**Recent Applications and Newly Developed Strategies of Solid-Phase Microextraction in Contaminant Analysis: Through the Environment to Humans**

#### **Attilio Naccarato 1,\* and Antonio Tagarelli 2,\***


Received: 10 October 2019; Accepted: 1 November 2019; Published: 6 November 2019

**Abstract:** The present review aims to describe the recent and most impactful applications in pollutant analysis using solid-phase microextraction (SPME) technology in environmental, food, and bio-clinical analysis. The covered papers were published in the last 5 years (2014–2019) thus providing the reader with information about the current state-of-the-art and the future potential directions of the research in pollutant monitoring using SPME. To this end, we revised the studies focused on the investigation of persistent organic pollutants (POPs), pesticides, and emerging pollutants (EPs) including personal care products (PPCPs), in different environmental, food, and bio-clinical matrices. We especially emphasized the role that SPME is having in contaminant surveys following the path that goes from the environment to humans passing through the food web. Besides, this review covers the last technological developments encompassing the use of novel extraction coatings (e.g., metal-organic frameworks, covalent organic frameworks, PDMS-overcoated fiber), geometries (e.g., Arrow-SPME, multiple monolithic fiber-SPME), approaches (e.g., vacuum and cold fiber SPME), and on-site devices. The applications of SPME hyphenated with ambient mass spectrometry have also been described.

**Keywords:** solid phase microextraction (SPME); air analysis; environmental waters analysis; soil analysis; food monitoring; on-site sampling; human biomonitoring (HBM); chromatography; ambient mass spectrometry; ultra-trace analysis

#### **1. Introduction**

The evolution of modern lifestyles due to the development of industrial civilization has influenced the relationship between humankind and his environment. If, on the one hand, the quality of life has generally changed considerably during the past decades, we may be facing a turnaround. Indeed, for many years, the impact that the current living habits have had on the environment has been underestimated and the receptiveness to topics such as sustainable development has only been recently established. The impact on the environment occurs in many ways, and with a different extent. Generally, less evident but equally important is the impact due to the inordinate use of chemical products. Although their daily use contributes to improving our quality of life, their uncontrollable dispersion in the environment associated with low biodegradability is undoubtedly the main mode of introduction of pollutants into the biological cycles, including those involving humans through nutrition and direct exposure. In this context, the identification and quantification of contaminants in environmental, food, and bio-clinical matrices find more and more space in both routine and research laboratories often driven by growing regulatory attention at the national and international level.

Solid-phase microextraction (SPME) is a well-established green technique for simultaneous extraction and pre-concentration of the compounds from a variety of matrices. Since its introduction

in the early 1990s, this technique has been experiencing rapid development and growth in terms of coating materials, geometries and applications. Although SPME is only part of the broad panorama of microextraction analytical techniques, it plays an important role. Given the simplicity, versatility, and availability in different formats, SPME addresses several challenges associated with the traditional sample preparation procedures and we expect a noteworthy impact on the methods of analysis in the next coming years. The purpose of this review is to highlight the role that SPME is having in contaminant monitoring through the path that goes from the environment to humans. To this end, we revised the studies focused on the analysis of persistent organic pollutants (POPs), pesticides, and emerging pollutants (EPs) including personal care products (PPCPs). We wanted to cover solely the most impactful advancements published in the last 5 years (2014–2019) giving to the reader a picture of the aspects towards the research is directed and at the same time highlighting as the broadly available panorama of SPME strategies can effectively solve even highly complex analytical problems.

#### **2. The Beginning Step: Environmental Matrices**

The journey undertaken by contaminants starts from their release into the environment in at least one of its components, i.e., air, water, soil. As constitutive elements of the biosphere, they are factors in a continuous dynamic equilibrium characterized by exchanges between them and make up a complex system with the life forms they host. In the environmental field, the SPME is a technique that includes a high number of studies: in particular in the analysis of environmental waters, whereas to a less extent are the applications to air and soil associated matrices (Figure 1 and Table 1).

**Figure 1.** Main solid-phase microextraction—geometries and strategies used in the analysis of environmental matrices during the covered period (2014–2019).

#### *2.1. Air*

Although ambient air may seem a simple matrix for less experienced analysts, it presents important analytical challenges because of the difficulty in sampling and calibration procedures. The air comprises many components of both organic and inorganic nature present in the gaseous and particulate phase which are in continuous mixing. In air can occur complex reactions the may convert the pollutants into other chemical species. Unlike the other environmental components, in the air, the analytes can be quickly transported covering even long distances around the world. In addition, phenomena such as release, volatilization, deposition, and resuspension driven by weather conditions lead to cycles of reduction and increase of pollutants and contribute to introducing of new contaminants from soil or water. To further complicate the matrix, due to the presence of suspended particulate matter, the analytes can be present as gaseous compounds as well as bound to the particles, depending on their chemical–physical properties.

In air sampling, the SPME has been exploited in different geometries and strategies (Figure 1, Table 1). The areas in which the research has mainly been addressed during the last five years have regarded the use of commercially available coatings, as well as the development of new coatings and their application. Many studies have been conducted in indoor environments in order to monitor exposure to volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs) that are released into the air by many activities [1–3], but also by used furnishing [4,5]. Commercially available (divinylbenzene/carboxen/polydimethylsiloxane) DVB/CAR/PDMS fiber has been successfully employed in a method for the simultaneous analysis of trihalomethanes (THMs) such as trichloromethane, bromodichloromethane, dibromochloromethane, and tribromomethane, in ambient air [1]. This method was validated for swimming pool air, bearing improvements over previous methods such as a decrease in instrumental runtime, minimization of sample contamination, and elimination of the need for harsh chemicals. Similarly, other commercially available SPME fibers, such as a carboxen/polydimethylsiloxane (CAR/PDMS) and polydimethylsiloxane-divinylbenzene (PDMS/DVB) fiber, were used in the investigation of some VOCs present in a laboratory environment [2] and in the implementation of a method for the analysis of trace carbonyls based on on-fiber derivatization with (pentafluorobenzyl)hydroxylamine (PFBHA) and gas chromatographic (GC) analysis, which was then applied to the monitoring in dated office rooms [5].

As regards the application in an outdoor environment, among the most interesting applications there is the use of the SPME fibers in the arrow version (Figure 2). SPME Arrow consists of a steel rod coated with a larger amount of sorbent material than the traditional SPME fiber, while still being compatible with desorption in a standard GC liner due to its dimensions and sharp, closed tip. The sorbent-coated rod can be withdrawn in a steel tube when the device is not in use. Because of the steel rod, the device is less fragile than a conventional SPME fiber [6].

**Figure 2.** The Arrow SPME system with sorbent exposed (left) and with sorbent covered by a steel tube (right) [6].

The SPME Arrow device coated with commercial and self-made functionalized mesoporous silica materials was used on board an aerial drone, which was used for the simultaneous collection of air samples in two difficult access places such as boreal forest and wetland [7]. The sampled VOCs were detected by gas chromatography with a polar column and mass spectrometry. Established pollutants such as polycyclic aromatic hydrocarbons (PAHs), and other hazardous substances including phthalates, adipates, vulcanizing agents and antioxidants were successfully analyzed in the air generated above synthetic turf football fields following heating of the rubber material using a PDMS/DVB as fiber coating in headspace mode [8]. Benzene, toluene, ethylbenzene, and xylenes (BTEX) in ambient air have been the target of one research with the aim of developing a simpler, low-budget, and accurate method for their quantification based on SPME and gas chromatography coupled with mass spectrometry (GC–MS) [9]. This article shows how in urban areas with highly contaminated air, 20 mL vials can be used for the collection of air samples, and a 100 μm polydimethylsiloxane (PDMS) fiber can be used to sufficiently preconcentrate the analytes before the analysis with GC–MS. Exposure to ambient HCN concentration in a residential area close to a gold mine was assessed by SPME using 75 μm CAR/PDMS fiber [10]. Given the low detection limit, this method provides promising results for monitoring ambient HCN concentrations, and at the same time, minimizes the work for sample preparation, reducing the time and cost of the monitoring.

