Undersized (12.5mm diameter) glass beads with minimal amount (11mg) of geochemical and archeological silicic samples for X-ray fluorescence determination of major oxides

The small glass bead technique was developed to assay precious silicic samples for geochemical and archeological analyses. Undersized (12.5mm diameter) glass beads were prepared for wavelength dispersive X-ray fluorescence determination of major oxides (Na ₂O, MgO, Al ₂O ₃, SiO ₂, P ₂O ₅, K ₂O, CaO, TiO ₂, MnO, and total Fe ₂O ₃). Synthetic calibration standards were prepared by compounding chemical reagents (oxides, carbonates, and diphosphate). For reliable calibration, recipe compositions of standard specimens were constructed on the basis of the evenness of plot intervals on a calibration curve. Calibration curves showed good linearity (correlation coefficient: r>0.998). Analytical results of major oxides in granitic and basaltic rocks, obsidian, and ancient pottery were obtained with good precision (relative standard deviations were the following: <3% for more than 10.0 mass% of analyte, <5% for 1.0-10.0 mass% of analyte, and <15% for 0.1-1.0 mass% of analyte). Lower limits of detection were roughly a sub-percentage of analyte in an unprepared sample: 0.3 mass% for Na ₂O, 1.0 mass% for SiO ₂, 0.01 mass% for MnO, and so on. The present preparation reduced the following analytical scales: (1) amount of sample from 400mg to 11mg (97% cutting out), (2) amount of Li ₂B ₄O ₇ as an alkali flux from 4000mg to 396mg (90% cutting out), and (3) weight of platinum crucible from 80g to 12.5g (84% cutting out). This small-scale preparation might enable us to conduct destructive pretreatment of valuable and limited silicic samples such as archeological ceramics and stone tools, geochemical minerals in rock, and sediments and sand.