Ultra-Fast Determination of the Hydrocarbon Oil Index by Gas Chromatography using a Modular Accelerated Column Heater (MACH)

Significance of the Topic

The hydrocarbon oil index (HOI) is a critical environmental parameter defined by ISO 9377-2 for assessing mineral oil contamination in water.
Traditional FTIR and conventional GC methods deliver reliable data but suffer from long cycle times and limited throughput.
Accelerated analytical techniques that maintain compliance and performance can greatly improve laboratory productivity in environmental monitoring and industrial quality control.

Objectives and Study Overview

This study aimed to develop and evaluate ultra‐fast gas chromatographic methods for determining HOI according to ISO 9377-2 using a modular accelerated column heater (MACH).
Key goals included:

• Reducing total analysis time while preserving separation, sensitivity, and linearity.
• Implementing fully compliant procedures and specialized methods for subgroup fractionation (n‐C10–C20, n‐C20–C30, n‐C30–C40).
• Assessing method robustness and potential for rapid screening.

Methodology and Instrumentation

Analyses were performed on an Agilent 6890 GC fitted with:

• GERSTEL PTV injector (CIS 4) and MPS 2 autosampler
• Low Thermal Mass (LTM) MACH module on the oven door
• Capillary columns: 5 m × 0.32 mm i.d., 0.25 µm Rtx-1 (Restek) or 5 m × 0.18 mm i.d., 0.18 µm DB-5 MS
• Flame ionization detector at 350 °C, acquisition frequency 200 Hz

Typical temperature programs:

• ISO‐compliant fast method: 40 °C (0.75 min) to 200 °C at 200 °C/min, then to 350 °C (3 min)
• Ultra‐fast screening: 35 °C (3 s) to 60 °C at 123 °C/min, then to 350 °C at 369 °C/min; hydrogen carrier
• Enhanced resolution: 35 °C (10 s) to 350 °C at 100 °C/s

Calibration used mineral oil standards (diesel and lubricant) and n‐alkane mixes; quantitation was based on external standards covering 50–1000 ng/µL.

Main Results and Discussion

System performance with n-alkane mixtures demonstrated baseline separation from C10 to C40 and C40/C20 response ratios > 0.9.
Comparison of conventional and MACH methods:

• Conventional GC cycle: ≈ 39 min (including oven cool‐down)
• MACH ISO‐compliant method: 5.5 min total cycle (splitless, bakeout, rapid cooling)
• Ultra‐fast screening: complete elution < 2 min (split 1:20)
• Subgroup fractionation method: total cycle ≈ 4.5 min with discrete n-C10–20, n-C20–30, n-C30–40 peaks

Linearity was excellent (R² > 0.99985), and a robustness test (three standards, 100 high-concentration injections) yielded peak area RSD < 1%.

Benefits and Practical Applications

The MACH‐based methods deliver:

• Up to 18-fold reduction in cycle time for HOI analysis
• Maintained or improved sensitivity and linearity compared to conventional GC
• Options for rapid screening or detailed subgroup profiling
• Full compliance with ISO 9377-2 performance criteria

These advantages support high‐throughput environmental monitoring, industrial QA/QC, and regulatory laboratories.

Future Trends and Applications

Further developments may include:

• Integration with mass spectrometric detection for compound‐specific identification
• Automated method translation tools for diverse column chemistries
• Application of MACH technology to other environmental and petrochemical indices
• Miniaturized, field-deployable GC systems for on-site analysis

Conclusion

The modular accelerated column heater (MACH) enables ultra‐fast, ISO-compliant determination of the hydrocarbon oil index with cycle times reduced from tens of minutes to 2–5.5 minutes.
The approach maintains analytical performance, offers flexible screening or detailed subgroup analysis, and significantly enhances laboratory throughput.

Instrumentation Used

• Agilent 6890 GC with GERSTEL MACH LTM module
• GERSTEL PTV (CIS 4) and MPS 2 autosampler
• Capillary columns: 5 m Rtx-1 (0.32 mm i.d., 0.25 µm) or 5 m DB-5 MS (0.18 mm i.d., 0.18 µm)
• Flame Ionization Detector at 350 °C, 200 Hz acquisition