Theory and Key Principles Series: Session 4 – Advanced Liquid Injection Techniques

Significance of the Topic

Advanced liquid injection techniques in gas chromatography address challenges encountered with standard split/splitless inlets. They enable accurate analysis of thermally labile compounds, broad boiling-point mixtures and trace-level analytes. These approaches expand the applicability of GC in environmental, petrochemical, pharmaceutical and food analysis.

Study Objectives and Overview

The presentation aims to identify situations where split/splitless inlets are inadequate and to compare alternative injection methods: programmable temperature vaporisation (PTV), on-column injection (OCI or COC), and multi-mode inlets (MMI) with large volume injection (LVI). It outlines their operating principles, hardware needs and performance benefits.

Methodology and Instrumentation

The discussed techniques were illustrated using Shimadzu GC systems equipped with the OPTIC-4 multi-mode inlet. Key features include:

• PTV mode: cold injection followed by programmable heating (up to 250 °C/min) in a narrow liner (<100 µL).
• OCI/COC mode: direct sample introduction into the column, requiring a 0.53 mm i.d. retention gap or column and a compatible syringe needle (26 gauge or tapered).
• MMI modes: hot/cold split, splitless, OCI, pyrolysis, thermal desorption, in-system derivatisation and Difficult Matrix Introduction (DMI).
• Large Volume Injection (LVI): high split ratios (>100:1) during solvent evaporation, followed by splitless analyte transfer.

Main Results and Discussion

Split/splitless inlets perform well for routine analyses but exhibit thermal degradation of labile species and mass discrimination across wide boiling-point ranges. PTV prevents decomposition and reduces discrimination by injecting samples cold and applying a controlled temperature ramp. OCI offers direct cold transfer without liner interactions but lacks split capabilities. Modern MMIs combine multiple modes in one inlet, providing flexibility for diverse sample types and enabling ultra-trace analysis via LVI.

Benefits and Practical Applications

• Thermal stability: PTV and OCI preserve thermolabile analytes such as certain pesticides and high-boiling hydrocarbons.
• Wide range coverage: Programmable temperature and OCI ensure consistent response from C5 to C100 compounds, critical for simulated distillation.
• Trace-level sensitivity: LVI on MMIs concentrates analytes for detection at ultra-low concentrations.
• Versatility: MMIs accommodate liquids, solids and complex matrices, reducing method development time.

Future Trends and Potential Applications

Advances in inlet design will focus on higher heating rates, automated liner exchange and real-time pressure control to further minimise discrimination. Integration with in-line sample preparation and enhanced compatibility with mass spectrometric detectors will broaden applications in metabolomics, petrochemical fingerprinting and contaminant screening. Emerging areas include coupling OCI with two-dimensional GC and implementing AI-driven method optimisation.

Conclusion

Alternative injection techniques extend GC’s analytical capabilities beyond what split/splitless inlets offer. Programmable temperature vaporisation and on-column injection address thermal degradation and discrimination issues, while multi-mode inlets consolidate functions into a single platform. Large volume injection further enhances sensitivity for trace analysis. Together, these approaches support robust, versatile and high-sensitivity GC methods across a wide range of industries.

Used Instrumentation

• Shimadzu GC system with OPTIC-4 Multi-Mode Inlet
• PTV liner (<100 µL volume)
• 0.53 mm i.d. retention gap or analytical column for OCI
• 26 gauge or tapered syringe needles

References

No formal references were provided in the source text.