SPME is not only exploited for direct sampling in the air but it is also used as a convenient strategy for the introduction of the analytes into a gas chromatographic system after the sample preparation step. This strategy was recently used to quantify a class of largely studied pollutants such as organophosphate esters (OPEs) [11,12] bound to airborne particulate matter by a green protocol based on the combination of microwave-assisted extraction (MAE) and SPME followed by gas chromatography coupled with tandem mass spectrometry (GC-MS/MS) analysis [13]. Similarly, aliphatic and aromatic amines (AA) in indoor air were investigated by SPME-GC-MS after the ultrasonic extraction of XAD-2 adsorbent without derivatization [3]. In this paper, the author reached detection limits lower from about a factor of five in comparison to direct injection. This result, consequence of the solvent-less nature of SPME injection, is consistent with the improvement in chromatographic performance reported in the literature, also in the analysis of polar compounds with SPME [14]. The use of commercial SPME fibers has also recently found space in the quantification of some pesticides in atmospheric samples. Thirty-one pesticides belonging to different chemical classes (urea, phenoxy acids, pyrethrenoïds, etc.) and commonly used in non-agricultural areas such as public or private gardens were investigated. After the sampling on glass fiber filters and on XAD-2 resin traps followed by the extraction using the accelerated solvent extraction (ASE) technique, an 85 μm polyacrylate (PA) fiber was immersed in the ASE extract to preconcentrate the analytes before GC-MS analysis with on-injector derivatization using N-methyl-N-(tert.-butyldimethylsilyl) trifluoroacetamide (MtBSTFA) [15].

Porous materials are attracting the attention of the scientific community in numerous scientific fields due to their properties and different possible uses. In this connection, also in the development of SPME coatings these so-called advanced porous materials and cavitands find space, and their use for the analysis of pollutants in the air constitutes a robust line of research still growing [16]. In particular, interesting articles have recently been published on the development of new coatings based on materials such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), and their application in air analysis. These were followed by other papers focused on the use of microporous organic network (MON), or other hybrids materials with MON, which made it possible to improve the extraction efficiency by countering the tendency of some competitors such as water molecules to occupy the pores of MOF [17,18]. Most of the recent applications of these materials have focused on the analysis of established pollutants in particulate matter after a previous extraction of the analytes from the solid matrix [17–19], whereas a COFs coated fiber-based with gas chromatography-mass spectrometric detection was used in the investigation of some volatile and harmful benzene homologs by exploiting the favorable π-π and hydrophobic interactions between the π-conjugated aromatic groups of the coating and the aromatic rings of the analytes [20]. This material was followed by the use of functionalized cavitand that, given its molecular structure, allowed to obtain high enrichment factors, very low detection limits and increased selectivity toward the benzene. For this reason, this coating was proposed as a selective SPME coating for the determination of BTEX at trace levels even for the assessment of short-term exposure in workers [21].

Analytical approaches that are not yet fully explored are the needle-trap and cold-fiber SPME, partly because of the difficulty in commercially obtaining the necessary equipment for the analytic set-up [22,23]. Recently, some studies have been published on SPME fibers used as a passive sampling device to get the Time-Weighted Average (TWA) concentration of VOCs. These papers aim to improve the technique in terms of accuracy, particularly with longer sampling times [24,25]. Advances in this field could contribute in the coming years to the effective use of the SPME in the environmental monitoring programs targeted to the chronic exposure or background concentrations.

#### *2.2. Water*

The presence of organic and inorganic pollutants in the environmental waters is a serious problem for human health and the well-being of ecosystems. Protecting this important environmental sector involves national and international monitoring interventions along with regulatory legislative actions, which are necessarily based on the results of analytical studies in the environmental field. Using microextraction techniques, such as the SPME, have offered a significant contribution to the identification and quantification of established and emerging pollutants in environmental waters by offering reliable analysis tools with growing selectivity and sensitivity [26]. In the last 5 years, the number of published articles in which the SPME is employed to address analytical issues in environmental waters is remarkable (Table 1). Most of these investigations (over one hundred in the covered period) were directed on the development of new coating materials and their use in real case scenarios (Figure 1). These studies, in terms of numbers, are followed by works in which new strategies in using SPME are presented, e.g., new coupling with mass spectrometer equipment [27–29] or the development of a new mechanically robust SPME sampler for the on-site sampling [30]. The involved classes of molecules are various and include both pollutants traditionally investigated such as PAHs [19,31–41] and polychlorinated biphenyls (PCBs) [33,42–45], and new polluting molecules such as dyes [46], additives for materials [14], new pesticides [47–53], antibiotics [54–57], pollutants coming from a number of polluting processes including those in industries [58–60], and last but not least, ultraviolet filters, whose determination in natural water is one of the issues that is attracting greater interest [37,61–73].

The investigation of PAHs is still current, and despite the numerous studies already present in the literature, PAHs are the target in many studies because of their high relevance as pollutants, and for this reason, they are also used as reference compounds to test new SPME uses and coating materials [74,75]. Some noteworthy applications report the successful analysis in environmental waters using coating based on polymer materials [31], MOF [19,32–34], mesoporous carbon materials [35], polyhedral oligomeric silsesquioxanes (POSSs) [36], titanium dioxide-nanosheets [37], coatings based on nanoparticles [38,75], nanotubes (MONTs) [39,40], and material derived from low-cost waste such as biochar [41]. The latter analytical strategy seems particularly interesting as it opens new scenarios involving the recovery of waste materials and their exploitation in an analytical context. Similarly, BTEX were also taken as a reference for verifying the analytical performance of a new SPME coating in environmental waters. These studies were mainly carried out by prof. Ouyang's group, which has achieved satisfactory results for analysis of river, lake, pond and seawater by developing SPME coatings that exploit different materials such as tri-metal centered MOFs (tM-MOFs) [76], graphene–carbon nanotubes (G-CNTs) [77], Prussian blue nanoparticles-doped graphene oxide [75], knitting aromatic polymers (KAPs) [78] and porous organic polymers (POPs) [79] including Scholl-coupling microporous polymers [80].

Among the most impactful trends, there is the application of the newly developed geometries to real environmental case studies. Multiple monolithic fiber solid-phase microextraction (MMF-SPME) was developed as a new extraction approach for aqueous samples; considering its peculiarities, it is proposed as an improved alternative to the traditional SPME in terms of extraction capacity, mass-transfer speed and flexibility [48,81–84]. MMF-SPME consists of four independent thin fibers bound together to form a fiber bunch (Figure 3).

**Figure 3.** Schematic diagram of MMF-SPME (**a**) and the microscope image of the gap between thin fibers (**b**) [81].

