Accurate determination of sulfur in biodiesel using Isotope Dilution-Triple Quadrupole ICP-MS (ID-ICP-QQQ)

Importance of the Topic

Accurate quantification of sulfur at trace levels in biodiesel and other organic matrices is critical to comply with stringent fuel emission regulations and to safeguard modern engine performance. Traditional techniques such as ICP-OES and FAAS face severe matrix interferences, while conventional ICP-MS struggles with polyatomic overlaps. The described isotope dilution triple quadrupole ICP-MS (ID-ICP-QQQ) approach offers enhanced sensitivity and interference removal, meeting industry demands for rapid, reliable sulfur analysis.

Objectives and Study Overview

This study aimed to develop and validate an ID-ICP-QQQ method for precise sulfur determination in biodiesel, demonstrating its applicability to other sulfur‐containing organic samples. Key goals included creating robust calibration protocols, reducing spectral interferences, and assessing method accuracy using a certified reference biodiesel (NIST SRM 2773).

Methodology and Instrumentation

A commercially available Agilent 8800 Triple Quadrupole ICP-MS was configured for organic solvents with a Peltier-cooled spray chamber at –5 °C, an organics torch (1 mm i.d. injector), and O₂ addition to minimize carbon build-up. The instrument was operated in three modes:

• Single-quad ion guide (SQ ion guide)
• Single-quad bandpass with O₂ reaction (SQ bandpass)
• MS/MS mass-shift mode (precursor S⁺ → product SO⁺)

Isotope dilution utilized a 98.8% ³⁴S spike, and calibration standards ranged from 0 to 850 µg/kg S in ethanol with 0.14 M HNO₃. Sample preparation involved direct dilution of biodiesel plus spike in ethanol.

Key Results and Discussion

Calibration performance revealed severe nonlinearity in SQ ion guide mode due to matrix overlaps. SQ bandpass improved linearity but retained offsets from residual interferences. In contrast, MS/MS mode achieved fully interference-free detection of ³²S¹⁶O⁺, ³³S¹⁶O⁺, and ³⁴S¹⁶O⁺, with limits of detection of 0.4–5 µg/L, outperforming reported values for conventional ICP-MS and rivaling HR-ICP-MS sensitivity. Spectral scans confirmed that MS/MS mode effectively excludes Ca⁺ and Ti⁺ interferences. Analysis of NIST SRM 2773 by ID-ICP-QQQ yielded 7.231 ± 0.015 µg/g S (95% confidence), in excellent agreement with the certified 7.39 ± 0.39 µg/g.

Benefits and Practical Applications

This ID-ICP-QQQ method provides:

• High accuracy and precision for low‐level sulfur in complex fuels
• Robust interference removal without high-resolution instruments
• Rapid analysis compatible with routine QA/QC and regulatory compliance
• Flexibility to extend to other sulfur‐containing matrices such as biological samples or pharmaceuticals

Future Trends and Potential Applications

Advances may include on-line species-unspecific isotope dilution coupling with HPLC-ICP-MS for speciation studies, expansion to multi-element isotope dilution workflows, and integration into automated high‐throughput fuel testing platforms. Further exploration of alternative reaction gases and cell chemistries could broaden the scope of interference-free analyses in challenging matrices.

Conclusion

The isotope dilution-triple quadrupole ICP-MS technique effectively eliminates polyatomic overlaps and matrix interferences in sulfur analysis, delivering trace‐level detection and high accuracy in biodiesel. Its flexibility and performance make it a valuable tool for regulatory laboratories and research settings focused on fuel quality and environmental monitoring.

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