(c) Time-of-flight secondary ion mass spectrometry

The fragment of chert containing the R. gwynne-vaughanii was mounted directly onto a sample holder using stainless steel screws and clips for ToF-SIMS analysis. Static SIMS analyses were carried out using an ION-TOF ‘TOF-SIMS IV-200’ instrument (ION-TOF GmbH, Münster, Germany) of single-stage reflectron design [19]. Figure 2 shows a schematic illustration of the ION-TOF ‘TOF-SIMS IV-200’ instrument. Positive ion spectra and images of the sample were obtained using a Bi₃⁺ focused liquid metal ion gun at 25 keV energy, incident at 45° to the surface normal and operated in ‘bunched’ mode for high mass resolution. This mode used 20 ns wide ion pulses at 10 kHz repetition rate. Charge compensation was effected by low-energy (approx. 20 eV) electrons provided by a flood gun. The total ion dose density was less than 1 × 10¹⁶ ions m⁻². The topography of the sample surface and the ion gun mode of operation limited the mass resolution in this work to approximately m/Δm = 2000. The spatial resolution was limited by the primary ion beam diameter to approximately 4 µm.

Schematic view of ToF-SIMS spectrometer.

Positive and negative ion static SIMS spectra and images were recorded from the outermost approximately 1 nm of the sample surface at room temperature. Raw data containing the secondary ions recorded at each pixel were acquired with a 256 × 256 pixel raster and a field of view of 500 µm × 500 µm. Four adjacent 500 µm × 500 µm analysis areas were mapped starting at the approximate centre of the specimen and then moving the sample radially outward by 500 µm each time. The third area included the epidermis of the R. gwynne-vaughanii and some matrix material, while the final area included only the matrix for comparison.