Similarly, monolithic fiber has also found application in the in-tube SPME (IT-SPME) through the development of novel materials such as the MOF-monolithic adsorbent with enhanced surface area, which was successfully used to detect fluoroquinolones at ultra-trace level in river water [54], but also the development of new strategies such as magnetism-enhanced monolith-based in-tube solid-phase microextraction (ME-MB/IT-SPME). This technique used modified absorbent material with Fe3O4 nanoparticles and the exertion of a variable magnetic field to overcome the main drawbacks of the traditional monolith-based in-tube solid-phase microextraction (MB/IT-SPME), that is the low extraction efficiency; this method was used for the extraction of triazines from river and lake water [85].

Recently, Kabir and Furton proposed the fabric phase sorptive extraction (FPSE) [86] which is a new promising member of the sorbent-based sorptive microextraction group. This technique overcomes some of the major shortcomings of traditional SPME related to the sorbent coating technique and the geometry of the microextraction device such as the small primary contact surface area and the low amount (typically ~0.5 μL) of sorbent loading that often results in poor extraction sensitivity. Despite its recent development, FPSE has not only been used for the analysis of pollutants such as non-steroidal anti-inflammatory drugs [87], and cytostatic compounds in environmental water [88], but it has already been the topic of some studies focused on its own evolution that suggest interesting scenarios in the next years [89,90].

#### *2.3. Soil*

The monitoring of pollutants in the soil is very important to ensure environmental and food safety. The main pollutants that are being investigated are pesticides, petroleum hydrocarbons, PAHs, PCBs, pharmaceuticals, and industrial residuals. Although soil is one of the environmental compartments together with water and air, unlike them, the presence of pollutants in the soil presents different analytical challenges determined by the complexity of the matrix. Compared with the pollution of air and water, soil pollution is very nonhomogeneous and often local because of a slower diffusion of chemicals. Moreover, soil samples have different physicochemical properties and affinity to analytes [91]. In fact, the main issue in soil analysis using SPME is certainly related to the matrix effect, which can affect the reliability of the analysis and lead to the fouling of the fiber coating caused by irreversible adsorption of macromolecules on the extraction phase. This occurrence can result in the changes in fiber properties and the extraction performance of the fiber upon subsequent use. Currently, despite the recent technical developments, the use of SPME in soil analysis is still a challenge and requires, at least during method development, robust expertise in the field.

During the last five years, SPME has found a place in the analysis of soil pollutants in several respects. Similar to what we highlighted for the study in environmental waters, a significant part of the research is aimed at the development and use of new, better-performing coatings, above all in terms of sensitivity and specificity (Figure 1, Table 1). A cavitand-based SPME coating was synthesized and used for the selective determination of nitroaromatic explosives and explosive taggants at ultratrace levels [92]. Among the most used materials, the MOFs have a leading role. Their application as SPME coating for soil analysis has been extensively investigated in recent years, both as regards the use of new structural variants and their synthesis. After their preparation, these SPME fibers were successfully used in the analysis of trace levels of fungicides [93], PAHs [19,94–96], PCBs [97,98], polybrominated diphenyl ethers (PBDEs) [99].

As for commercial fibers, their use in soil analysis still has many potentials and they have been used not only in method development for new harmful contaminants [100–102] but rather as an analytical tool to carry out other studies, principally with ecological–environmental purposes. Indeed, given its analytical features, SPME allows for the determination of analyte free concentration and for this reason it has been used in bioavailability [103], bio-accessibility [104], and soil sorption studies involving the binding potential of dissolved organic matter (DOM) and the effects of bioremediation on contaminated industrial and agricultural soils [105,106].

Of particular interest is also the deployment of untraditional SPME approaches including the fabric-phase sorptive extraction used for alkyl phenols determination [107], MMF-SPME exploited for the sulfonylurea herbicides assay [47], ionic liquid-coated PTFE tube used for mercury extraction before cold vapor atomic absorption spectrometry (CV-AAS) determination [108], and vacuum-assisted headspace solid-phase microextraction (Vac-HSSPME). The latter was recently reviewed by Psillakis [109] and appears to be a promising route for the analysis of pollutants in solid matrices such as soil because increasing the volatility of the analytes, it allows the analysis in HS-SPME with the consequent reduction of the matrix effect and improvement of the fiber lifetime.


*Separations* **2019** ,

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#### **3. The Spread Step: Food Matrices and Drinking Water**

Chemical pollution of food is perceived as a global safety issue. Most times, the source of contaminants is the environment where the food has been produced. This is the situation of several organic substances coming from industrial processes or, in general, from human activities that persist in the environment and become globally dispersed in air and water sources, and therefore concentrate on fruits, vegetables, and animals through the food chain. Because of the type of contamination, some food products may be more contaminated than others owing to factors such as the specific exposure to pollutants or the differences in plant uptake systems from the ecosystem.

Food samples are complex matrices which often consist of compounds such as proteins, fats, acids, bases, and salts that can largely interfere with the analysis. Moreover, for solid foods, SPME is usually forerun by other previous extraction procedures to get an analyzable liquid extract.

#### *3.1. Drinking Water*

Intensive industrialization of the world has resulted in an increased input of organic pollutants, which may significantly affect the quality of surface waters that are often sources of drinking water for a large part of the world's population. Since most water contaminants are toxic and dangerous, not only to humans but also to animals and plants, these pollutants should be monitored in tap and surface waters. As with environmental water samples, microextraction techniques and in particular SPME have been giving a significant contribution to the determination of several contaminants in drinking water. Given the presence of drawbacks in the use of a traditional commercial coating, such as limited variety of the fibers available, insufficient thermal or solvent instability and limited selectivity, the greatest efforts have been made in the development of new coating materials to offer reliable analysis tools with better selectivity and sensitivity (Figure 4, Table 2).

**Figure 4.** Main solid-phase microextraction geometries and strategies used in the analysis of food matrices during the covered period (2014–2019).

The use of carbon nanotubes (CNTs) has been exploited for the extraction of benzene derivatives [110,111], and phthalates [112] from drinking water samples. Owing to the carbon backbone provided by a two-dimensional structure with unique structural and chemical properties, CNTs are highly hydrophobic and mostly applied to extract hydrophobic analytes, through π-π stacking interactions. However, the surface of CNTs can be modified by chemical and physical functionalization to introduce different functional groups, resulting in enhanced dispersibility and accessible surface. In above-cited papers, CNTs have been combined with a polypyrrole/titanium oxide composite [111], multiwalled carbon nanotubes (MWCNT) functionalized with polyimidazolium ionic liquid was coated on an electrodeposited polyaniline film supported by a stainless-steel fiber [110], and MWCNTs on polystyrene (PS) microspheres have been designed and prepared by layer-by-layer assembly via electrostatic interaction [112]. In general, these coatings offered excellent thermal stability, high EFs and low LODs compared with other analytical methods. The MWCNTs composite microspheres could be of substantial scientific and technological interest with potential applications for the extraction of pollutants in environmental samples. Conductive polymers (CPs) have been also exploited as SPME coating for the extraction of contaminants from drinking water. In particular, thermal and mechanical stability of the polypyrrole-based SPME fibers have been evaluated for the analysis of haloanisoles

in tap water [113] whereas polyaniline/silver (PANI/Ag) composites were prepared, and fabricated into SPME coatings for the extraction of bifenthrin from tap water and the following matrix-free laser desorption/ionization (LDI) of the target analyte [114].

