Task-specific solid-phase microextraction based on ionic liquid/polyhedral oligomeric silsesquioxane hybrid coating for sensitive analysis of polycyclic aromatic hydrocarbons by gas chromatography–mass spectrometry

Highlights

• A selective fiber coating based on ionic liquid (ILs) and POSS.

• Rapidly photopolymerization of hybrid coating on stainless steel fiber for SPME.

• ILs and POSS endow the coating with high extraction performance and stability.

• A simple, sensitive and eco-friendly SPME-GC–MS method for PAHs determination.

Abstract

A direct immersion solid-phase microextraction (DI-SPME) approach for gas chromatography-mass spectrometry (GC–MS) based on hybrid fiber coating of ionic liquid and polyhedral oligomeric silsesquioxane (POSS) is presented. To fabricate the task-specific coating for the enrichment of polycyclic aromatic hydrocarbons (PAHs), 1-butyl-3-vinylimidazolium bis[(trifluoromethyl)sulfonyl]imide (IL) and POSS were rapidly photoinitiated copolymerized within 5 min on a stainless steel fiber. The high efficient extraction of target analytes can be attributed to a combined result of multiple interactions including the strong CF⋯HC pseudohydrogen bonding, π-π stacking, hydrophobic force, and molecular sieve effect. A wide linear range (0.04–400 ng L⁻¹) with low detection limits in the range of 0.004 and 0.5 ng L⁻¹ were obtained for PAHs by GC–MS. The applicability of this coupling method was successfully demonstrated by the analysis of trace PAHs in real river water and soil samples, with satisfied recoveries (84.2–108.6%) and relative standard deviations (<8.1%). Compared to the other commercial fiber-based SPME methods, the IL/POSS hybrid coating-based SPME is much cheaper, thermally stable and capable of eliminating possible deleterious effects as well.

Introduction

Green sample preparation is generally aimed at minimizing the consumption of organic solvents and operation costs, reducing the analysis time and the detection limit as well. Solid-phase microextraction has played a significant role in the cutting edge of sample preparation since it integrates sampling, sample preconcentration and injection into a single one step [[1], [2], [3]]. Currently, the commercial polymer coated fibers-based SPME has become a mature technique and achieved widespread application in GC. The extraction performance of SPME fibers is greatly relied on the intermolecular and steric interactions between the coating and the target analytes, which are offered by coating chemistry [4]. However, some major bottlenecks such as the inborn analyte discrimination or desorption problems, as well as the unsatisfied mechanical stability and operating costs, have been found in the real application of these commercial coatings. Fast synthesis or modification of task-specific coating materials that possess a variety of chemistry and excellent stability under extreme conditions are highly required [5]. Several types of fiber coatings employing new materials, e.g., ionic liquids (ILs), polymeric ionic liquids (PILs) and nanomaterials, or synthesis strategies, have been prepared for target studies.

Ionic liquids and PILs have been characterized as a new class of designable sorbent coatings in SPME, owing to their inherent physicochemical properties, including negligible vapor pressure, tunable solvation capability, high viscosity, and thermal stability [[6], [7], [8]]. ILs-based SPME can be considered as an ideal alternative to traditional extraction methods, both in headspace and direct-immersion (DI) modes. Using the ILs as the solvent can impart chemical functional groups to the fiber coatings and undergo unique solvation interactions with analytes [9], or even enhance thermal stability of coating [10]. Exploiting these features for SPME-GC is suggested to demonstrate exceptional benefits because selective extractions can be easily realized.

However, the widely used ILs coatings that physically coated on a silica fiber are inherently unstable when exposing to the high temperature of the injection port of GC–MS (>200 °C), due to the decrease in viscosity [11]. Frequent cleaning of the liner assembly with organic solvent and re-coating of the fiber after every run are likely needed. Sometimes the deleterious effects such as GC-column blocking occur [12], resulting in a reducing extraction capacity and sensitivity [13]. Chemically bonded ILs/PILs-coatings [5,14,15], which are supported on solid supports [[16], [17], [18], [19], [20], [21], [22]], have been demonstrated to be promising alternatives to physical coatings, owing to their higher extraction efficiency and larger surface area [9]. Synthesis of such coatings was usually executed by refluxing, free radical copolymerization or sol-gel technology that are cumbersome and time-consuming (>12 h) [[23], [24], [25]]. Apart from their fragile or costly counterparts, the stainless steel fiber has recently attracted particular attention since it is cheaper, easy-to-obtain, strong and durable [24,25]. To meet the requirement of complicated sample analysis in GC–MS, the design of new bonded ILs-based SPME coatings that can withstand high temperature and extreme matrix composition (such as high content of salt or organic modifier and low/high pH), as well as displaying good selectivity for analytes, is highly needed.

The incorporation of polymeric networks with polyhedral oligomeric silsesquioxane (POSS), a kind of cage-like silsesquioxane with nano size and reactive organofunctionalized groups, can result in dramatic improvement in polymer properties [26,27] and therefore has significantly attracted interests in monolith preparation [28,29]. The synthesis process of POSS-based monolith was simple, in comparison to the sol–gel method, since the hydrolysis and condensation reactions of siloxane are no longer needed. Several attempts to introduce ILs as the functionalized monomer in the free radical copolymerization preparation of POSS-based monoliths have been proposed, demonstrating increased specific surface areas and multiple interactions [[30], [31], [32]]. Our recent work also indicated that the photopolymerization was a preferable route for fast preparation of hybrid material [33]. A particular case of POSS-incorporated PILs SPME fiber coating was proposed by Yao et al. [28] via a thermally-initiated copolymerization of PILs and POSS on Ti wire at 60 °C for 12 h. To fulfill the needs of further HPLC-MS/MS analysis of perfluorinated compounds, the use of toxic organic solvent and prolonged desorption procedure in SPME is evitable, which has weakened the advantage of this technique.

Polycyclic aromatic hydrocarbons (PAHs) are a group of persistent organic pollutants with enormous environmental and food concern, due to their strong carcinogenic and mutagenic [34,35]. Since many PAHs exhibit toxicity even at low levels of exposure, some of them have been formulated as priority pollutants by the United States Environmental Protection Agency (U.S. EPA) and other official agencies [36]. Sensitive analysis of PAHs residue in air, water or soil is of great importance for risk assessment and environmental or food safety. This work focuses on the development of a thermally stable, selective and efficient DI-SPME approach for GC–MS, by introducing the POSS as a multifunctional crosslinker and the 1-butyl-3-vinylimidazolium bis[(trifluoromethyl)sulfonyl]imide (VBIMNTF₂; IL) as a task-specific monomer in a PEG200/1-propanol porogenic system. The chemically bonded IL/POSS hybrid SPME coatings were rapidly cured via free radical polymerization on the pretreated stainless steel fibers under UV light at 365 nm within 5 min. The feasibility of the prepared fiber coating in DI-SPME coupled to GC–MS has been proved by using PAHs as the model analytes. The improved mechanical stability, matrix tolerance and specific surface area of fiber coating greatly contribute to the excellent analytical performance of coupling method for real samples. To the best of our knowledge, this work presents the first example of ILs/POSS hybrid coating that can be used in SPME-GC–MS. It also provides a simple photoinitiated synthesis strategy that can be extended to the design of novel sorbent coatings.