EA-IRMS: Fast and Precise Isotope Analysis of Liquids on a Delta V Isotope Ratio MS with a High Temperature Conversion Elemental Analyzer

Significance of the Topic

The determination of hydrogen (δ2H) and oxygen (δ18O) isotope ratios in aqueous and other liquid samples is crucial across environmental studies, food authenticity, clinical diagnostics and industrial quality control. Fast, accurate and low-volume analysis enables laboratories to handle large sample volumes, reduce costs and improve data reliability.

Study Objectives and Overview

This application note evaluates the performance of a high temperature conversion elemental analyzer (TC/EA) coupled to a Delta V isotope ratio mass spectrometer (IRMS) for continuous‐flow analysis of liquid samples. The goals are to demonstrate precision, throughput, memory effects and applicability to diverse sample types including water, biological fluids, beverages and organic solvents.

Methodology and Instrumentation

• Conversion Reaction and Reactor Design

• Water samples react with glassy carbon at 1 400 °C to yield H2 and CO.
• A tube-in-tube design (glassy carbon inside a ceramic sleeve) prevents oxidation of the reactor.

• Carrier and Separation

• Helium carrier flow at 100 mL/min transports reaction gases through a GC column to separate H2 from CO.

• Mass Spectrometry

• Delta V IRMS collects H2 ions (m/z 2, 3) followed by CO ions (m/z 28, 30) via rapid magnet switching.

• Autosampler Settings

• Thermo Scientific AS 3000 autosampler using 0.5 µL syringe with triplicate strokes of 0.1 µL each.
• No rinses; post-injection dwell time of 16 s to minimize memory.

Results and Discussion

• Precision and Accuracy

• International water standards (GISP, SLAP, SMOW) and lab standards yielded routine precision better than 1‰ for δ2H and 0.1‰ for δ18O.
• Raw delta values agree with accepted values without need for post-run correction.

• Memory Effects

• First injections following large isotope shifts may show slight carry-over; a 16 s post-injection dwell time in the hot zone effectively minimizes this.

• Throughput

• Single-ratio measurement: δ18O in 6 min per sample, enabling ~240 samples per day.
• Dual-ratio mode: δ2H and δ18O in under 6 min with two replicate injections (triplicate strokes), yielding ~72 samples per day.

• Extension to Other Liquids

• Ethanol, urine, blood plasma and juices were successfully analyzed, demonstrating broad applicability.

Benefits and Practical Applications

• Rapid sub-µL analysis reduces sample consumption.
• High sample throughput lowers per-analysis cost.
• Direct conversion avoids complex equilibration or reduction chemistries.
• Single system integration with EA IsoLink IRMS allows seamless switching between C/N/S and H/O isotope analyses.

Future Trends and Opportunities

• Wider adoption in food authenticity testing and environmental monitoring.
• Interlaboratory standardization using reference compounds such as tetramethylurea.
• Further miniaturization of reactors to enhance throughput and lower detection limits.
• Integration with automated sample preparation and data evaluation platforms.

Conclusion

The TC/EA-IRMS system offers a fast, precise and versatile solution for δ2H and δ18O analysis of liquid samples. Its robust reactor design, minimal memory effects and high throughput support demanding applications in research and routine laboratories, while integration with multi-element isotope analysis broadens its utility.

References

1. Koziet J.J. Isotope Ratio Mass Spectrometric Method for the Online Determination of Oxygen-18 in Organic Matter. J. Mass Spectrom. 1997, 32, 103–108.
2. Coplen T. Normalization of oxygen and hydrogen isotope data. Chem. Geol. 1988, 72, 293–297.
3. Nelson S.T. A simple, practical methodology for routine VSMOW/SLAP normalization of water samples analyzed by continuous flow methods. RCM 2000, 14, 1044–1046.
4. Werner R. & Brand W.A. Referencing strategies and techniques in stable isotope ratio analysis. RCM 2001, 15, 501–519.
5. Calderone G., Reniero F. & Guillou C. 18O/16O measurements on ethanol. RCM 2006, 20, 937–940.
6. Bréas O. et al. Performance evaluation of EA-IRMS methods for D/H ratio in tetramethylurea and other compounds – intercomparison results. RCM 2007, 21, 1555–1560.