Low-Pressure Retention Time Locking with the 7890A GC

Importance of the Topic

The ability to maintain consistent retention times despite column maintenance or instrument-to-instrument variation is essential for reliable qualitative and quantitative gas chromatographic analyses. Retention time locking (RTL) simplifies method transfer, reduces the need for revalidation after column trimming or replacement, and ensures reproducible identification of target analytes in complex samples.

Study Objectives and Overview

This application note evaluates the performance of low-pressure RTL using an Agilent 7890A GC equipped with a fifth-generation electronic pneumatic control (EPC) on a wide-bore 0.32 mm id column. The primary goals were to:

• Translate an existing method from a 0.25 mm id column to a 0.32 mm id column using Agilent Method Translator software.
• Demonstrate precise retention time locking at sub-5 psi head pressures.
• Assess retention time stability before and after simulated column maintenance (45 cm trim) for a 16-component polycyclic aromatic hydrocarbon (PAH) mixture.

Methodology and Instrumentation

The translated method employed a 25 m × 0.32 mm id HP-5 column with 0.52 µm film and helium in constant pressure mode. Fluoranthene (11.112 min) served as the locking compound. Key chromatographic parameters included:

• Injection: 1 µL pulsed splitless at 7 psi for 0.3 min, 300 °C inlet.
• Oven program: 55 °C (1.1 min), ramp at 22.88 °C/min to 320 °C (6.5 min hold).
• MSD scan range: 45–400 u; source 230 °C, quad 150 °C, transfer line 280 °C.

Used Instrumentation

• Agilent 7890A GC with fifth-generation EPC
• Agilent 5975C GC/MSD
• HP-5 25 m × 0.32 mm id × 0.52 µm film column (p/n 19091J-112)

Main Results and Discussion

Calibration curves for 16 PAHs over 10 ppb–5 ppm showed excellent linearity (r2 0.995–0.997). Retention time repeatability for replicate injections was outstanding (RSD ≤0.09%). Low-pressure RTL using three-decimal-place EPC control achieved sub-0.5 s deviations (<0.016 min) across all analytes after a 45 cm column trim. Chromatographic overlays before and after trimming confirmed minimal shift, ensuring reliable peak identification without full method revalidation.

Benefits and Practical Applications of the Method

• Maintains high confidence in analyte identification after routine column maintenance.
• Eliminates frequent manual updates to retention time windows.
• Facilitates method transfer between instruments with consistent peak retention.
• Improves laboratory productivity and data comparability for QA/QC and research applications.

Future Trends and Possibilities for Use

Advances may include broader application of low-pressure RTL to other column chemistries and dimensions, integration with laboratory information management systems for automatic method translation, and expansion to highly complex matrices. Continuous EPC refinements will further enhance locking precision, supporting ultratrace analysis and high-throughput workflows.

Conclusion

Agilent 7890A GC’s enhanced EPC enables robust retention time locking at low head pressures on wide-bore columns. The demonstrated PAH study achieved retention time stability within 0.5 s after column trimming, streamlining GC method maintenance and ensuring reproducible results across instruments.

References

• Agilent Technologies. Reliable transfer of existing Agilent 6890/5973 GC/MSD methods to the new 7890/5975 GC/MSD, publication 5989-6569EN.
• Agilent Technologies. Low-Pressure Retention Time Locking with the 7890A GC, application note 5989-8366EN.