The accurate measurement of selenium in twelve diverse reference materials using on-line isotope dilution with the 8800 Triple Quadrupole ICP-MS in MS/MS mode

Importance of the Topic

The precise measurement of selenium (Se) at ultratrace levels is critical in environmental monitoring, agricultural research and health studies. Se is both an essential micronutrient and a potential toxin at higher concentrations. Accurate quantification of Se species supports regulatory compliance, risk assessment and investigations into Se’s role in disease prevention. However, Se analysis by conventional ICP-MS is hindered by poor ionization efficiency, matrix-induced signal suppression and multiple spectral overlaps.

Objectives and Study Overview

This study evaluates the performance of the Agilent 8800 Triple Quadrupole ICP-MS (ICP-QQQ) operating in MS/MS mode for reliable ultratrace Se determination in a variety of high-matrix sample types. The goals were to:

• Compare collision and reaction gas strategies (He, H₂, O₂/H₂, NH₃/H₂) using both on-mass and mass-shift measurements for ^78Se, ^80Se and ^82Se.
• Demonstrate the removal of challenging polyatomic and doubly charged interferences from Ca, Br and rare-earth elements.
• Apply on-line isotope dilution analysis (OIDA) with an ^82Se-enriched spike to certified reference materials (CRMs) without mass-bias correction.

Methodology and Instrumentation

The Agilent 8800 ICP-QQQ was configured with a glass concentric nebulizer, quartz double-pass spray chamber and quartz torch (2.5 mm ID). The triple-quad arrangement—Q1 mass filter, ORS3 collision/reaction cell, and Q2 mass filter—operated in MS/MS mode to reject non-target ions prior to the cell. Four cell gases or mixtures were tested:

• Helium (collision mode with KED)
• Hydrogen
• Oxygen/Hydrogen (mass-shift to SeO⁺ at m/z 94–98)
• Ammonia/Hydrogen

Quantification used on-line isotope dilution: an ^82Se-enriched spike (97.4% ^82Se) was mixed via the ISTD kit with each sample. The final sample contained ~1% isopropyl alcohol to improve Se ionization. Se concentrations in unknowns were calculated from measured isotope ratios without external mass-bias correction.

Main Results and Discussion

• Cell Gas Comparison in Synthetic Matrices
The background equivalent concentrations (BECs) for ^78Se, ^80Se and ^82Se were measured in four matrices (1% HNO₃, 500 ppb Br, 1 ppm mixed rare-earth elements, 200 ppm Ca in 0.5% HCl). Only the O₂/H₂ mixture in mass-shift mode provided interference-free measurement of all three Se isotopes across all matrices.

• Quantification Error without Mass-Bias Correction
An error study over 0.025–20 ppb Se (using a 2 ppb reference standard) showed that OIDA delivers 95–105% accuracy for Se concentrations between 0.1 ppb and 5 ppb, even without applying mass-bias correction.

• Analysis of Certified Reference Materials
Twelve CRMs covering waters, sediments, soils, plant and animal tissues were measured by OIDA with the 8800 ICP-QQQ in O₂/H₂ mass-shift mode. Recoveries for both ^78Se/^82Se and ^80Se/^82Se isotope pairs ranged from 90% to 112%, demonstrating robust, matrix-independent performance.

Benefits and Practical Applications

• Complete removal of isobaric, polyatomic and doubly charged interferences under a single set of conditions.
• High sensitivity and reliability for ultratrace Se in complex matrices without laborious method development.
• Availability of multiple interference-free isotopes enables precise isotope dilution quantification without internal standards or mass-bias correction.
• Streamlined routine analysis with turnkey, preset methods and minimal tuning effort.

Future Trends and Possible Uses

The demonstrated MS/MS approach can be extended to other challenging elements and speciation workflows. Emerging applications include automation of on-line isotope dilution for high-throughput laboratories, development of novel reaction gases to target specific interferences and coupling with separation techniques for detailed speciation studies. Continued advances in triple-quadrupole ICP-MS technology will further simplify ultratrace analysis in environmental, food and life-science fields.

Conclusion

The Agilent 8800 ICP-QQQ in MS/MS mode overcomes the traditional limitations of Se analysis by eliminating spectral overlaps and matrix effects in a single, robust method. Combined with on-line isotope dilution, it delivers accurate, interference-free quantification of Se at ultratrace levels across diverse sample types, enhancing data quality and laboratory efficiency.

Used Instrumentation

• Agilent 8800 Triple Quadrupole ICP-MS
• Glass concentric nebulizer
• Quartz double-pass Scott-type spray chamber
• Quartz torch (2.5 mm ID)
• ORS3 collision/reaction cell

References

1. Centineo, G., Angel, J., Castrillon, R. & Munoz Agudo, E. (2011). Application of on-line isotope dilution for trace element analysis by ICP-MS. Agilent Technologies Publication 5990-9171EN.
2. Henrion, A. (1994). Accuracy of isotope dilution without mass-bias correction. Fresenius Journal of Analytical Chemistry, 350, 657–658.