Purge and Trap - The Gases

Importance of the Topic

This study addresses the critical need to detect trace levels of volatile organic compounds in water samples, a key requirement in environmental monitoring and regulatory compliance.

Study Objectives and Overview

The application note evaluates the performance of the CDS Model 7000 Purge & Trap system coupled to an Agilent 6890 gas chromatograph with a 5975B mass spectrometer for the analysis of six light gases at sub-ppb concentrations. It focuses on chromatographic resolution, reproducibility, and quantitative linearity.

Methodology

Water samples spiked with target analytes were purged using helium at 40 ml/min for 11 minutes. A multi-bed sorbent trap captured the volatile compounds, which were rapidly desorbed via a timed trap preheat step followed by thermal transfer to the GC/MS system. No cryogenic cooling was required before chromatography.

Used Instrumentation

• CDS Model 7000 Purge & Trap with trap preheat and bake functions
• Agilent 6890 Gas Chromatograph coupled to 5975B Mass Spectrometer
• Varian CP 624 CB capillary column (30 m × 0.25 mm, 1.4 µm film)
• Helium carrier gas, 1.3 ml/min flow, split ratio 40:1
• Oven temperature program: 40 °C (5 min hold) to 180 °C at 10 °C/min

Main Results and Discussion

Chromatograms of the six gases at 0.5 ppb demonstrated sharp, well-separated peaks with seven overlaid runs showing average reproducibility of approximately 6 % RSD. Linearity tests from 0.5 to 50 ppb yielded correlation coefficients (R²) ≥ 0.999 for all targets. Method detection limits ranged from 0.03 to 0.25 ppb, confirming high sensitivity.

Benefits and Practical Applications

• High sensitivity for sub-ppb level detection without cryogenics
• Rapid trap preheat ensuring efficient collection and release of early eluting compounds
• Robust reproducibility supporting routine environmental and quality control analyses

Future Trends and Applications

• Integration with automated sampling systems for higher throughput
• Development of advanced sorbent materials to enhance selectivity and capacity
• Miniaturization and field-portable devices for on-site monitoring
• Advanced data processing and coupling with tandem mass spectrometry for increased specificity

Conclusion

The CDS Model 7000 Purge & Trap combined with GC/MS delivers reliable, sensitive, and reproducible analysis of volatile gases in water at sub-ppb levels. Its excellent linearity and lack of cryogenics streamline routine laboratory operations, supporting diverse environmental and industrial applications.

References

• CDS Analytical, Inc. Application Note #117: Purge and Trap – The Gases