The Role of Selective Exclusion in the Analysis of Specific Hydrocarbons in n-Alkane Waxes

Significance of the Topic

Selective exclusion in gas chromatography enables analysts to target specific n-alkane fractions within complex wax samples, significantly reducing run times and minimizing column overloading. By discriminating sample components based on injector temperature programming and split valve control, this approach streamlines routine monitoring of petroleum waxes in crude oils, condensates, and deposit analyses.

Objectives and Study Overview

This application note describes a three-fold investigation of selective exclusion using the ATAS Optic 2-200 programmable injector to:

1. Transfer all volatile hydrocarbons up to C20 while excluding involatile components
2. Transfer heavy hydrocarbons C40 and above while excluding volatiles
3. Isolate and transfer exclusively C30 alkanes, excluding both lighter and heavier species

Initial theoretical modelling established injector temperature thresholds for transfer and exclusion prior to experimental validation.

Methodology and Instrumentation

The procedure relies on a three-step injector temperature program synchronized with split valve operations:

• Step 1: Inject at low temperature with split open to purge unwanted volatiles (optional)
• Step 2: Close split valve and heat to an isothermal temperature to transfer target hydrocarbons onto the column
• Step 3: Reopen split valve and ramp to final temperature to vent non-target involatile residues (optional)

The exclusion envelope, a calibration graph correlating injector isothermal temperature with analyte boiling point, was used to calculate specific temperature set-points. Experimental conditions were fine-tuned on the bench.

Applied Instrumentation

• ATAS Optic 2-200 programmable injector in expert mode
• HP 5890 gas chromatograph with flame ionization detector (FID)
• SGE HT-5 column (25 m × 0.32 mm, 0.1 µm film thickness)

Main Results and Discussion

Using selective exclusion, the authors achieved:

• 100 % transfer of C20 and lighter alkanes with less than 2 % involatile carryover
• Complete transfer of C40 and above with negligible volatile contamination
• Selective isolation of C30 only, by excluding lighter and heavier homologues

Chromatograms confirmed sharp target peaks and minimal baseline noise, demonstrating efficient sample discrimination and reduced analysis times compared to full-range injections.

Benefits and Practical Applications

Selective exclusion offers several advantages for industrial and research laboratories:

• Shorter chromatographic runs focused on analytes of interest
• Reduced reagent and column contamination from non-target components
• Enhanced throughput for routine quality control of wax deposits and crude oils

Future Trends and Potential Uses

Advancements may include automation of exclusion envelope generation, integration with direct matrix introduction (DMI) systems for solid or viscous samples, and extension to other compound classes beyond n-alkanes. Improved injector designs and software-driven optimization could further streamline targeted hydrocarbon profiling.

Conclusion

This study demonstrates that programmable selective exclusion in GC inlets can precisely isolate specific hydrocarbon ranges within complex wax matrices. By combining theoretical modelling and injector valve programming, the method delivers efficient targeted analyses, offering significant time savings and improved chromatographic performance.