Graphene, functional graphene materials or graphene composites have been successfully used as SPME coatings to detect various organic contaminants. In particular, several functionalized graphene coatings have been developed in order to enhance its extraction capacity and, above all, its selectivity for specific analytes. For example, hydroxy-terminated poly(ethylene glycol) (PEG) grafted onto graphene sheets was prepared via a covalent interaction and then PEG-g-graphene was coated on the etched stainless steel wire by a sol-gel process [115]. This coating was tested for the determination of volatile aromatic compounds (VACs) in tap and mineral waters. In another work, polyethylene glycol–graphene oxide (PEG–GO) sol-gel coating was successfully applied for the analysis of aromatic amines in aqueous samples as target compounds [116]. Due to the adsorptive and inherent advantages of GO nano-sheets as well as the performance of the sol-gel coating technology, this fiber exhibited a porous surface structure, good precision and accuracy, high selectivity and sensitivity, longer life span (over 200 uses) and high thermal stability. Similarly, graphene coatings have also found application in the IT-SPME for the determination of triazines [117,118]. In one case, a method was developed based on the use of a packed column containing graphene oxide supported on aminopropyl silica and following analysis by high performance liquid chromatography and tandem mass spectrometry (HPLC-MS/MS) [117], whereas in the other one a coated column was prepared by the covalent modification of monolayer graphene oxide sheets onto the inner wall of a fused-silica capillary [118]. Other porous carbons have been selected and developed as novel SPME coatings focusing on the two key factors for the choice of suitable porous carbons: surface area and pore structure. Li et al. proposed a C18 composite fiber, prepared with a new method and applied to the analysis of organochlorine pesticides (OCPs) in drinking water samples [119]. The prepared fiber showed excellent thermal stability, solvent resistance and extraction performance more than seven times higher than those of commercial fibers. Ordered mesoporous carbon film [120] and different activated carbon-polymer monoliths [120] were also used as a coating for the quantification of BTEX and phthalates in drinking waters, respectively.

MOFs are porous hybrid materials composed of metal ions at their center, and organic linkers. In the last years, they have become very popular as SPME coatings, due to the easy modification of their pore surfaces, which can lead to enhanced selectivity toward specific analytes. A wide variety of MOFs was proposed in the literature depending upon fiber format, the method used for assembling MOFs onto the fiber surface, the metal ion and ligand in the synthesis of MOFs. Metal-organic aerogel (MOA)/MIL-53(Fe) was fabricated by gluing them on a nichrome wire using silicone glue [121] and the resulting fiber was applied to the analysis of chlorobenzenes in tap water. In another application for the determination of chlorobenzene in HS mode, three types of MOFs with Zn and Cd as metal centers were synthesized with 4,4-biphenyldicarboxylate, terephthalic acid, and 2,6-naphthalene dicarboxylate as ligands [122]. MOF coatings were also prepared by using 2-aminoterephthalic acid and Al salts and termed MIL-53 (Al). These MOFs were employed for the extraction of organochlorine pesticides [123] and PAHs [95]. Experimental results showed that the NH2-MIL-53(Al) SPME coating was solvent resistant and thermostable, and its efficiency for organochlorine pesticides was higher than that of commercially available SPME fiber coatings such as polydimethylsiloxane, polydimethylsiloxane-divinylbenzene, and polyacrylate. A sol-gel coating technique was applied for the preparation of a solid-phase microextraction fiber by coating the metal-organic framework UiO-67 onto a stainless-steel wire. The prepared fiber was explored for the HS-SPME of nitrobenzene compounds from tap water [124]. Again, the sol-gel technique was used for the preparation of fiber by coating MIL-101 onto stainless steel wires. The prepared fiber was explored for the HS-SPME of seven VACs [125]. Metal-organic nanotubes (MONTs) are a novel class of hybrid materials that bridge inorganic and organic nanotubes and possess the advantages of CNTs and MOFs. The study conducted by Li et al. investigates the feasibility of using [Cu3(μ3-O)(μ-OH)(triazolate)2] +

MONTs (Cu-MONTs) as an SPME coating material to extract PCBs from water samples for GC-MS/MS detection [45]. The novel fibers achieved good thermal stability (∼340 ◦C), high enhancement factors (396–1343), excellent repeatability (2.12–7.22%), wide linear ranges (0.1–500 ng/L) and low LODs (3.9–21.7 pg/L) for PCBs. A particular subfamily of MOFs, i.e., zeolitic imidazolate frameworks (ZIFs), was prepared in chemically resistant plastic microtubes for SPME on a polydopamine layer [126]. The extraction efficiency of ZIFs modified microtube was systematically investigated and more than 500-fold enrichment was obtained for analysis of PAHs in tap water.

Ionic liquids (ILs) seem to be an excellent alternative for the currently used SPME fiber sorptive materials, due to the change of their physical and chemical properties by the selection of appropriate ions. Several ILs-based SPME coatings have been prepared to achieve enhanced robustness and therefore a longer lifetime of fiber. A new stationary phase microextraction with physically fixed ionic ILs as sorptive coatings was proposed by Kang et al. for the determination of phosphorus flame retardants (PFR) in tap water [127]. Among the four ILs tested, the coating based on 1-octyl-3-methyloimidazole hexafluorophosphate demonstrated the best performances. Moreover, the prepared fiber has higher extraction capacity, better mechanical stability, lower cost, and comparable reproducibility compared with the commercially available SPME fibers. Several crosslinked polymeric ionic liquid (PIL)-based sorbent coatings of different natures were prepared for the determination of polar organic pollutants [128]. The PIL coatings contained either vinyl alkyl or vinyl benzyl imidazolium-based (ViCnIm- orViBCnIm−) IL monomers with different anions, as well as different dicationic IL crosslinkers. In the work proposed by Pena-Pereira et al., three bis(trifluoromethanesulfonyl)imide anion-based ILs were used to prepare ionogel fibers for the quantification of chlorinated organic pollutants [129]. Ionogels were a family of hybrid materials in which ionic liquids are confined in a sol-gel network. The ionogel based on 1-butyl-1methylpyrrolidinium bis(trifluoromethanesulfonyl)imide [C4C1Pyrr][TFSI] exhibited the best performances in terms of extractability of target analytes. Cross-linked PIL bucky gels were proposed by free-radical polymerization of polymerizable ionic liquids gelled with MWCNT [130]. The PIL bucky gel sorbent coatings demonstrated higher efficiency for the extraction of PAHs from tap water respect to the neat PIL-based sorbent coating, due to a significant enhancement of the π–π interaction between the sorbent coatings and the aromatic analytes. Neat crosslinked PIL sorbent compatible with high-performance liquid chromatography was proposed by Yu et al. [131]. Six structurally different PILs were crosslinked to nitinol supports and applied for the determination of phenols, and multiclass insecticides. Superior extraction performance compared to the other studied PILs was achieved by the PIL-based sorbent coating polymerized from the IL monomer 1-vinyl-3-(10-hydroxydecyl) imidazolium chloride [VC10OHIM][Cl] and IL crosslinker 1,12-di(3-vinylbenzylimidazolium) dodecane dichloride [(VBIM)2C12] 2[Cl]. ILs were used as coatings also for the particular design of the capillary for IT- SPME system, called fiber-in-tube SPME. In particular, to improve the durability and extraction efficiency of an ionic liquid coating, 1-dodecyl-3-vinylimidazolium bromide was polymerized and grafted onto basalt fibers [132]. The tube was connected to an HPLC instrument and the system was applied to the extraction of phthalates from bottled water.

