Matrix effects on secondary ion emissions for H and OH from natural spinel-structured oxides: Implications for H2O quantification by secondary ion mass spectrometry

Abstract Spinel-structured oxides (hereafter “spinels”) are a group of nominally anhydrous minerals characterized by a wide range of solid solutions. To allow the analysis of hydrogen in spinels by secondary ion mass spectrometry, Relative Sensitivity Factors (RSFs), which are typically matrix-dependent, need to be determined. For this work, we have selected 11 natural spinels from the Mg-Al-Fe-Ti-Cr-Mn-Zn compositional space, ranging in density from 3.7 to >5.1 g cm–3. This is the first attempt to determine 2H and 16O2H RSF values in several crystals with different cation compositions. Samples were implanted with a fixed dose of deuterium (1 × 1015 atoms cm–2) using an ion energy of 40 keV. Subsequently, we performed depth profiling using a Cs+ primary beam on a CAMECA IMS 1280–HR ion microprobe, monitoring secondary ions of 2H– and 16O2H– until the implanted ion intensities dropped below background levels. From the reduced data, we calculated the RSFs for 2H and 16O2H relative to 18O as matrix elements. RSF values vary with Al2O3 content. RSFs for 2H yield 2.53 ± 0.99 (2SE) × 1022 atoms cm–3 at Al2O3 < 2 wt% and 1.49 ± 0.51 (2SE) × 1022 atoms cm–3 at Al2O3 ≥ 10 wt%. RSFs for 16O2H relative to 18O range from 2.59 × 1020 atoms cm–3 to 2.51 × 1021 atoms cm–3and increase with increasing Al2O3 content. Preliminary RSF data for 16O2H relative to 18O (Zellmer et al. 2025a) closely match the data presented here, and taken together, the RSF for 16O2H relative to 18O follows a second-order polynomial relationship with Al2O3 content: RSF= 1.516 × 1017Al2O32+ 2.121 × 1019Al2O3+ 2.758 × 1020where the RSF is in atoms cm–3 and Al2O3 is in wt%. Along this polynomial relationship, the relative standard error is about 23% (2SE). The overall lower RSF values of the dimer and the lower standard errors of the mean indicate that the analysis of hydrogen in spinels is best undertaken through the analysis of OH rather than H. We show how the concentration of hydrogen in spinels, C1H (in atoms cm−3), can be derived from their Al2O3 contents. This script, therefore, provides a method to calculate the hydrogen concentration, [H], in spinel-structured oxides, given by: [H] = C1H × 106 M/(ρ × NA) where square brackets denote concentration in μg/g, M is the molar mass of 1H (∼1.008 g mol–1), ρ is the density of the spinel in question (in g cm–3), and NA is the Avogadro constant (∼6.022 × 1023 atoms mol–1).