Apart from the previous works, the combination of MOF and polymer-coated m-NPs has also been found beneficial for the extraction of PAEs. In this sense, Li et al. [88] prepared Fe3O4@MIL-100 and Fe3O4@ SiO2@polythiophene as mixed sorbents for m-dSPE extraction of six PAEs (DMP, DEP, DBP, BBP, DEHP, and DNOP) from tap and mineral water samples. Although PAEs contain both benzene rings and alkyl chains, the use of the MOF-coated m-NPs alone did not show enough extraction efficiency particularly for both DMP and DEP. This was attributed to the greater water solubility of these low-molecular PAEs as well as lower hydrophobic interaction with this sorbent. When the polymer-coated m-NPs were used, it did not show good adsorption capacity for the PAEs that contain longer alkyl chains, especially DNOP. This was associated to the negative effect of these long alkyl chains on the π-π interactions with the sorbent. Instead, both sorbents in a 1:1 (w/w) ratio were combined under sonication, and the mixture was used as sorbent, giving satisfactory extraction recovery values for all the PAEs and matrices.
1.5 Others Minor Sorbent-Based Microextraction Techniques
Although dispersive versions of SPE have been widely used due to the well-known advantages they offer, as it has been already mentioned, trends in sorbent-based extraction techniques are also focused on the miniaturization of the extraction devices, which has given place to the appearance of new modifications of conventional SPE but with reduced amounts of sorbent or slight changes on extraction devices. Some of these alternative methodologies have also been successfully applied to the analysis of PAEs in water samples (see Table 1.3).
In this sense, microextraction by packed sorbent (MEPS) is considered as a miniaturized technique derived directly from SPE which can be coupled directly to the chromatographic systems without any additional modification. MEPS only use 1–2 mg of sorbent to adsorb the analytes successfully. Concretely, it is packed between frits inside a microsyringe and extraction is performed within it by subsequent suction. After the analytes are trapped, the packed sorbent is washed with water to remove the interferences. Finally, the target analytes are eluted by appropriate solvent aspiration into the microsyringe [89]. Amiri et al. [90] used only 2 mg of synthesized hydroxyapatite NPs packed inside a 0.5-ml microsyringe for the rapid extraction of five PAEs from river, mineral and tap water samples. To evaluate the extraction efficiency, the effect of multiples drawing-ejecting cycles in the range 10–60 cycles were performed in the same vial containing 8 ml of spiked sample at 50 μg/L. The results showed that the maximum peak areas were achieved using 40 cycles for all the target analytes and then kept constant. When the same process was repeated using 8 mL of spiked sample at 100 μg/L discarding each 0.5 mL load to another vial (16 cycles), the results did not improve. Therefore, 40 cycles of draw-eject in a same vial was the best approach in terms of simplicity. With it, the developed methodology showed good sensitivity, repeatability, and relative recovery values, being a really interesting alternative to conventional SPE procedures.
As it is well-known, thin film microextraction (TFME) initially emerged as an alternative to classical SPME, which provides a higher volume of extractive phase as well as a larger surface-to-volume ratio compared to SPME fibers, which results in an improved sensitivity with relatively shorter extraction times [91, 92]. However, such an interesting alternative extraction method has been adapted and applied in different ways which can be considered as SPE variants more than SPME modifications. As an example, Mehrani et al. [93] developed a membrane by peeling a piece of poly m-aminophenol-nylon6-GO nanofiber and supported it in a circular holder in order to emulate a miniaturized version of a conventional SPE device. Then, the analytes were extracted by pushing the sample through the membrane with a syringe. After a washing step, the analytes were eluted with 2-propanol. In this case, a GC-MS system was used for the separation and detection of the analytes. This methodology was successfully applied to the analysis of four PAEs from water and milk samples, with LODs in the range 0.1–0.15 μg/L for all the studied analytes.
Table 1.3 Some examples of the application of other sorbent-based extraction techniques for the analysis of PAEs in water samples.
| PAEs | Matrix (sample amount) | Sample pretreatment | Separation technique | LOQ | Recovery study | Residues found | Comments | Reference |
| DIBP, DBP, DMP, and DEP | Water (20 mL) | 4-mg poly m-aminophenol/nylon6/GO nanofiber previously conditioned with 2 mL 2-propanol, the sample was passed at 1 mL/ min, washing with 2 mL water, and desorption with 400 μL 2-propanol at 0.05 mL/min | GC-MS | 0.3–0.5 μg/L | 96–101% at 20 and 100 μg/L | Two samples were analyzed and no residues were detected | The poly m-aminophenol/nylon6/GO nanofiber gave better results than poly m-aminophenol/nylon6 nanofiber. 2-propanol showed higher extraction efficiency than MeOH, ethanol, chloroform and ACN as desorption solvent. Milk was also analyzed | [93] |
| DMP, DEP, DBP, BBP, DEHP, and DNOP | Bottle water (20 mL) | 10 mg of MIL-101(Cr) MOF were introduced in a porous polypropylene membrane and stirred for 35 min; desorption into 0.5 mL MeOH by sonication for 15 min | GC-MS | 0.01–0.07 μg/L | 76.8–111.6% at 1, 4, and 10 μg/L | Two samples were analyzed and contained at least 1 PAE at levels from 0.28 to 2.85 μg/L | MIL-101(Cr) showed higher extraction efficiency than AC and MIL-101(Fe) as extraction sorbent, and MeOH than acetone and ACN as desorption solvent. A computational modeling method accurately predicted the extraction efficiency of MOFsbased materials toward the PAEs | [94] |
| DMP, DEP, DIBP, DBP, and DEHP | River, bottled mineral and tap waters (8 ml plus 20% w/v NaCl) | MEPS using 2 mg of nanohydroxyapatite previously conditioned with 0.5 mL MeOH followed by 0.5 mL of deionized water, drawejecting of the sample for 40 cycles, washing with 1 mL water, and desorption with 60 μL dichloromethane by solvent aspiration into the syringe | GC-FID | 0.07 and 0.25 μg/L | 85.5–99.2% at 0.25, 5, and 50 μg/L | One sample of river and tap waters, and 3 mineral water samples were analyzed and contained at least 2 PAEs at levels from 0.5 to 5.3 μg/L | Dichloromethane showed higher extraction efficiency than MeOH, ethyl acetate, hexane, and ACN as desorption solvent | [90] |
μ-SPE, micro-solid-phase extraction; AC, activated carbon; ACN, acetonitrile; BBP, benzyl butyl phthalate; DBP, dibutyl phthalate; DEHP, di(2-ethylhexyl) phthalate; DEP, diethyl phthalate; DIBP, diisobutyl phthalate; DMP, dimethyl phthalate; DNOP, di-n-octyl phthalate; FID, flame ionization detector; GO, graphene oxide; GC, gas chromatography; LOQ, limit of quantification; MEPS, microextraction in packed syringe; MeOH, methanol; MIL, Material of Institute Lavoisier; m-NPMOF, metal organic framework; MS, mass spectrometry; PAE, phthalic acid ester; PSTFME, thin film microextraction.