The Determination of Sodium, Calcium and Silicon in Pure Water by Graphite Furnace AA

Importance of the Topic

In semiconductor manufacturing, trace metal contaminants in ultrapure rinse water can lead to electrical shorts and reduced device yield. Continuous monitoring of sodium, calcium and silicon at sub-ng/mL levels is critical for maintaining process integrity and product reliability.

Objectives and Study Overview

This application note applies graphite furnace atomic absorption spectroscopy (GFAAS) to quantify sodium, calcium and silicon in ultrapure water. The study aims to establish sensitive calibration procedures, determine detection limits under standard protocols (IUPAC and DIN 38 402), and evaluate reproducibility, including the effect of multiple injections.

Methodology and Instrumentation

Instrumentation:

• Atomic absorption spectrometer: Agilent SpectrAA-30A
• Graphite furnace atomizer: GTA-96 with PSD-96 programmable sampler
• Hollow cathode lamps for Na, Ca, Si
• Argon (99.999%) as purge gas
• Ultrapure water system: ion-exchange filter (Servo Mischbettfilter) and Nanopure Reinstwasseranlage D-1797

Key analytical settings included element-specific furnace temperature programs, peak-height absorbance mode, and a 0.5% v/v nitric acid modifier for calcium. Calibration standards ranged from 1 ng/mL Na, 5 ng/mL Ca to 100 ng/mL Si, dispensed automatically in up to five replicates.

Results and Discussion

Calibration for sodium and calcium was linear (r>0.996) with RSDs below 9%. Silicon required a second-order polynomial fit due to non-linearity. IUPAC detection limits were 0.06 ng/mL (Na), 0.13 ng/mL (Ca) and 2.1 ng/mL (Si). DIN 38 402 limits for Na and Ca were 0.05 ng/mL and 0.20 ng/mL, respectively. Multiple injections enhanced signal proportionally, suggesting a 10–20× improvement in detection capability.

Benefits and Practical Applications

The described GFAAS method delivers high sensitivity and reproducibility for ultratrace analysis in ultrapure water. Automated sampling and multiple injection features support routine QA/QC in semiconductor fabs, reducing manual handling and increasing throughput.

Future Trends and Potential Applications

Future developments may focus on automated multi-element furnace sequences, tighter integration with inline process control, and further sensitivity gains via advanced chemical modifiers or laser-based atomization. Extending the approach to additional critical metals (e.g., iron) will broaden its utility in high-purity applications.

Conclusion

A robust GFAAS protocol has been validated for measuring sodium, calcium and silicon in ultrapure rinse water at sub-ng/mL levels. Optimized furnace programs, chemical modification and multiple injection strategies achieve the precision and detection limits required for semiconductor process monitoring.

Reference

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• G. L. Long, J. D. Winefordner, Anal. Chem. 55, 713A–724A (1983)
• Deutsche Einheitsverfahren DIN 38 402, Part 5: Water, wastewater and sludge analysis