7th International Glow Discharge Spectroscopy Symposium

Abstract:

Determination of trace metallic impurities in Electronic Grade Titanium via glow discharge mass spectrometry (GDMS) was conducted following the ASTM Standard Test Method F1710-08, developed on the previous generation of commercial GDMS instruments, Model VG-9000. The method was withdrawn in 2023, with no replacement. Since then, newer GDMS instruments have been introduced to the market, specifically, Ametek Nu Instruments Astrum family of GDMS instruments (Astrum, Astrum ES, and most recently Astrum Swift; referred to here as Astrum). The Astrum is a low-flow, direct current, cryogenically cooled GDMS. The Astrum was designed to replace the VG-9000, but there are key differences between the designs of the two instruments. The Astrum is a forward-geometry magnetic sector instrument. It utilizes an ion counter, as opposed to a Daly counter, which provides lower background noise due to the difference between the electron and photon multipliers. The newer optics in the Astrum also translate to better abundant sensitivity relative to the VG-9000 while maintaining comparable sensitivity. These differences warrant a thorough investigation of the Astrum’s applicability to analyse high purity Ti samples.

Method F1710-08 specifies the Ti sample should be in the co-axial pin configuration. Controlling the sample pin dimensions is extremely difficult, especially if chemical etching is used to clean the pin surface. This can result in different electron densities on the sample surface, which would alter the plasma conditions from the reference material used to tune the instrument. High purity Ti also has low thermal conductivity (22 W/mK). This low thermal conductivity results in a large temperature gradient between the plasma-exposed end to the cooled end in the sample holder. Both these are significantly affecting the measurements repeatability and reproducibility in high purity Ti measurements.

An elegant solution to these challenges is changing the analytical geometry from co-axial to flat. By utilizing the flat geometry, we mitigate the electron density difference between Ti samples because the area exposed to the plasma is the same diameter. Similarly, the analyzed surface no longer experiences a temperature gradient across it since the sample is exposed to the same plasma is a flat surface, and not a pin extending into the plasma with an increasing distance from the cooling contact.

We analyzed high purity Ti reference materials by flat geometry on the Astrum ES. The data show that the Astrum ES provides stable and reproducible results over multiple analytical days. These results show the importance of reproducibility of the analysis as the industry moves to qualify a new standard method.o