both obtained through electrochemical polymerization, for the extraction of mineral water samples. Song et al. [41] also prepared a MWCNTs-polystyrene (PS) material via electrostatic interactions as SPME coating, and Zhang et al. [43] developed a SiO2-PDMS-MWCNTs fiber by a sol-gel method. In both cases, drinking and environmental water samples were analyzed. All these fiber coatings presented a porous structure with very large surface areas where both phases (the MWCNTs and polymer) took part in the extraction procedure, enhancing the final adsorption capacity of the fiber. Moreover, in the last work, the organic-inorganic bilayer structure was designed to increase the stability and durability of the coating. In particular, a stainless-steel fiber was coated with a SiO2 layer, which was used as support for the chemical bonding of the second layer of PDMS-MWCNTs (see Figure 1.3). The first coating with a SiO2 layer is a general procedure widely used for coating different surfaces or particles [50, 51]. Compared with commercial PDMS, PA, and DVB-CAR-PDMS fibers, this new coating showed better extraction efficiency and longer lifetimes (150 vs. 50–100 times) for the extraction of DMP, DEP, DBP, and DEHP. It is also noteworthy to mention the extensive study that was conducted by these authors to evaluate the influence of salt addition on the extraction efficiency. It was observed that the addition of different kinds of salts such as NaCl, CaCO3, FeCl3, and MgCl2 at different concentrations (0%, 5%, 10%, and 15%, w/v) can have a negative or a negligible effect on the recovery values. Therefore, no salt was added to the samples, as in the vast majority of the published works on this topic (see Table 1.1).
Figure 1.3 Schematic representation of the SiO2-PDMS-MWNTs fiber preparation. Reprinted from [43] with permission from The Royal Society of Chemistry. MWNTs, multi-walled carbon nanotubes; TEOS, tetraethoxysilane; TSO-OH, hydroxyl terminated silicone oil.
Graphene is another of the allotropic forms of carbon that has been used as SPME coating. It consists of a monolayer of sp2 hybridized carbon atoms arranged in a 2D network. Like MWCNTs, graphene has a high surface area, high chemical and thermal stability as well as a high affinity for hydrophobic and aromatic compounds. Then, graphene-polymer nanocomposites have also been used as excellent SPME fiber coatings for the extraction of PAEs. Such is the case of the work developed by Amanzadeh et al. [20] in which a stainless-steel fiber was coated using a new graphene/polyvinylchloride (PVC) material and evaluated successfully as a SPME fiber for the extraction of dipropyl phthalate (DPP), DBP, DEHA, and DEHP from drinking waters and sunflower and olive oil samples. However, even though it was used in the HS mode, a single fiber could be used only 60 times without a significant decrease in the extraction efficiency. As a very interesting experiment, these authors also determined these PAEs in boiling water exposed to a polyethylene terephthalate (PET) bottle. Although the water used did not contain residues of any of the target PAEs at the beginning, residues of DPP and DBP were found at 2.1 and 1.8 μg/L, respectively, after filling this bottle with the same water just after boiling (it was analyzed after cooling). That is, the PAEs with low molecular weight (250.2 g/mol for DPP and 278.3 g/mol for DBP compared to 370.5 g/mol for DEHA and to 390.5 g/mol for DEHP) have larger water solubility, so these kinds of PAEs migrated more easily from PET bottles containing hot water.
Another example of the benefits of using graphene, is the work of Tashakkori et al. [52] who prepared SPME fibers based on the use of the ionic liquid (IL) 1-(3-aminopropyl)-3-vinyl imidazolium bromide and 1-(3-aminopropyl)-3-vinyl imidazolium tetrafluoroborate grafted onto graphene oxide (GO) previously deposited onto stainless-steel wires. On the one hand, GO disperses more easily for the first preparation step and inherits the mechanical properties of graphene but with a moderate decrease of mechanical parameters (Young’s modulus and intrinsic strength) due to the alterations produced in the sp2 structure [53, 54]. On the other hand, ILs can be structurally customized based on diverse procedures to tune the extraction performance [55]. In fact, ILs can establish a broader variety of interactions with the analytes such as π-π, dipolar, hydrogen bonding, and ionic/charge-charge [56]. As a result, they are also suitable for the extraction of hydrophobic compounds and aromatic analytes like PAEs. Consistently, the first GO-IL fibers showed better extraction efficiency for the analysis of DMP, DEHP, DBP, DNPP, BBP, and DNOP in tap and sea water samples (also in instant coffee samples) than other lab-made fiber, as well as commercial PA and CAR-PDMS fibers, using DI mode in all cases.
