Using the PSD for Backflushing on the Agilent 8890 GC System

Importance of the topic

Residual fuel oils contain a wide carbon range C10–C70 and sulfur species; heavy components cause carryover in gas chromatography. Implementing backflushing with a Deans switch and capillary flow technology ensures reproducible, high-throughput analysis of complex heavy hydrocarbon matrices.

Objectives and overview

This note evaluates an Agilent 8890 GC system equipped with a pneumatic switching device (PSD), capillary flow technology Deans switch, flame ionization and flame photometric detectors to analyze residual fuel oil (NBS 1622c). It aims to demonstrate backflush capability, reduced gas consumption, and enhanced analytical performance, alongside a simulated distillation of carbon chain distribution.

Methodology and instrumentation used

• GC system: Agilent 8890 with multimode inlet (MMI), Agilent 7693A autosampler, helium constant flow.
• Columns: Primary DB-1ms UI (30 m × 0.25 mm, 0.25 µm); Secondary DB-17ht (30 m × 0.25 mm, 0.15 µm); deactivated fused silica restrictor tubing.
• Pneumatic control: PSD module providing forward pressure for backflush (70 psi) and engineered purge bleed at 3 mL/min to stabilize low flow and minimize carrier gas waste.
• Detectors: FID for hydrocarbon profiling; FPD Plus for sulfur compounds, with heart-cut window at 20.1–20.4 minutes targeting 4,6-dimethyldibenzothiophene.
• Simulated distillation: J&W DB-HT Sim Dis column (5 m × 0.53 mm, 0.15 µm), 5 mL/min He, FID at 450 °C.
• Samples: NBS 1622c diluted 1:40 in toluene; Polywax 500 standard at 0.1 % in toluene; 1 µL splitless injections.

Main results and discussion

• Without backflush: baseline drift beyond C36, significant carryover, poor precision in FPD channel.
• With backflush via PSD: stable baselines, elimination of carryover, improved retention time and area precision for the heart-cut sulfur analyte.
• Gas savings: PSD maintained 3 mL/min bleed even at 70 psi, avoiding over 500 mL/min waste associated with fixed restrictors.
• Simulated distillation confirmed RFO carbon range up to C70, underscoring backflush necessity.

Benefits and practical applications

• Enhanced accuracy and reproducibility in heavy hydrocarbon and sulfur analysis.
• Reduced column bake-out cycles extends stationary phase life.
• Higher sample throughput due to shorter run-to-run times.
• Lower helium consumption through controlled purge flow.

Future trends and applications

• Standard inclusion of backflush in modern GC systems for routine high-boiling samples.
• Advances in multidimensional GC with automated CFT switching and intelligent flow control.
• Adaptive EPC modules optimizing carrier gas usage and method robustness.
• Coupling backflush-enabled GC systems with mass spectrometry for detailed heavy fraction speciation.

Conclusion

Implementing backflushing with a PSD on the Agilent 8890 GC and Deans switch effectively eliminates carryover of C10–C70 hydrocarbons, improves method reproducibility, reduces carrier gas consumption, and increases laboratory throughput for residual fuel oil analysis.

References

1. Tranchida, P. Q., et al. Heart-cutting multidimensional gas chromatography: A review of recent evolution, applications, and future prospects. J. Chromatogr. A, 2012, 716, 66–75.
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3. Meng, C.-K. Improving productivity and extending column life with backflush. Agilent Technologies Application Brief, 2006, 5989-6018EN.
4. Agilent CFT Backflush Brochure, 2013, 5989-9408EN.
5. Westland, J. Examining maximum residue levels for multiresidue pesticides in jasmine rice. Agilent Technologies Application Note, 2018, 5991-9393EN.
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