Another extensively exploited strategy in the development of new coating materials is the usage of metallic and metal oxides nanomaterials. For the analysis of drinking water, a palladium coating was fabricated on the SPME fiber by a simple in situ oxidation–reduction process to quantify PAHs and phthalates [133]. Moreover, new materials based on Co-Al bimetallic hydroxide nanocomposites [134] and gold nanoparticles [135] were proposed for fiber IT-SPME configuration in order to assay PAHs in tap water. Silica is another kind of inorganic material that has been widely used due to its features, such as the controllable morphology, excellent stability, and easy modification. VOCs were analyzed by ionogel fibers prepared by sol-gel processing using methyltrimethoxysilane as the silicon alkoxide precursor with the IL 1-methyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide ([C4MIM][TFSI]) confined within the hybrid network [136]. Enrichment factors in the range 275–7400 were obtained under optimum conditions. Remarkably, IL-rich SPME fibers yielded extractability up to 20-fold

higher than that of the SPME fiber obtained after removal of the IL by solvent extraction. The very promising results obtained with ionogel fibers make us consider them as an excellent option for the preparation of advanced SPME coatings. Another silica-based material was used for the determination of chlorophenols [137]. In this work, the prepared continuous ordered mesoporous silica film supported on the anodized titanium wire demonstrated higher extraction efficiency toward the selected chlorophenols compared to the commercial PA fiber. Electrospinning is the most commonly used technique for the fabrication of SPME nanofibers from a wide range of materials. A polyetherimide (PEI) nanofibrous coating was prepared for the assay of PAHs, demonstrating a high filtration capacity and an increased extraction capacity for the analytes [138]. The applicability of inorganic oxide nanoparticles on the extraction efficiency of polyethylene terephthalate-based nanocomposites was evaluated by HS-SPME of BTEX [139]. Four types of nanoparticles including Fe3O4, SiO2, CoO, and NiO were examined as the doping agents and, among them, the presence of SiO2 in the prepared nanocomposite was prominent. BTEX were also analyzed by using sol-gel hybrid organic-inorganic materials prepared in the presence of polyethyleneoxide (PEO) non-ionic surfactants (Triton X-100) [140]. After the analytical procedure was optimized, the dynamic linear range obtained using PDMS-TX100 and PDMS-no TX100 fibers was 4–200,000 and 100–200,000 pg/mL and the limits of detection were 1–3 and 30–300 pg/mL, respectively. Molecularly imprinted polymers (MIPs) are typically synthesized from the copolymerization of a complex (the template and functional monomer), which can form recognition sites being able to rebind a target molecule. In recent years, MIPs have been prepared as monoliths by in situ polymerization directly within micro-columns or capillaries. Recently, several kinds of MIPs have been prepared and used as SPME coatings. For drinking water, a nanostructured molecularly imprinted fiber based on methyltriethoxysilane as the sol-gel precursor was prepared for the extraction of simazine [141]. Actually, SPME can also be coupled with MS directly without chromatographic separation, producing workflows that are not only more robust, sensitive, and selective, but also faster, cheaper, and cleaner. In 2016, Mirabelli et al. proposed a new strategy for the direct coupling of SPME with mass spectrometry, based on thermal desorption of analytes extracted on the fibers, followed by ionization with a dielectric barrier discharge ionization (DBDI) source [142]. The coupling of SPME with mass spectrometry reported in this study has several novel aspects. The work is the only one employing a thermal desorption of SPME fibers for a direct coupling with ambient ionization. This system has allowed to achieve LODs in the low-ng/L range for pesticides belong to various classes in tap water samples using the commercial PDMS/DVB fiber. The advantage over chromatographic approaches, in addition to the much shorter analytical procedure, is the absence of suppression effects from chromatography solvents. Finally, the small overall size of the setup could also allow one to perform in situ analyses with portable MS instruments.

#### *3.2. Fruits and Vegetables*

Fruits and vegetables, as a group of crops from the horticulture class, have very wide importance both as a source of food and health care. Fruits and vegetables are among the most studied food kind in terms of both nutritional principles and contaminants. Also for this food category, SPME has been contributing to the determination of several pollutants in various matrices. As for the water matrix, in the last five years, most of the studies have been focused on preparing new coating materials to be used instead of commercial fibers (Table 2).

A crosslinked PIL on a functionalized stainless steel wire through the oxidized MWCNT was fabricated by Feng et al. for the analysis of naphthol in pomelo and orange samples [143]. The following SPME extraction was carried out in multiple HS mode involving several consecutive extraction cycles of the same sample. The CNTs-based coating was also proposed for the determination of carbamate pesticides in apple and lettuce matrices [144]. In this work, a poly(3,4-ethylenedioxythiophene)-ionic liquid polymer functionalized multiwalled carbon nanotubes (PEDOT-PIL/MWCNTs) composite coating was fabricated which was successively dipped in Nafion solution. The outer layer Nafion enhances the durability and stability of the coating, being robust enough for replicated extraction for at least 150 times. Authors reported the direct GC analysis of carbamates without derivatization, in analogy to other studies [145,146]. Nanohybrids of CNTs/metal oxides have been applied to the determination of pollutants in vegetables. In particular, the synthesis of CNTs@SiO2 nanohybrids with high surface area as an SPME coating was carried out and the obtained fiber was used for the assay of four organophosphorus pesticides in pear, grape, and eggplant [147]. The extracted compounds were detected using gas chromatography–corona discharge ion mobility spectrometry (GC-CD-IMS), demonstrating that the developed CNTs@SiO2 fiber presented better extraction efficiency than the commercial SPME fibers (PA, PDMS, and PDMS/DVB). The same instrumental approach was employed for the analysis of organophosphorus pesticides in cucumber, lettuce, and apple by extracting the analytes with a polypyrrole/montmorillonite nanocomposites coating directly deposited on a Ni-Cr wire [148]. Polypyrrole/montmorillonite nanocomposites film provided a high specific surface due to non-smooth porous structure, resulting in the higher loading capacity and thermal stability. A comparison of the prepared fiber with commercial fibers revealed that the fabricated fiber has higher extraction efficiency for the extraction of OPPs relative to commercial PDMS and PDMS/DVB fibers and pure Ppy. Polypyrrole was also considered for the fabrication of polypyrrole nanowire (PPy NW) which was utilized for the quantification in headspace mode of bisphenol A in canned beans, corn, and peas [149]. The developed protocol involves the detection of the analyte by ion mobility spectrometry (IMS) without chromatographic separation. In a work based on MIP coating, an SPME fiber was developed on silica fiber via sol-gel using calixarene as a functional monomer for extraction of organophosphorus pesticides from apple and pineapple samples [150]. A comparison of MIP-SPME was made with liquid-liquid extraction coupled with gas chromatography demonstrating that much lower limits of detection and better recoveries were achieved by the SPME approach. The porous carbon can be directly obtained by the carbonization of different materials to achieve higher surface area and more abundant pores. In the work carried out by Liang et al., the carbon precursor was a barley husk biomaterial [151]. The obtained carbon was coated onto a stainless-steel wire through the sol-gel technique to prepare a solid-phase microextraction fiber for the extraction of trace levels of twelve multiclass pesticides from vegetable samples. COFs are analogs of metal-organic frameworks without metal in their structure. Wu et al. have designed a novel hydrazone COF and developed a polydopamine (PDA) based method to immobilize COF on a stainless steel fiber for carrying out the HS-SPME quantitative analysis of pyrethroids in apple and cucumber samples [152]. The authors ascribe the very satisfactory performances of this new coating firstly to hydrophobic nature of COFs and, secondly, to the fact of the hydrazone COF possesses abundant phenyl rings and –C=N groups, which provide strong π-π stacking interaction with pyrehroids. In another study reported by the same group, cross-linked hydrazone COFs were prepared via the thiol-ene "click" reaction and applied for the extraction of pesticides in cucumber [153]. The comparison of the cross-linked hydrazone COFs-based fiber with PDMS (7, 85, 100 μm), Car/PDMS, and PA fibers demonstrated that the extraction efficiency of the new coating is superior respect to commercial ones.

