Addressing the challenge of measuring difficult elements using triple quadrupole ICP-MS

Importance of the Topic

The accurate measurement of silicon, phosphorus, sulfur, arsenic and selenium at trace levels is critical in environmental monitoring, industrial quality control and life science research. These elements present challenges in inductively coupled plasma mass spectrometry (ICP-MS) due to high ionization potentials and isobaric or polyatomic interferences, leading to elevated detection limits and compromised data quality.

Objectives and Study Overview

This technical note demonstrates how a triple quadrupole ICP-MS system equipped with an oxygen reaction cell and a PLUS torch can overcome these obstacles. The goal is to achieve sub-ng·mL⁻¹ detection limits for five traditionally difficult analytes (Si, P, S, As, Se) by applying mass-shift reactions to separate analyte signals from interferences.

Methodology and Instrumentation

• Instrument: Thermo Scientific iCAP TQ ICP-MS with PLUS torch and quartz cyclonic spray chamber cooled to 2.7 °C.
• Nebulizer and Interface: Borosilicate glass Micromist nebulizer (400 µL·min⁻¹), nickel sampler and skimmer cones.
• Operating Conditions: Plasma power 1550 W, nebulizer gas 1.09 L·min⁻¹, cell gas O₂ at 0.34 mL·min⁻¹, QCell in TQ-O₂ mode, dwell time 0.1 s per isotope.
• Mass-Shift Strategy: Conversion of target ions into oxide forms (e.g. 28Si→28Si¹⁶O⁺ at m/z 44, 31P→31P¹⁶O⁺ at m/z 47) to remove isobaric and polyatomic overlaps.
• Calibration: Gravimetric dilution of single-element SPEX CertiPrep standards in 2% HNO₃; ranges spanned 0.005 µg·L⁻¹ to 10 µg·L⁻¹ depending on analyte.

Main Results and Discussion

• All analytes exhibited linear calibration (R²>0.999) after mass-shift in the reaction cell.
• Survey scans for selenium (m/z 90–100) confirmed complete interference removal, matching the natural isotopic pattern of ⁸⁰Se¹⁶O⁺, ⁷⁸Se¹⁶O⁺, etc.
• Instrumental detection limits ranged from 0.0004 µg·L⁻¹ for arsenic to 0.026 µg·L⁻¹ for sulfur; blank equivalent concentrations remained at or below 1.17 µg·L⁻¹.
• Precision tests at near-background levels (1 µg·L⁻¹ for Si, P, S; 5 ng·L⁻¹ for As, Se) yielded RSDs below 8%, typically under 2% for most elements.

Benefits and Practical Applications

• Substantial reduction of spectral interferences and background signals via mass-shift reactions.
• Sub-µg·L⁻¹ detection limits support ultra-trace analysis in complex matrices.
• Enhanced data quality and reproducibility facilitate reliable monitoring in environmental, industrial and life science laboratories.

Future Trends and Potential Applications

The combination of reactive cell gases and advanced torch designs paves the way for analyzing additional high-ionization-potential elements. Integration with automated sample preparation, expanded speciation studies and routine ultra-trace screening in clinical and battery research are promising future directions.

Conclusion

Triple quadrupole ICP-MS with oxygen reaction gas and PLUS torch technology offers a robust, sensitive and interference-free solution for the determination of traditionally challenging elements. Demonstrated sub-ng·mL⁻¹ detection limits and high precision underscore its value for modern analytical workflows.

References

• Thermo Scientific Application Note 44465: Analytical testing of trace elements in refinery products using a robust ICP-MS approach.
• Thermo Scientific Application Note 44484: Overcome unexpected interferences and accelerate environmental analysis using triple quadrupole ICP-MS.
• Thermo Scientific Application Note 44478: Effectively increase sample turnover and boost data confidence using triple quadrupole ICP-MS.
• Thermo Scientific Product Spotlight 44485: Thermo Scientific iCAP Qnova Series ICP-MS PLUS Torch for improved ICP-MS analysis of challenging samples.