Thermal Desorption GC Analysis of High Boiling, High Molecular Weight Hydrocarbons

Significance of the Topic

The analysis of high boiling, high molecular weight hydrocarbons is critical in applications ranging from environmental monitoring to quality control in polymer and wax production. Thermal desorption coupled to gas chromatography (GC) provides a solvent-free method to introduce volatile and semi-volatile analytes efficiently, overcoming limitations of direct liquid injection for non-amenable samples.

Objectives and Overview of the Study

This application note evaluates the performance of a direct-transfer Thermal Desorption System (TDS) for quantitative GC analysis of hydrocarbons from C5 to C40. Key aims include:

• Comparing transfer efficiency of thermal desorption versus manual liquid injection
• Assessing the effect of desorption flow rates (50 vs. 80 mL/min) on high-boiling analyte recovery
• Determining carry-over and trapping efficiency across a wide boiling-point range

Methodology and Instrumentation

Solid-phase sampling and concentration were performed by spiking 1 µL of a C5–C40 hydrocarbon standard (Agilent boiling-point calibration mix) onto empty glass TDS tubes. Two thermal desorption protocols were tested:

• 50 mL/min helium at 300 °C for 10 min
• 80 mL/min helium at 300 °C for 10 min

Analytes were cryo-focused in the Cooled Injection System (CIS 4) at –120 °C and transferred to an HP-1 column via rapid heating to 400 °C. Peak areas were compared against manual 1 µL injections at 300 °C.

Used Instrumentation

• GERSTEL TDS 2/TDS A thermal desorption system
• CIS 4 cooled injection system (Peltier or LN₂ cooling)
• Agilent 6890 GC with FID detector
• 15 m HP-1 column, 0.53 mm ID, 0.15 µm film

Main Results and Discussion

Thermal desorption at 50 mL/min achieved complete transfer up to C32, while 80 mL/min was required for quantitative recovery of components up to C40. Overlay chromatograms showed nearly identical peak areas for C15–C40 when comparing thermal desorption and manual injection, with minimal retention time shifts attributable to inlet heating delay. Blank-tube runs confirmed negligible carry-over of high-boiling analytes. Optimization of flow, temperature, and trapping conditions is recommended for each target boiling-point range.

Benefits and Practical Applications

Thermal desorption on the GERSTEL TDS offers:

• Valveless, septumless flow path for leak-free operation
• Highly inert, short transfer lines minimizing adsorption losses
• Efficient transfer of analytes from C5 to C40 and beyond
• Elimination of solvent use and reduced sample preparation complexity

This method is well suited for trace and ultra-trace analysis in environmental monitoring, polymer characterization, food and cosmetic quality control, and pharmaceutical residual testing.

Future Trends and Applications

Emerging directions include:

• Integration with mass spectrometry for enhanced compound identification
• Development of novel sorbent materials to target ultra-high boiling analytes
• Miniaturized and portable thermal desorption devices for field analysis
• Automation of multi-phase sample extraction combining stir-bar sorptive extraction and thermal desorption

Conclusion

The GERSTEL Thermal Desorption System demonstrates robust, quantitative transfer of high-boiling hydrocarbons up to C40, matching manual injection performance while extending the range of amenable analytes. Its direct-flow, inert design and flexible cooling options make it a versatile tool for modern GC analysis.

Reference

Edward A. Pfannkoch, Jacqueline A. Whitecavage, Jeffrey Christenson. Thermal Desorption GC Analysis of High Boiling, High Molecular Weight Hydrocarbons. Gerstel AppNote 6/2004.