MOFs represent a suitable material as SPME coatings also in combination with graphene oxide. In particular, a hybrid material of the zinc-based metal-organic framework-5 and graphene oxide was prepared as a novel fiber coating material and the prepared fiber was used for the extraction of five triazole fungicides from grape, apple, cucumber, celery cabbage, pear, cabbage, and tomato samples [154]. This new fiber was stable enough for 120 extraction cycles without a significant loss of extraction efficiency. The same authors prepared a new coating material, a zeolitic imidazole framework-67 (ZIF-67) templated nanoporous carbon, Co-NPC, by one-step direct carbonization of ZIF-67 without using any other carbon precursors [155]. Successively, the SPME fiber prepared by coating the Co-NPC onto a functionalized stainless-steel wire was used for the determination of five organochlorine pesticides.

ILs, as stable coating compounds in SPME, were also employed together with other materials. For the analysis of vegetable samples, a new ionic liquid 1-vinyl-3-butylimidazolium ditrifluoro methyl sulfimide -calixarene coated fiber has been synthesized on the surface of quartz fiber by the sol-gel

method [156]. The fiber was then applied to the assay of triazines in garlic sprout, cherry tomato, strawberry, cole, cabbage, cucumber, and tomato. Although potato is a common food in many countries and, as a consequence, is extensively studied [157], only one study is reported about the determination of contaminants in this matrix [158]. In this application, graphene oxide was firstly chemical-bonded on the support and then the novel 1-aminoethyl-3-methylimidazolium bromide (C2NH2MIm<sup>+</sup>Br−) or polymeric 1-vinyl-3-hexylimidazolium bromide (poly(VHIm+Br−)) was assembled to the surface of GO by surface radical chain-transfer reaction, respectively. Afterward, C2NH2MIm<sup>+</sup>NTf2 −-GO-coated and poly(VHIm<sup>+</sup>NTf2 −)-GO-coated fibers were obtained through on-fiber anion exchange. Extraction performance of ILs and PILs coatings with different anions were investigated with PAHs and PAEs as the model analytes. Finally, the proposed poly(VHIm<sup>+</sup>NTf2 −)-GO-SPME fiber was used to determine several PAHs and phthalates in potatoes and food-wrap samples, respectively.

The metal oxides can be hybrid with organic/inorganic materials, leading to higher extraction efficiency and great convenience for the coating preparation. A biocompatible environmentally friendly SPME fiber coating (halloysite nanotubes-titanium dioxide (HNTs-TiO2)) was prepared by Saraji et al. [159]. HNTs-TiO2 was chemically coated on the surface of a fused-silica fiber using a sol-gel process and the fiber was evaluated in the determination of parathion in apple, strawberry, and celery. The HNTs-TiO2 fiber was compared in terms of extraction efficiency with bare-silica (sol-gel based coating without HNTs-TiO2), HNTs, carbon nanotubes and commercial SPME fibers (PA, PDMS, and PDMS-DVB). The HNTs-TiO2 fiber showed the highest extraction efficiency among the studied fibers. In another application, zinc oxide hybridized with graphite-like C3N4 (ZnO/g-C3N4) nanoflowers based SPME coating was fabricated [160]. This fiber was successfully used for the simultaneous determination of nine pesticides in cucumber, pear, and green tea. The high extraction performance of ZnO/g-C3N4 nanoflowers fiber towards the pesticide residues may be due to its high dispersion in solution and to the pesticide hydrogen bonding interaction force and the π-π stacking interaction force between the analytes and the organic linker of the ZnO/g-C3N4.

Recently, a new matrix-compatible fiber was developed by Pawliszyn and coworkers by modification of the solid coating of a commercial SPME fiberwith a thinlayer of polydimethylsiloxane [161]. In particular, the outer PDMS layer was employed to protect the polydimethylsiloxane/divinylbenzene (PDMS/DVB), thereby merging the proven suitability of PDMS for sampling in complex matrixes and the extraction capability associated with the original solid porous coating. In the last five years, this fiber was tested for the determination of pesticides and contaminants in various fruits and vegetables in direct immersion mode [162–164]. In an application of DI-SPME for the extraction of multiclass pesticides and other contaminants in the avocado pulp, the polydimethylsiloxane-divinylbenzene-polydimethylsiloxane (PDMS/DVB/PDMS) compared to the DVB/PDMS coating, showed excellent robustness and matrix compatibility for oily matrices [162]. The optimized extraction conditions involving dilution of avocado puree with water and cleaning steps pre- and post-desorption extended the coating lifetime, with enhanced reproducibility for more than 100 consecutive extraction cycles. The robustness and endurance of this new coating were evaluated for analyses of vegetables (i.e., spinach, tomato, and carrot) that present different analytical challenges such as pigmentation, water content, interfering matrix compounds, and vegetable texture [163]. Multiclass pesticides and other contaminants were selected as target analytes to test PDMS/DVB/PDMS fiber in new challenging analytical scenarios. The PDMS/DVB/PDMS fiber was shown to be suitable for use for more than one hundred extractions in raw blended vegetable samples. Moreover, the overcoated fiber was revealed to provide advancements not only in terms of robustness and durability, but also in cleanability and sensitivity in respect to the commercial coating (Figure 5).

**Figure 5.** PDMS/DVB/PDMS fiber lifetime evaluation in spinach: stereomicroscope images of the fiber before (**left**) and after (**right**) manual cleaning of the coating [163].

In another application, a method for the determination of 40 pesticides, belonging to 21 different classes, was optimized for extractions from the grape pulp by means of Design of Experimental (DoE), using the matrix compatible PDMS/DVB/PDMS coating in direct immersion mode [164]. In the broad area of chemometrics, which plays an important role in many scientific disciplines [165–167] experimental design is a multivariate approach to achieve the optimization of a procedure by evaluating the factors simultaneously. The following instrumental analysis was carried out by GC-ToFMS. Commercial PDMS fiber was used by Abdulra'uf et al. for the assay of pesticides belong to various classes in apple, tomato, cucumber, and cabbage [168]. In this study, the authors proposed the sonication of samples for 5 min prior to HS-SPME extraction in order to improve the partition of the targeted analytes into the sample headspace.

#### *3.3. Juices*

Juices are also among the most analyzed food commodities by SPME for safety assessment. A CNTs-based coating was prepared for in-tube and glass capillary SPME configurations [28,169]. In the first work, IT-SPME directly coupled with DART-MS has been reported [28] and applied to the determination of triazine herbicides in orange juice samples. The extraction device consisted of a syringe barrel coupled to one end of the syringe pinhead. A capillary column replaced the other end of the pinhead. This method greatly improved the detection sensitivity, compared with the offline mode, because the majority of the desorption solvent was introduced into DART-MS for detection. The limits of detection (LODs) of the proposed method for the six triazine herbicides were only 0.02–0.14 ng/mL, and the negative matrix effect was minimized due to the IT-SPME procedures. In the study reported by Wang et al. a polymer monolith microextraction (PMME) procedure coupled to plasma-assisted laser desorption ionization mass spectrometry (PALDI-MS) was developed [169]. The extraction device used a "Dip-it" sampler coated with an MWNT incorporated monolith, and laser effectively desorbed the triazines adsorbed on monoliths. In another two works, the direct interface of microextraction technologies to mass spectrometry (SPME-transmission mode, SPME-TM) was proposed and applied to the analysis of multiclass pesticides in various matrices such as grape juice, orange juice, cow milk, and surface water [27,170]. These studies demonstrated the great potential of SPME-TM as a tool for fast concomitant screening and quantitation of agrochemicals in complex matrices. Limits of quantitation in the sub-nanogram-per-milliliter range were attained and the total analysis time did not exceed 2 min per sample [27]. Moreover, it was demonstrated the suitability of SPME-TM for on-site semi-quantitative analyses of target analytes in complex matrices via DART coupled to portable mass spectrometry [170].

