Modelling an OPTIC 2 as an Alternative to Multidimensional Chromatography

Importance of the Topic

Multidimensional chromatography allows selective introduction of analytes and removal of solvents or major components, enhancing sensitivity and protecting columns and detectors. However, conventional systems are complex, costly, and require frequent replacement of expensive columns due to heart–cutting techniques.

Objectives and Overview of the Study

This study evaluated the ATAS OPTIC 2 programmable injector as a simplified alternative to multidimensional chromatography. Key goals were to optimize injector parameters to achieve selective discrimination based on volatility differences, develop a predictive model correlating injector isothermal temperature with selective transfer/exclusion, and propose a procedure for estimating conditions for unknown compounds.

Methodology

A mixture of C8–C36 n-alkanes in hexane was used to investigate discrimination under varying injector isothermal temperatures (40–350 °C) and times (10–240 s). Expert-mode parameters, especially isothermal temperature (parameter 5) and time (parameter 6), were optimized while split flow, injection volume, and liner choice were held constant. Effective boiling points were defined as injector temperatures at 50 % transfer.

Used Instrumentation

• Gas chromatograph (HP-6890) with autosampler, FID and ATAS OPTIC 2-200 injector
• CP Sil 8CB column (25 m × 0.25 mm × 0.25 µm)
• Oven program: 35 °C (2 min hold) to 350 °C at 40 °C/min
• Injector program: 40 °C (45 s), ramp to set temperature at 12 °C/s (60 s), final ramp to 350 °C
• Split flow: 200 mL/min; liner: fritted; split line rerouted through Tenax trap
• Additional HP-5890 GC (CP Sil 5CB column) used to determine retention-boiling point correlations

Main Results and Discussion

Selective discrimination depended strongly on isothermal temperature. Up to 98 % of less volatile components could be excluded, while volatile analytes transferred quantitatively well below their literature boiling points. A polynomial correlation (R2 ≈ 0.9999) was established between effective and literature boiling points. The exclusion envelope defined upper and lower temperature limits for 100 % transfer of the volatile and 100 % exclusion of the involatile component. A boiling point difference of ≥ 77 °C ensures total separation. Isothermal time of ~ 1 min balanced complete volatile transfer and minimal involatile transfer.

Benefits and Practical Applications

• Cost-effective and simpler than conventional two-dimensional GC
• Enhanced detection limits by enabling injection of highly concentrated samples
• Protection of columns and mass detectors from heavy contaminants
• Rapid optimization of only two key parameters
• Applicability across a wide range of volatile compounds

Future Trends and Potential Applications

• Extension to diverse compound classes and complex matrices
• Integration with mass spectrometry and real-time monitoring
• Development of predictive software tools for automated parameter selection
• Adaptation to other programmable injector designs

Conclusion

The OPTIC 2 programmable injector provides a versatile, cost-effective alternative to multidimensional chromatography through selective discrimination based on volatility. Predictive models and simple procedures enable precise control of analyte introduction, improving sensitivity while protecting chromatography hardware. This approach can be broadly applied to trace analysis in concentrated samples.

Reference

• IAMBEC MSc project by Diane Nicholas, University of Warwick 1999