Detecting Nitro-PAHs in Aquatic Environments Using Solid-Phase Microextraction

A recent study done by researchers at Nanjing University in Jiangsu, China developed a solid-phase microextraction (SPME)-based system for detecting nitrated polycyclic aromatic hydrocarbons (nitro-PAHs) in aquatic environments. Their findings were published in the Journal of Chromatography A (1).

Freshwater Aquatic Weeds and Algae | Image Credit: © mustbeyou - stock.adobe.com

Polycyclic aromatic hydrocarbons (PAHs) are a class of chemicals that naturally occur in coal, crude oil, and gasoline (2). They can also be produced when coal, oil, gas, and other materials are burnt. Nitrated PAHs (nitro-PAHs) are derivatives of PAHs with the aromatic rings substituted by at least one nitro-functional group. They are commonly formed from incomplete combustion of fossil fuels or biomass, as well as reactions between PAHs and highly reactive free radicals or molecules, such as nitrous oxides and nitric acid. They have also been frequently detected in various aquatic environments, like drinking water and domestic sewage. While nitro-PAHs in waters are typically lower than corresponding unsubstituted PAHs, they demonstrate higher toxicity and mutagenicity. It has become vital to create highly sensitive determination methods for nitro-PAHs in waters.

Analyzing nitro-PAHs in environmental samples usually requires enrichment pretreatments, with solid-phase microextraction (SPME) often used for this process. This non-exhaustive sample preparation technique involves a small volume of an extractive phase, which is immobilized on a solid support, and then exposed to a sample for a certain amount of time (3). The technique has been praised as a green, versatile, non-exhaustive sample pretreatment method, integrating sampling, isolation, enrichment, and sample introduction into a single step. However, wide practical implementation of SPME in nitro-PAH analysis faces various obstacles, including the scarcity and limited stability/extraction efficiency of commercially available coatings.

For this study, the scientists introduced a novel and carbonaceous SPME coating derived from metal-organic framework (MOF), namely a spherical assembly consisting of carbon nanorods with hierarchical porosity (HP-MOF-C), for extracting and determining nitro-PAHs in waters. The HP-MOF-C coated fiber showed superior nitro-PAH extraction efficiencies, with enrichment factors approximately 2–70 times higher than commercial fibers. This enhancement stems from its strongly hydrophobic nature, π-π electron coupling/stacking, and π-π electron donor-acceptor interactions between the carbonaceous framework of HP-MOF-C and the nitro-PAHs. Further, the HP-MOF-C system’s unique hierarchical porous structure accelerated the diffusion of nitro-PAHs, better facilitating their enrichment.

The HP-MOF-C showed a spherical structure formed from the self-assembly of 1D carbon nanorods, providing a large surface area and a hierarchical porous structure with pore sizes ranging from micro- to macro-scale. The coated fiber had markedly high enrichment factors for nitro-PAHs, owing to the strong hydrophobic and π-π interactions (including π-π electron coupling/stacking and EDA interactions) and facilitated nitro-PAH diffusion in the meso-/macropores. Overall, the determination method based on the HP-MOF-C coating afforded low detection limits for the nitro-PAHs, while also showing satisfactory thermal and chemical stabilities, long service life, and good applicability in real water samples. This study showed a new, efficient, and reliable method for detecting nitro-PAHs in water samples, while simultaneously highlighting the great potential of hierarchical carbons derived from MOFs in environmental analyses.

References

(1) Tao, Q.; Ma, P.; Chen, B.; Qu, X.; Fu, H. Hierarchically Spherical Assembly of Carbon Nanorods Derived from Metal-Organic Framework as Solid-Phase Microextraction Coating for Nitrated Polycyclic Aromatic Hydrocarbon Analysis. J. Chromatogr. A 2024, 1736, 465352. DOI: 10.1016/j.chroma.2024.465352

(2) Polycyclic Aromatic Hydrocarbons (PAHs). CDC Environmental Health 2025. https://www.epa.gov/sites/default/files/2014-03/documents/pahs_factsheet_cdc_2013.pdf (accessed 2025-1-28)

(3) Solid Phase Microextraction. ScienceDirect 2015. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/solid-phase-microextraction (accessed 2025-1-28)