In another application, a poly(methacrylic acid-co-ethylene dimethacrylate) monolith was prepared and used as the sorbent of multiple monolithic fiber solid-phase microextraction (MMF-SPME) [82]. The combination of MMF-SPME with high-performance liquid chromatography-diode array detection (MMF-SPME-HPLC-DAD) showed good analytical performance for the quantification of five benzoylurea insecticides in grape and orange juice samples. HPLC-UV was also used for the analysis of parabens in orange, lemon, and peach juices, milk, and waters after the extraction by an in-tube fiber with a nanostructured polyaniline-polypyrrole composite as a coating [171]. Under the optimized conditions, part-per-trillion (ppt) level detection limits were achieved for the analytes. Besides, shorter sample analysis time, more accurate quantification, and satisfactory reproducibility were achieved by in-tube SPME-HPLC, which are favorable for routine analysis of the parabens in various matrices.

Molecularly imprinted fibers were prepared by de Souza Freitas et al. to extract triazole fungicides from grape juice samples followed by gas chromatography-mass spectrometry analysis [172]. This coating, synthesized from methacrylic acid, ethylene glycol dimethacrylate, and triadimenol as a template, is able to renew its selective binding sites because of the gradual thermal decomposition of the polymeric network.

A novel hybrid material incorporating porous aromatic frameworks and an ionic liquid, 1-triethoxy silyl)-propyl-3-aminopropyl imidazole hexafluorophosphate, was developed and applied to the analysis of organochlorine pesticides in apple juice, peach juice, and milk [173]. These materials, prepared in general by combining gel and ionic liquids, are made up of highly conjugated repeating aromatic monomers that involve a strong enrichment ability for benzene homologs. The work reported by Pelit et al. describes the novel usage of polythiophene–ionic liquid modified clay surfaces for fiber production and the following application to the analysis of multiclass pesticides in grape juice samples [174]. Among the fibers developed, the polythiophene fiber co-deposited with C12mimBr modified clay has given the best results in terms of recoveries and sensitivities.

The metal-oxide coating was used also for fruit juice analysis. In the study published by Vinas et al. was described the use of magnetic nanoparticles of cobalt ferrite, with oleic acid as the surfactant (CoFe2O4/oleic acid) for the determination of alkylphenols in orange, pineapple, apple, peach, and grapefruit juices [175]. This demonstrated that appropriate modification of the magnetic core with oleic acid as a surfactant is useful for overcoming some limitations such as the chemical stability and oxidation of magnetic nanoparticles.

Forty pesticides pertaining to various classes were determined by a fast and sensitive direct immersion–solid-phase microextraction gas chromatography time-of-flight mass spectrometry (DI-SPME-GC-ToFMS) method in grapes employing a PDMS/DVB/PDMS fiber (i.e., PDMS-overcoated fiber) [164]. The validated method yielded good accuracy, precision, and sensitivity. With regard to the limitations of the proposed approach, the DI-SPME method did not provide a satisfactory performance toward more polar pesticides and highly hydrophobic pesticides, such as pyrethroids. The PDMS/DVB/PDMS coating was successfully used for the quantitative determination of multiclass pesticides in grape juice by direct coupling to mass spectrometry through dielectric barrier discharge ionization (DBDI) [176]. The use of a matrix compatible SPME coating is critical especially when a direct-MS analysis is carried out for complex food matrices since the occurrence of matrix effects is drastically minimized or avoided. Moreover, the throughput of the analytical procedure is extremely high, since no matrix pre-treatment is necessary due to the anti-fouling properties of the SPME coating used.

#### *3.4. Other Foods*

Various contaminants were determined by SPME technique in other food commodities (Table 2), such as milk [29,177–182], fish [183–187], honey [188–190], tea [191–193], coffee [194,195], oil [196,197], butter [198], rice [199], chili [166,200,201], and seaweeds [202]. In particular, in milk SPME has been exploited in different coatings. The MOF-based coating was prepared for the analysis of bisphenol A [177], commercial fibers were used to assay organochlorine pesticides [178] and PAHs [179], whereas graphene-based fiber was fabricated for the determination of PCBs [180]. A very interesting approach for the analysis of contaminants in complex matrices was developed by Deng et al. [29]. In this work, a surface coated wooden tip was proposed and realized for the first time to form a novel SPME probe that was used for a new SPME and ambient mass spectrometry hyphenated strategy. The proposed SPME-AMS method was applied to the direct ultra-trace analysis of perfluorinated compounds which has been recognized as one of the hot research topics in recent years [203,204].

Analysis of several pollutants (i.e., PAHs, anesthetics, trihalomethanes, furan, organophosphorus pesticides, personal care products, PCBs) in fish was carried out by the conventional ex vivo SPME extraction by using various coatings [185–187] and by the in vivo SPME approach. In vivo SPME is introduced as a low-invasive technique that utilizes biocompatible probes and has been used in a number of animal studies, and the results have shown that it is capable of extracting trace endogenous components from a living system [205,206]. In the last five years, in vivo SPME was applied to the analysis in living fish of benzo[a]pyrene in order to examine cellular responses to BaP exposure [183] and of anesthetics by using a biocompatible custom PDMS fiber [184].

One of the strategies of coupling SPME with ambient mass spectrometry is the surface desorption/ionization of analytes from SPME for AMS. One of the reported ambient ionization techniques coupled with SPME via this strategy is the desorption corona beam ionization (DCBI) [207]. SPME coupled with DCBI is performed by placing an SPME in the stream position of reactive species, and the analytes enriched on SPME was directly desorbed/ionized after the reactive species impacted on the surface of SPME. The reported SPME coupled with DCBI-MS methods were based on thin-film microextraction (TFME). This configuration was proposed by Chen et al. for the determination of Sudan dyes (I-IV) and Rhodamine B in chili oil and chili powder [200]. A small piece of commercial carbon nanotube film was used for the extraction of 1 mL of the sample solution. After extraction, the carbon nanotube film was attached to the front end of a glass capillary, and then transferred to the visible plasma beam of the DCBI source for surface desorption/ionization of the enriched analytes for MS analysis. In another application, the coupling of SPME with ambient mass spectrometry was again proposed for the analysis of Sudan dyes in chili powder [201]. In this case, a bifunctional monolith dip-it, prepared by in situ polymerization of poly(BMA–EDMA–MAA) monolith in the glass capillary of dip-it, was used as an SPME device for direct analysis in real-time mass spectrometry (DART-MS). This sample loading device showed a strong affinity to four Sudan dyes through hydrophilic interaction and hydrogen bond interaction and could be directly analyzed by DART-MS without organic solvent elution or laser desorption.


