A Trial Modification of a Surface Ionization Ion Source for Direct Sample Loading for Isotope Analysis.

Abstract

A trial modification was carried out to the conventional surface (thermal) ionization ion source. A direct sample loading of milligram sized chip of solid sample completely free from terrestrial chemical additional reagents is adopted in the modification. A milligram sized block sample is set into a kind of boat filament made of tantalum, heated and evaporated in the form of atomic vapor with no addition of silica gel or phosphoric acid. This point is perfectly different from other previous works of direct loading methods. The atoms are ionized at the surface of the side filament made of rhenium. The direct loading ionization system was installed in a virtual image compact double focusing mass spectrometer whose mass resolving power is reached over 5,000. As an example of isotope analysis, a 0.5 mg weighed chip of commercially sold pure magnesium metal was loaded as a sample. It gave reasonable isotope ratio 25Mg/24Mg=0.12472±0.00045 and 26Mg/24Mg=0.13564±0.00050; δ26=0.15‰. Several terrestrial mineral samples, forsterite, enstatite, hypersthene, hornblende, and serpentine were also analyzed for magnesium isotope ratios, and δ26's for those samples were distributed -1.7‰ to +4.9‰. Although the reproducibility of isotope analysis is ±3.5‰, and may be comparatively too large to apply the detection of minute isotope anomaly, the newly developed ion source was found to be usable for the isotope analysis of a milligram sized solid samples by the method of direct loading completely free from the additives of chemical reagents.