Rapid analysis using inactivated fused silica tube (a.k.a. “guard column”) as a sample inlet (2)

Significance of the topic

Average molecular weight and distribution are key parameters for characterizing complex mixtures such as crude oils and synthetic polymers. Soft ionization techniques like field ionization (FI) and field desorption (FD) generate predominantly molecular ions, enabling direct calculation of molecular weight metrics and group-type classification based on functional additions or unsaturations.

Objectives and overview of the study

This application note compares three mass spectrometric approaches—blank fused silica tube inlet with FI, capillary GC/FI, and FD—for rapid group-type analysis of kerosene, diesel fuel, and crude oil. The goal is to assess analysis speed, molecular weight determination, polydispersity, and detection of low- and high-boiling components.

Methodology and instrumentation

Three commercially available samples (kerosene, diesel fuel, Middle Eastern crude oil) were analyzed under standardized parameters. Key operating conditions included:

• Sample volume: 1 µL for inlet methods
• Blank fused silica tube: 3 m × 0.1 mm; GC column: ZB-5ms, 30 m × 0.25 mm, 0.25 µm film
• Inlet temperature: 280 °C; GC temperature program for capillary method: 40 °C to 300 °C at 10 °C/min
• Mass analyzer: Time-of-flight MS, positive FI or FD mode, m/z 35–800
• FI emitter current: 5 mA; FD emitter current ramp: 0–50 mA

Instrumentation used

• JEOL time-of-flight mass spectrometer
• Fused silica guard inlet tube
• ZB-5ms GC capillary column
• Field ionization/field desorption emitters (5 µm FI wire, 10 µm FD wire)
• Helium carrier gas at 0.1 mL/min (inlet) or 1 mL/min (GC)

Main results and discussion

Analysis times varied significantly:

• Blank tube inlet/FI: 0.2–1.0 min per sample
• Capillary GC/FI: 18–>30 min
• FD: ~0.5–0.7 min

Group-type analysis yielded number-average molecular weights (Mn) and polydispersity indices (PD):

• Kerosene: Mn ~179–349, PD ~1.02–1.04
• Diesel: Mn ~248–300, PD ~1.02–1.04
• Crude oil: Mn ~265–403, PD ~1.09–1.12

The blank tube inlet/FI method outperformed FD for low-boiling compounds and outperformed GC/FI for high-boiling and trace constituents. All detected ions contributed to group-type metrics without chromatographic separation.

Benefits and practical applications of the method

• High throughput: analysis in under one minute per sample
• Comprehensive detection across boiling ranges without fractionation
• Simplified sample preparation and larger injection volumes versus GC
• Direct determination of Mn, PD, and group abundances for QA/QC in petrochemical laboratories

Future trends and applications

Emerging directions include coupling guard inlet FI with automated sampling for continuous process monitoring, expanding to other complex matrices (polymers, bio-oils), integration with data analytics for real-time molecular profiling, and miniaturized inlet designs for field- deployable MS systems.

Conclusion

The blank fused silica tube inlet combined with field ionization provides a rapid, robust alternative to GC/FI and FD for determining molecular weight distributions in petroleum samples. It enables efficient group-type analysis with minimal sample handling, suitable for high-throughput quality control and research applications.

References

• M. Ubukata et al., Journal of the Mass Spectrometry Society of Japan, 56, 13–19 (2008).