Sub-ppt level automated analysis of impurities in nitric acid–hydrogen peroxide solution via HR-ICP-MS

Significance of the Topic

High purity mineral acids such as nitric acid are fundamental in ultra-trace element analysis by ICP-MS, particularly in academic research, commercial laboratories, and semiconductor manufacturing. Even minute metal impurities can compromise analytical accuracy and process quality, making sensitive, interference-free screening methods essential.

Objectives and Study Overview

This study evaluates automated, sub-ppt detection of 68 elemental impurities in a 2% HNO₃/2% H₂O₂ matrix using a Thermo Scientific Element 2 High Resolution ICP-MS. The aim is to determine Limits of Detection (LODs) and Background Equivalent Concentrations (BECs) under hot and cold plasma modes, demonstrating suitability for high-purity reagent quality control.

Methodology and Instrumentation

An Element 2 HR-ICP-MS equipped with magnetic sector technology was used, offering low (LR = 300), medium (MR = 4000), and high (HR = 10000) resolution settings. A PFA nebulizer and double-pass spray chamber, sapphire injector, and platinum/nickel cones minimized contamination. Samples and blanks were introduced via an autosampler. Hot plasma (1200 W) maximized sensitivity, while cold plasma (600 W) reduced argon-based interferences and torch-derived backgrounds. LODs and BECs were calculated from five blank replicates and a multi-element standard, covering analytes from ⁹Be to ²³⁸U. The total analysis time per sample was approximately six minutes, including automatic switching between plasma modes.

Main Results and Discussion

Most elements achieved LODs below 1 ppt in hot plasma. Cold plasma further lowered LODs for species affected by argon interferences: for example, 44Ca improved from 16 ppt (HP, MR) to 5 ppt (CP, LR), and 39K, 56Fe, 58Ni all reached ≤1 ppt in CP, MR. The absence of 88Sr²⁺ under cold plasma allowed interference-free measurement of 44Ca. High resolution eliminated the need for reaction gases, simplifying the workflow. Background stability and low BECs confirmed the robustness of the method for ultra-pure acid screening.

Benefits and Practical Applications

• Rapid, fully automated multi-element screening with minimal user intervention.
• Interference-free quantification across a broad mass range without reaction gases.
• Customizable resolution and plasma power to optimize LODs for specific elements.
• Ideal for semiconductor, pharmaceutical, and environmental laboratories requiring ultra-pure reagents.

Future Trends and Potential Uses

Innovations may include real-time plasma tuning, enhanced automation for high-throughput workflows, and integration with hyphenated separation techniques for speciation analysis. Advances in sector field technology could extend detection capabilities to novel isotopic and molecular species, supporting evolving analytical challenges.

Conclusion

The Element 2 HR-ICP-MS, paired with optimized hot and cold plasma conditions and an inert sample introduction system, delivers fast, automated detection of sub-ppt impurities in nitric acid–hydrogen peroxide matrices. This approach meets stringent purity requirements, providing reliable quality control for high-purity reagents in research and industrial settings.