Secondary Ion Mass Spectrometry Reference Materials for Lithium in Carbonaceous Matrices

Secondary ion mass spectrometry techniques are used to study trace elements in organic samples where matrix compositions vary spatially. This study was conducted to develop calibrations for lithium content and lithium isotope measurements in kerogen. Known concentrations of Li ions (⁶Li and ⁷Li) were implanted into organic polymers, with a range of H/C and O/C ratios similar to kerogen, along with glassy carbon (SPI Glas-22) and silicate glass (NIST SRM 612). Results show that Li content calibration factors (K*) are similar for carbonaceous samples when analysed using a 5 kV secondary ion accelerating voltage. Using a 9 kV secondary ion accelerating voltage, K* factors are negatively correlated with the sample O content, changing ~ 30% between 0 and 15 oxygen atomic %. Thus, to avoid the matrix effect related to O content, using a 5 kV secondary ion accelerating voltage is best for quantification of Li contents based on ⁷Li⁺/¹²C⁺ ratios. Under these analytical conditions, Li ppm (atomic) = (132 (± 8) × ⁷Li⁺/¹²C⁺) × ¹²C atom fraction of the sample measured. Lithium isotope ratio measurements of SPI Glas-22 and NIST SRM 612 are within uncertainty; however, the organic polymer samples as a group show a 10‰ higher δ⁷Li than NIST SRM 612.