GD-MS analysis of sulphur at the ultra-trace level

Importance of the Topic

Precise determination of sulphur at sub-ppm levels in nickel alloys is critical for maintaining high-temperature oxidation resistance and mechanical integrity of aerospace turbine components.

Objectives and Overview

This application note demonstrates the use of high-resolution glow discharge mass spectrometry (GD-MS) for ultra-trace analysis of sulphur in solid nickel alloy samples. The study aims to establish reliable quality control methods for production processes and final products.

Methodology

Sample preparation involved wet grinding with grit 80 silicon carbide paper followed by a finish with grit 120 to remove surface contamination. Analysis used a pre-sputter of 12 minutes and an acquisition time of 8 minutes. Sulphur 32S was separated from the 16O2 interference in medium resolution (R=4000). Quantification was based on ion beam ratios corrected for isotopic abundance, applying relative sensitivity factors derived from reference materials. Calibration utilized a single point with NIST SRM 1249.

Applied Instrumentation

• Thermo Scientific Element GD Plus high-resolution glow discharge mass spectrometer
• Argon discharge gas flow: 450 mL/min at ~800 V discharge voltage and 45 mA current
• Steel anode, graphite cap, flow tube, and cone
• Gas purification kit and coated transfer capillaries to minimize background sulphur

Main Results and Discussion

Three nickel alloy samples with expected sulphur levels from sub-ppm to single ppm were analyzed on different days. Results yielded:

• Sample 1: 0.061 ± 0.003 ppm S (RSD 5.7%)
• Sample 2: 0.350 ± 0.010 ppm S (RSD 1.7%)
• Sample 3: 4.00 ± 0.10 ppm S (RSD 1.7%)

Background contributions from stainless steel gas lines were mitigated by implementing a gas purification cartridge and specialized capillaries.

Benefits and Practical Applications

The GD-MS method provides:

• Accurate sulphur quantification at ppb to low ppm levels
• Excellent repeatability (2% RSD at ~0.3–4 ppm, 6% RSD at sub-ppm)
• Direct solid sampling without complex chemical digestion
• Rapid analysis suitable for quality assurance in aerospace alloy production

Future Trends and Potential Applications

Upcoming developments may include:

• Further reduction of detection limits via advanced gas purification and instrument optimization
• Extension of the approach to other challenging elements and matrices
• Integration with automated sample handling and data processing workflows
• Retrofitting existing GD-MS systems with low-background configurations

Conclusion

The Thermo Scientific Element GD Plus mass spectrometer enables precise and reliable ultra-trace sulphur analysis in nickel alloys. The method achieves low-ppm to ppb quantification with high repeatability, supporting stringent quality control requirements in high-performance materials.

References

• NIST SRM 1249 Nickel-base superalloy Inconel 718, certified sulfur content 6.4 ± 1.0 ppm
• Thermo Fisher Scientific Application Note 30237-EN (2018)