Selected SPME applications in food matrices.

> **Table 2.**


#### *Separations* **2019** , *6*, 54



*Separations* **2019** , *6*, 54




#### **4. The Final Endpoint: Bio-Clinical Matrices**

Human biomonitoring (HBM), that is the measurement of chemical and/or their metabolites in human tissues and fluids, is a significant means for assessing cumulative exposure to complex mixtures of chemicals and for monitoring chemical hazards in the population. It is also important for understanding the health effects of environmental pollutions and public susceptibility to these compounds. HBM can be used in epidemiological studies in combination with other medical data to show an association between the body burden of pollutants and their consequences on humans [208]. Besides, HBM is a powerful tool for tracking the effectiveness of public health interventions, such as, for example, restrictions on smoking in public places [209]. Considering its usefulness to perform an integrated assessment of environmental exposure, there is increasing awareness for HBM in developed countries that have already started national programs to improve the prevention of disease, injury, and harmful exposures in populations [209]. For these important screening programs, to develop simple, rapid, and reliable methods to quantify the markers of exposure in human body tissues and fluids represents an essential goal. Moreover, it is very necessary to realize automated procedures entailing the benefit of using a small quantity of toxic chemicals, which are not friendly to the environment. In this sense, microextraction approaches can provide a significant contribution to achieving this aim. In fact, many protocols developed in recent years involved the use of microextraction techniques, mainly SPME (Figure 6, Table 3).

**Figure 6.** Main solid-phase microextraction geometries and strategies used in the analysis of bio-clinical matrices during the covered period (2014–2019).

Generally speaking, urine is undoubtedly the preferred matrix among the analyzed biological fluids because of its greater biological half-life and its less invasive sampling. This trend is confirmed in the last lustrum since most of the published works on this topic have been directed, as well as to the assay of markers of several pathological conditions [210–212], toward the determination of several contaminants in urine samples [14,74,83,213–219]. Few studies focused on hair [220–222], three papers were published about analysis in blood/serum [29,223,224], and one regarding meconium [225]. Some studies reporting the application of SPME for the analysis of urine have employed commercially available coatings such as Car/PDMS [214,216], PDMS/DVB [215], PDMS [217], and PA [14] for the extraction and further analysis of different contaminants such as VOCs [214,216], aromatic amines [215], organochlorine pesticides [217], and benzothiazoles, benzotriazoles, benzosulfonamides [14]. In all these works, except that regarding the analysis of the polar compounds benzothiazoles, benzotriazoles, benzosulfonamides, SPME was applied in headspace mode in order to minimize the coextraction of matrix interferences and extend the lifetime of employed coatings. In the DI-SPME application proposed by Naccarato et al. a six-fold dilution of urine sample was carried out to avoid the degradation of polyacrylate coating [14]. In another study by Naccarato et al., the overcoated fiber

(i.e., PDMS/DVB/PDMS) was evaluated as an analytical sampling tool for the first time in human urine and urinary polycyclic aromatic hydrocarbons with 2–6 aromatic rings were considered as target compounds [74]. The satisfactory results achieved in terms of the lifetime of coatings and validation parameters created not only new alternatives for polycyclic aromatic hydrocarbon exposure assessment but also opened new perspectives for the application of direct immersion solid-phase microextraction to the analysis of bio-clinical matrices. As for food analysis, various coatings were proposed for the analysis of urine, such as C18/PAN [213], layered double hydroxide/graphene (LDH-G) [218], inorganic-organic hybrid nanocomposite [219], and cross-linked PIL [83] for the determination of hydroxylated PAHs as metabolites of PAHs [213,219], organochlorine pesticides [218], and endocrine-disrupting chemicals [83]. In the study proposed by Yang et al. [213], an improved low-cost direct SPME-MS technique with glass capillary was further developed for the determination of five typical OH-PAHs in urine samples using a C18-SPME fiber, which then was inserted into a prefilled glass-capillary with spray solvent to generate ions for MS analysis.

Human hair as an indicator in assessing exposure to organic pollutants is less considered than urine. Recently, PAHs [221], multiclass pesticides, DDTs and PCBs [220,222] were determined in hair by commercial PDMS/DVB fiber [220,222] and a CP-Sil 19CB-based in-tube fiber [221].

As regards other biological fluids, the coupling of SPME with ambient mass spectrometry using a surface coated wooden-tip probe proposed by Deng for the analysis of PFCs in milk samples and already cited in the food section [29] was also employed for the analysis in whole blood. For this matrix, the porous structural surface together with the dual extraction mechanisms demonstrated that the SPME probe has an outstanding enrichment capacity, enhancing sensitivity by approximately 100–500 folds. In another application regarding the analysis in serum, a novel mesoporous graphitic carbon nitride@NiCo2O4 nanocomposite-based fiber was prepared and used for sensitive determination of PCBs and PAHs in headspace mode [224], whereas the commercial PDMS fiber was employed in order to analyze organochlorine pesticides and PCBs levels again in headspace mode [223]. To investigate fetal exposure to volatile organic compounds, a method was developed to identify and quantify BTEX and two chlorinated solvents (trichloroethylene and tetrachloroethylene) in meconium [225]. The protocol is based on SPME extraction in headspace mode carried out by commercial Car/PDMS fiber and following gas chromatography–mass spectrometry analysis.


Selected SPME applications in bio-clinical matrices.

> **Table 3.**

#### **5. Concluding Remarks and Future Directions**

This review described the most impactful SPME applications in pollutant analysis published in the last five years (2014–2019). The goal of our paper is giving to the reader information about the ongoing research fields involving SPME and its potential future directions. The surveyed papers cover studies that focused on pollutant determination in the path that goes from the different environmental comparts to the bio-clinical matrices passing through the foods. We revised the studies aimed to determine different chemicals including persistent organic pollutants, pesticides, emerging pollutants, and personal care products. These applications underline the growing demands for green and sensitive analytical methods, which have also fostered the development of new SPME devices, coatings, and geometries. As a results, our paper points out how SPME is an analytical approach constantly evolving in multiple directions, that can satisfy the requirements of green analytical chemistry (GAC) [226], while providing ease of use, high-throughput, extraction efficiency for trace analysis, robustness, suitability for in vivo and on-site analysis, and easy coupling to various separation techniques and direct MS analysis.

Regarding the environmental field, the researchers have mainly focused their efforts on the analysis of environmental waters followed by a smaller number of applications on soil and air. In these areas, the SPME has not only been used to set-up new methods, but it was also used as a proper tool in the analyst's hands to carry out eco-biological and health control studies driven by its unique features such as the capability to determine the concentration of the analytes unbound to the matrix and the advancements in on-site sampling devices and modes.

In food analysis, SPME is a widely used technique particularly for drinking water and in other water matrices. The development in the last five years of new coatings that allow improving the fibers in terms of mechanical robustness, selectivity, and resistance to the matrix effect together with the possibility to perform in vivo and ex vivo analysis opened new research directions for pollutant analysis for the next years.

As for the monitoring of pollutants in bio-clinical matrices, SPME has not yet expressed its full potential. The bio-clinical matrices are very complex and although in the last 5 years the SPME was mainly used for pollutant monitoring in urine, the in vivo mode, and to a larger extent, the new direct couplings with the ambient mass spectrometry, are expected to open exciting scenarios in the near future.

**Author Contributions:** Conceptualization, A.N.; writing—original draft preparation, A.N. and A.T.; writing—review and editing, A.N. and A.T.; visualization, A.N. and A.T.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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