MIPs also provide a great improvement in selectivity since they have cavities specifically designed for a particular compound or group of analogous compounds [57, 58]. That is to say, retention occurs through a molecular recognition mechanism based on their size, shape and three-dimensional distribution of functional groups [59]. He et al. [45] demonstrated that MIPs are quite suitable as SPME fiber coatings for the successful extraction of low (DMP, DEP, DBP, and diallyl phthalate -DAP-) and high-molecular PAEs (DNOP) simultaneously, from bottled, tap, and reservoir water samples, although it is true that the latter was poorly extracted since DBP was used as template molecule during the synthesis of the polymer. Moreover, the peak areas obtained using the MIP fiber were much higher than those using a non-imprinted fiber prepared with the same protocol (without the addition of the template molecule), but also better compared to commercial PDMS, PA, and CW-DVB fibers (see Figure 1.4). These results indicate that the MIP fiber provided a better selectivity for the structural analogues of DBP, while commercial SPME coatings are more susceptible to undesirable interferences in the extraction process.
Another variant of SPME which has also been applied for PAEs extraction is in-tube (IT)-SPME [60]. In this format, a very thin tube is coated in its inner walls and the extraction and desorption are carried by the introduction and extraction of the sample inside the tube several times [61]. As in conventional SPME, the combination of materials with high surface areas and polymers afford a high extraction capability, while its porous structure provides suitable dynamic transport during extraction. As examples, Wang et al. [23] used poly(dopamine) (PDA) to functionalize melamine formaldehyde aerogel on carbon fibers and were packed inside IT-SPME tubes for the extraction of seven PAEs from drinkable and surface water, while the performance of this process embedding activated carbon (without any chemical modification) in different polymers (e.g., poly(butyl methacrylate-co-ethylene dimethacrylate) (poly(BMA-EDMA)) and PS-DVB) was investigated by Lirio et al. [62]. In this last work, low extraction recovery values were obtained when a solution containing eight PAEs was collected using monolithic columns with native poly(BMA-EDMA) and PS-DVB. On the contrary, the presence of increasing amounts of activated carbon provided a higher extraction efficiency under the same conditions. Moreover, the activated carbon-PS-DVB monolithic column exhibited better extraction performance than the activated carbon-poly(BMA-EDMA) one. Therefore, the first was applied in the IT-SPME of mineral water samples obtaining recovery values in the range of 78.8%–104.6% at 50 μg/L.
Figure 1.4 Extraction yields with different fibers (MI-SPME, PDMS, CW/DVB, and PA) in water samples. Extraction conditions: 12 ml of spiked pure water including NaCl content of 10% w/v, stirring at 60°C in DI, adsorption time 30 min, desorption at 250°C for 10 min. Reprinted from [45] with permission from Elsevier. CW, carbowax; DAP, diallyl phthalate; DBP, dibutyl phthalate; DEP, diethyl phthalate; DMP, dimethyl phthalate; DNOP, di-n-octyl phthalate; DVB, divinylbenzene; MI, molecular imprinted polymer; PA, polyacrylate; PDMS, polydimethylsiloxane; SPME, solid-phase microextraction.
1.3 Stir Bar Sorptive Extraction
As a technique derived directly from SPME, SBSE is based on the same principles of distribution of the analytes between the sorbent and the sample. This technique, introduced by Baltussen et al. [63], is based on the use of a small device consisting of a magnetic bar which is introduced in a glass tube coated by PDMS in most cases, generally using around 50–300 times larger PDMS amounts as coating, so SBSE significantly increases the enrichment factors compared to SPME, providing at the same time a higher extraction efficiency as