to statistical analysis (analysis of mean) the intensities
of the prominent secondary ions in the ToF-SIMS spectra were
normalised to the total ion yield.