Analysis of Wet Samples by Direct Ther mal Desorption GC

Significance of the topic

Direct thermal desorption with cryofocusing has emerged as a powerful technique for trace-level analysis of volatile compounds in solid and semisolid matrices. Samples such as foods, textiles and polymers often contain high moisture, which can impair chromatographic performance and shorten column life when water enters the GC system. Effective water removal strategies are therefore essential to maintain sensitivity, reproducibility and instrument longevity.

Study objectives and overview

This application note evaluates three strategies for managing water during direct thermal desorption–GC analysis of wet samples: offline thermal extraction with Tenax TA™ adsorbent, low-temperature sample drying prior to desorption, and use of Tenax TA™ packed inlet liners for online water venting. The goal is to define conditions that eliminate water without losing volatiles and to demonstrate high-sensitivity profiling of aroma compounds in herbs and peppers.

Methodology

• Model drying studies: Cotton terrycloth spiked with known water quantities and a C5–C18 hydrocarbon mix were dried at controlled helium flow and temperatures (20–60 °C) to assess water removal efficiency and analyte retention.
• Offline thermal extraction: Fresh samples (herbs and peppers) were extracted under helium flow onto Tenax TA™ tubes at ambient or elevated temperatures, followed by tube heating to remove residual water before GC desorption and cryotrapping.
• Online direct desorption: Drying and volatile trapping were integrated by packing Tenax TA™ into the PTV inlet liner (CIS 4). Inlet temperature (5–60 °C) and solvent-venting conditions were optimized to vent water while retaining target volatiles.

Used instrumentation

• GC system: Agilent 6890 with flame ionization detector (FID)
• Thermal desorption units: Gerstel TDS 2 and TDS A with autosampler
• Programmable temperature vaporization inlet: Gerstel CIS 4
• Offline thermal extractor: Gerstel TE unit

Key results and discussion

• Drying cotton at 30 °C under helium removed >98% of 10 mg water in 10 min with minimal loss of analytes boiling >200 °C.
• Tenax TA™ tubes retained significant water at low temperatures; heating at 60 °C for ≥20 min was required to prevent inlet freezing and chromatographic distortions.
• Offline extraction of fresh peppers and basil on Tenax TA™ delivered comprehensive volatile profiles but required extended extraction and drying times (up to 60 min) and careful water elimination prior to desorption.
• Online direct desorption using Tenax TA™ packed inlet liners at ~40 °C achieved optimal volatile trapping and continuous water venting, improving peak shape and sensitivity and reducing total analysis time to ~30 min.

Benefits and practical applications

This approach enables rapid and sensitive analysis of volatile aroma compounds, fragrances and other organics in high-moisture matrices. It minimizes instrument downtime from water condensation, preserves column longevity and supports quality control in food, fragrance and polymer industries.

Future trends and potential applications

• Development of advanced hydrophobic sorbents with enhanced water resistance for streamlined direct desorption workflows.
• Miniaturization and automation of sample handling and data processing to boost throughput.
• Coupling with mass spectrometry and advanced deconvolution algorithms to expand qualitative and quantitative capabilities in complex matrices.

Conclusion

Optimized water elimination through controlled drying and Tenax TA™ assisted trapping in the GC inlet enables reliable, high-sensitivity analysis of volatiles from wet samples. Online direct thermal desorption at moderate inlet temperatures offers a rapid, robust solution for laboratories confronting high-moisture analytical challenges.