Methanol showed higher extraction efficiency than ACN and chloroform as elution solvent. The use of poly(1 -vinylimidazole)-carboxy-latocalix[4] arene m-NPs showed higher enrichment capacity than Fe3O4 and poly(1-vinylimidazole) separately. m-dSPE using poly(1-vinylimidazole)-carboxy-latocalix[4] arene m-NPs showed better results compared with SPE with C18 and Cleanert SCX cartridges. Drinks, tonic lotions, and human serum were also analyzed
MeOHMeOHMeOHMeOHMeOHACN, acetonitrile; BBP, benzylbutyl phthalate; BMPP, bis(4-methyl-2-pentyl) phthalate; DAD, diode-array detector; DAP, diallyl phthalate; DBEP, di(2-butoxyethyl) phthalate; DBP, dibutyl phthalate; DCHP, dicyclohexyl phthalate; DEEP, di(2-ethoxyethyl) phthalate; DEHA, di(2-ethylhexyl) adipate; DEHP, di(2-ethylhexyl) phthalate; DEP, diethyl phthalate; DHP, diheptyl phthalate; DHXP, dihexyl phthalate; DIBP, diisobutyl phthalate; DIDP, diisodecyl phthalate; DINP, diisononyl phthalate; DIPP, diisopentyl phthalate; DMEP, di(2-methoxyethyl) phthalate; DMIMs, dummy molecularly imprinted microbeads; DMP, dimethyl phthalate; DNOP, di-n-octyl phthalate; DNP, dinonyl phthalate; DNPP, di-n-pentyl phthalate; DPhP, diphenyl phthalate; DPP, dipropyl phthalate; dSPE, dispersive solid-phase extraction; G, graphene; GC, gas chromatography; GO, graphene oxide; HCF, hierarchical carbon framework; HPLC, high-performance liquid chromatography; LOQ, limit of quantification; m-dSPE, magnetic solid-phase extraction; MeOH, methanol; MIL, Material of Institute Lavoisier; MIP, molecularly imprinted polymer; m-NPs, magnetic nanoparticles; MOF, metal organic framework; MS/MS, tandem mass spectrometry; MS, mass spectrometry; MWCNTs, multiwalled carbon nanotubes; PAE, phthalic acid ester; PDA, poly(dopamine); PS, polystyrene; SPE, solid-phase extraction; UV, ultraviolet; ZIF, Zeolitic Imidazolate Framework.
MeOHMeOHMeOHMeOHMeOH
Figure 1.5 Scheme of a similar extraction procedure carried out by Chen et al. [75] using the same (β-CD-PNIPAM temperature-sensitive polymer as extractant for the determination of phenolic compounds in river water samples. Reprinted from [75] with permission of Elsevier. Peak identification: phenol (BP), 2,4-dichlorophenol (2,4-DCP), (β-naphthol ((β-NP), and bisphenol A (BPA).
Despite the simplicity of the dSPE, the whole procedure can be even improved and simplified if the sorbent particles can be manipulated with a magnet. Such dSPE mode is called m-dSPE and is based on the use of magnetic NPs (m-NPs), which can be applied as synthesized (though in very few cases), although they are generally functionalized or coated with other chemical species or materials, resulting in many cases a “coreshell” structure, in order to improve their selectivity, or even embedded them in the extraction sorbent to provide it with magnetic properties [50]. Despite the extraction step is performed in a similar way as in dSPE, in this case, the magnetic sorbent containing the analytes is retained in the extraction recipient using an external magnet while the sample matrix is easily discarded without the need of an additional centrifugation step or the retention of the sorbent in an empty column. Finally, the analytes are desorbed from the magnetic sorbent using a suitable solvent and, once more, the sorbent is retained with the magnet to separate the solvent containing the analytes by decantation for their determination with a suitable technique.
Although a wide variety of metals, metal oxides and alloys can be used to provide magnetic properties to the sorbent, Fe3O4 NPs have been used as the main support with this purpose due to their extraordinary magnetic features and the possibility of being modified by anchoring certain chemical species or materials to obtain sorbents that provide different selectivity. Therefore, as in the rest of the sorbent-based microextraction techniques, the preparation of appropriate sorbents is one of the most important aspects to be considered. In this regard, carbon-based nanomaterials like MWCNTs [76, 77] and graphene [26, 78, 79], combined with m-NPs have once again been one of the most used for the extraction of PAEs from water samples due to their well-known properties in terms of high surface area-to-volume ratio that guarantees high extraction efficiency. As an example, MWCNTs were functionalized via chemical modification by Jiao et al. [76]. Figure 1.6 shows the scanning electron microscopy and transmission electron microscopy images of m-NPs successfully combined with MWCNTs. The authors used this sorbent to study a large set of thirteen PAEs, paying special attention to the elution step because PAEs may not be easily desorbed due to the strong interaction between the sorbent and analytes. For this purpose, different common organic elution solvents (i.e., acetone, MeOH, n-hexane, ethyl acetate, and toluene) were evaluated, finding that the use of toluene provided higher recovery percentages compared to non-aromatic solvents, particularly for BBP and diphenyl phthalate (DPhP), since they contain two and three benzene rings, respectively. However, since toluene is highly toxic, toluene-acetone mixtures were also evaluated in different proportions (1:1, 1:4, and 1:9, v/v), obtaining that toluene-acetone (1:4, v/v) kept satisfactory recovery values, while the use of toluene-acetone (1:9, v/v) decreased them. Finally, the viability of this method was demonstrated for the extraction of the selected PAEs from drinking water, showing an extraordinary extraction capacity which, in combination with the inherent advantages of m-dSPE, becomes this methodology an excellent alternative to be explored for the analysis of other pollutants. Similarly, Luo et al. [77] immobilized m-NPs onto MWCNTs by ultrasound application as a simpler alternative to chemical functionalization (see Figure 1.7). In particular, appropriate amounts of the previously synthesized Fe3O4 NPs and MWCNTs were dispersed in dimethylformamide and sonicated, achieving the spontaneous assembling of both materials resulting in a magnetic composite. Finally, it was washed with water and suspended in water at 40 mg/mL, taking a certain volume of it for the extraction procedure. Next, the usual m-dSPE procedure was carried out for the enrichment of sixteen PAEs from mineral and tap water, juice and carbonated drinks as well as one perfume sample. The combination of this extraction procedure with GC-MS allowed obtaining a good extraction capacity in relatively short times as well as high sensitivity, becoming this methodology an interesting alternative to be considered.
Figure 1.6 Scanning electron microscope image (A) and transmission electron microscope image (B) of the MWCNTs coated m-NPs. Reprinted from [76] with permission from Royal Society of Chemistry. m-NPs, magnetic nanoparticles.
