Stoichiometric Measurement of Hydroxyapatite by Atom Probe Tomography: Effects of UV and Deep UV Laser-assisted Analytical Conditions

Application of atom probe tomography to electrically non-conductive materials is typically enabled by pulsing a laser onto a sample under strong electric fields to induce field evaporation. The measured composition depends on the laser-material interaction, necessitating systematic optimization experiments. This is particularly important for hydroxyapatite (Ca10(PO4)6(OH)2), a biologically and geologically relevant mineral for which subtle compositional changes can have significant implications. Therefore, we performed a series of experiments on synthetic hydroxyapatite to systematically assess how the laser pulse energy, definition of ranges in the mass-to-charge state spectrum, and calcium charge state ratio have an impact on the measured calcium-to-phosphorous ratio and mechanism of field evaporation on separate atom probe systems equipped with ultraviolet (355 nm wavelength) and deep ultraviolet (257.5 nm) lasers. We also evaluated the stoichiometric accuracy of the simultaneous voltage pulsing mode on the deep ultraviolet system, which both reduces the background and introduces artifacts into the mass-to-charge state spectrum. Correlations between the calcium-to-phosphorus ratio and the charge state ratio and fraction of ions ranged were identified. In turn, these analyses provide guidance for improving measurement accuracy of hydroxyapatite and other insulating materials using atom probe tomography.