Analysis of Polycyclic Aromatic Hydrocarbons in Petroleum Vacuum Residues by Multiple Heart-Cutting LC Using the Agilent 1290 Infinity 2D-LC Solution

Significance of the Topic

Polycyclic aromatic hydrocarbons (PAHs) are toxic and potentially carcinogenic compounds monitored in high-boiling petroleum products such as vacuum distillation residues and bitumen. Regulatory limits on PAH content aim to reduce environmental emissions and health risks in applications like road construction, combustion processes, and accidental releases. The complex matrices and low analyte concentrations in these samples demand highly selective and sensitive analytical techniques.

Objectives and Study Overview

This study demonstrates the use of the Agilent 1290 Infinity Multiple Heart-Cutting (MHC) two-dimensional liquid chromatography (2D-LC) system for automated, quantitative determination of PAHs in petroleum vacuum residue. The primary goals were to separate PAHs from bulk hydrocarbons and polar interferences, to resolve individual PAHs from each other and from alkylated homologs, and to validate the method for linearity, precision, and quantitation.

Methodology and Instrumentation

The workflow couples normal-phase LC (NPLC) in the first dimension and reversed-phase LC (RPLC) in the second dimension via the MHC valve configuration. Key steps:

• Sample Preparation: Vacuum residue dissolved at 200 mg/mL in iso-octane/cyclohexane (1:9 v/v).
• First-Dimension (NPLC): Aminopropyl column (Agilent Polaris NH2, 2×150 mm, 3 μm) with heptane mobile phase at 120 μL/min, 30 °C. A six-loop selector parks four narrow time windows (heart-cuts) covering PAH elution (e.g., 5.4–15.2 min) while backflushing polar fractions to waste.
• Second-Dimension (RPLC): Dedicated PAH column (Agilent ZORBAX Eclipse Plus PAH, 4.6×100 mm, 3.5 μm) using water/acetonitrile gradient at 0.3 mL/min, 30 °C. Fluorescence detection with wavelength settings optimized per PAH; diode-array detection after the first dimension monitors fraction placement.
• Instrumentation: Agilent 1260 Infinity first-dim pump, Agilent 1290 Infinity second-dim pump, autosampler, thermostatted column compartment, valve drive, and MHC upgrade kit controlled by OpenLAB CDS software.

Main Results and Discussion

The first-dimension NPLC effectively separated PAHs from saturated hydrocarbons, grouping them by ring number and degree of fusion, while polar components were removed by backflush. Direct RPLC of the complex sample produced a broad unresolved hump, but targeted second-dimension analysis of heart-cut fractions yielded sharp peaks for PAHs amid clustered alkylated species. Representative heart-cuts demonstrated clear separation of compounds such as fluoranthene, pyrene, benzo(a)anthracene, chrysene, and higher-molecular-weight PAHs. Method validation for PAHs 9–13 (benzo(a)anthracene to benzo(a)pyrene) showed excellent linearity (R2>0.9999) over 0.05–1 ppm and injection precision with RSDs below 2% at 1 ppm.

Benefits and Practical Applications

This online 2D-LC approach delivers several advantages:

• Automated fractionation and cleanup, reducing sample handling and labor compared to offline methods.
• High selectivity and orthogonality improve resolution of PAHs from bulk matrix and from each other.
• Sensitive quantitation via fluorescence detection tailored for each PAH.
• Ability to detect low-level PAHs in challenging petroleum residues in a single run.

Future Trends and Potential Uses

Further developments may integrate mass spectrometric detection for structural confirmation and broader compound coverage. Miniaturized or accelerated 2D-LC configurations could enhance throughput for quality control labs. Advances in automated software control and data interpretation are expected to facilitate real-time monitoring of PAHs in petrochemical processing and environmental samples.

Conclusion

The Agilent 1290 Infinity MHC 2D-LC solution offers a robust, automated workflow for selective and quantitative PAH analysis in complex petroleum vacuum residues. By combining NPLC cleanup with RPLC separation and optimized detection, the method achieves high resolution, sensitivity, and reproducibility, meeting regulatory and industrial requirements.

Reference

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