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 a consequence of its larger volume and surface area. Moreover, SBSE desorption can be directly developed in a thermal desorption unit in many GC systems, so it maintains one of the main advantages derived from the use of SPME [64]. Since then, this technique has evolved and a wide variety of laboratory made coatings have been developed in order to extend the field of application of SBSE, especially to the extraction of polar compounds, as well as to improve its selectivity [64]. Up to now, few works have used SBSE for the determination of PAEs in water samples (see Table 1.1). This is the case of the work of Prieto et al. [65], who analyzed six PAEs together with sixteen polycyclic aromatic hydrocarbons (PAHs), twelve polychlorinated biphenyls (PCBs) and three nonylphenols in sea and estuarine waters, and that of Si et al. [66], who extracted fifteen PAEs from sea water. In both works, commercial PDMS-coated stir bars of 0.5- and 1-mm thickness were used, respectively. In both cases, the addition of methanol (MeOH) to the water sample (20% for 0.5-mm thickness fibers and 10% for 1-mm thickness fibers, v/v) was crucial to avoid PAEs adsorption on the glass walls and to improve the extraction efficiency, particularly for long chain PAEs. In the last work, four commercially available stir bars containing different amounts of PDMS were evaluated in terms of extraction capacity. The results showed that the stir bar containing the highest amount (150 μL vs. 50, 75, and 150 μL over carbon) provided the largest peak areas, which was consistent with the principles of the process. However, the sensitivity for the higher-molecular weight PAEs (i.e., DNOP and DEHP) was notably lower, which is probably caused by the fact that this kind of nonpolar compounds establish strong interactions with the PDMS coating which could result in an incomplete desorption and the consequent higher LODs and LOQs. The combination of SBSE with GC-MS systems using single quadrupoles operated in the SIM mode allowed obtaining high extraction efficiency as well as high sensitivity (0.1–489 ng/L) in general.
1.4 Solid-Phase Extraction
One of the most important advantages of SPE compared to SPME is the availability of a wider range of relatively cheap commercial sorbents. However, the low rate of diffusion and mass transfer between the packed sorbents and the target analytes generally results in a slow extraction process. Moreover, in some cases, cartridges can be blocked when complex matrices are analyzed, leading to process failure. Besides, a previous cartridge conditioning is required which also lengthens the extraction process. In order to solve these problems, dSPE—as a miniaturized version of SPE in many cases—emerged as an alternative based on the direct addition of the sorbents (without preconditioning) to samples or extracts [67, 68], improving the contact area between sorbent and the sample/extract solution. Consequently, the analytes are extracted much faster by simple manual, vortex, or ultrasound agitation, while most interferences remain in the solution (depending on the selectivity of the sorbent). Then, the sorbent can be separated by its retention into a SPE column [25] or by centrifugation and the subsequent supernatant decantation [69]. In the first case, the analytes are later eluted as in conventional SPE, while in the second, the analytes have to be desorbed under agitation, centrifugation, and decantation. Therefore, this variant saves time, efforts, and solvents, with the consequent economic savings that this entails [70, 71]. The few dSPE methods developed for the extraction of PAEs from water samples (see Table 1.2) have been based on the application of graphene [72], GO-MIP [69], MIPs microbeads [73], a commercial
