Programmed-temperature vaporiser injector as a new analytical tool for combined thermal desorption-pyrolysis of solid samples Application to geochemical analysis

Significance of the topic

The molecular characterization of hydrocarbon source rocks and coals underpins petroleum exploration and reservoir evaluation. Conventional thermal desorption–pyrolysis systems often require dedicated hardware, heated transfer lines and complex calibration, risking loss of high-molecular-weight compounds. Adapting a standard programmed-temperature vaporizer (PTV) injector for combined in-situ desorption and pyrolysis offers a cost-effective, flexible alternative that preserves sample integrity and simplifies operation.

Objectives and study overview

This study introduces a modified PTV injector as an integrated desorption–pyrolysis unit for geochemical analysis. The goals were to (1) develop a sequential multi-temperature protocol within the PTV liner, (2) evaluate its performance on various kerogen types (I, II and III) from source rocks and coal at different maturities, and (3) compare the results qualitatively with those from a conventional resistively heated pyroprobe system.

Used instrumentation

• Modified OPTIC 600 PTV injector (ambient to 630 °C) with internal glass frit and liquid CO₂ cooling
• Shimadzu 17A GC with high-temperature SimDist CB column (10 m × 0.25 mm I.D., 0.15 µm film) and FID
• Laboratory-built glass cold trap (–100 °C to 450 °C) for cryogenic refocusing of volatiles
• CDS Pyroprobe 100 coupled to HP 5890 GC with HP-1 column (25 m × 0.32 mm I.D., 1.05 µm film) and Perkin-Elmer AED for simultaneous C (193 nm) and S (181 nm) detection

Methodology and instrumentation used

• Sample loading: 1–10 mg rock or coal ground directly into PTV liner, then flushed with He
• Sequential temperature programme: 200 °C (5 min desorption), 400 °C and 600 °C at 8 °C/s heating rate
• Carrier gas: He at ~135 mL/min through liner, split ratios 1:54 to prevent column overload
• GC oven: 50 °C (6 min) to 425 °C at 10 °C/min, FID at 435 °C
• For AED runs: initial oven –20 °C (2 min) to 300 °C at 4 °C/min, no cold trap required with thick-film column
• Comparison pyrolysis: single-step PTV at 530 °C (2 min) vs. CDS Pyroprobe at 530 °C (20 s)

Main results and discussion

• 200 °C desorption released C₁–C₁₆ alkanes, aromatics and naphthenes representative of generated hydrocarbons.
• 400 °C treatment liberated medium-chain alkanes (C₂₀–C₃₀) and biomarker isoprenoids, indicating early kerogen decomposition.
• 600 °C pyrolysis yielded characteristic n-alkene/n-alkane doublets (C₄–C₄₀) and light gases (CH₄–C₄H₁₀), reflecting extensive cracking.
• AED profiles displayed bimodal carbon distributions and sulfur heterocycles consistent with kerogen type and maturity variations.
• PTV-based pyrograms aligned closely with those from the CDS Pyroprobe 100, validating analytical equivalence.

Benefits and practical applications

• Utilizes standard GC injector hardware—cost-effective and easy to maintain.
• Highly flexible temperature programming without additional transfer lines or valves.
• Eliminates heated transfer lines, reducing loss of heavy analytes.
• Minimal sample handling and straightforward calibration via liquid standard injection.
• Adaptable to a wide range of sample sizes and kerogen maturities.

Future trends and applications

• Coupling PTV desorption–pyrolysis with tandem mass spectrometry for detailed structural analysis.
• Automation of multi-step temperature protocols for high-throughput geochemical screening.
• Extension to environmental and forensic matrices requiring combined thermal pretreatment.
• Establishment of standardized PTV-based workflows for petroleum exploration and QA/QC.

Conclusion

The modified PTV injector presents a robust, versatile platform for integrated thermal desorption and pyrolysis in geochemical analysis. It delivers comprehensive molecular insights across a broad temperature range, matches conventional pyroprobe performance, and streamlines calibration and maintenance. Its simplicity and cost-effectiveness make it well suited for both research laboratories and industrial applications in petroleum geochemistry.

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