Direct analysis of a 25% sodium chloride sample matrix using the Thermo Scientific iCAP RQ ICP-MS with argon gas dilution

Significance of the Topic

Ultra-high matrix samples such as 25% (w/w) sodium chloride pose significant challenges for inductively coupled plasma mass spectrometry (ICP-MS) due to matrix suppression, instrument contamination and signal instability. Developing robust, cost-effective methods for direct analysis enables laboratories to maintain throughput and accuracy without extensive sample pretreatment.

Objectives and Study Overview

This application brief presents a method for the direct determination of trace elements in a 25% NaCl matrix using the Thermo Scientific iCAP RQ ICP-MS with argon gas dilution (AGD) and a segmented flow sample introduction system. The goals were to:

• Minimize matrix effects and memory carryover.
• Maintain stable internal standard responses over extended runs.
• Achieve detection limits below 1 μg·L⁻¹ for most analytes.

Methodology and Instrumentation

Samples were introduced via an SC-2 DX autosampler equipped with a FAST valve, enabling online 14× dilution with an internal standard solution (Ge, In, Tb, Ir, Bi at 20 μg·L⁻¹). The iCAP RQ ICP-MS was operated in kinetic energy discrimination mode using helium collision gas, coupled with AGD to reduce the effective total dissolved solids. Daily autotuning ensured consistent plasma conditions. A four-point calibration (0–100 μg·L⁻¹, except Hg 0–4 μg·L⁻¹) was performed in 2% HNO₃.

Instrumentation Used

• Thermo Scientific iCAP RQ ICP-MS
• Argon gas dilution module
• SC-2 DX Autosampler with FAST valve (Elemental Scientific)
• Peristaltic pump for sample loop loading

Results and Discussion

Internal standard signals remained stable across more than 50 injections over 4.5 hours, indicating minimal suppression when switching from calibration to 25% NaCl matrix. Spike recoveries (25 μg·L⁻¹; Hg at 1 μg·L⁻¹) ranged from 85 to 116% with RSDs below 5.3%. Instrument detection limits in the high-salt matrix were typically below 1 μg·L⁻¹. Elements with higher first ionization potentials (e.g., Zn, As, Se, Cd) exhibited slightly reduced sensitivity, partially mitigated by argon humidification.

Benefits and Practical Applications

Direct analysis of ultra-high salt matrices without extensive dilution or sample prep reduces consumable use and turnaround time. Stable internal standard performance under AGD conditions enhances quantitation accuracy for trace elements in brines, seawater concentrates and salt-rich industrial streams.

Future Trends and Potential Applications

Advancements in automated gas dilution and segmented flow introduction may enable higher dilution factors and broader matrix compatibility. Integration with high-throughput autosamplers and advanced collision-reaction cell chemistries could further lower detection limits and expand analyte scope in challenging matrices.

Conclusion

The combination of AGD, segmented flow introduction and a robust ICP-MS platform enables reliable, direct analysis of 25% NaCl samples. This approach offers accurate trace element quantification with low detection limits and minimal instrument maintenance.