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.
Finally, the use of miniaturized extraction devices based on membrane-protected sorbents has also been explored for the analysis of PAEs in water. That is the case of Wang and co-workers [94], who developed a small device (1 cm × 1.5 cm) using a polypropylene membrane sheet as a kind of heat-sealed bag containing 10 mg of sorbent. The extraction was developed introducing the extraction device in the sample and stirring for a certain time while the device tumbled freely. Then, the device was taken out with a pair of tweezers, dried with a tissue paper and the analytes were desorbed with MeOH in a vial. After that, the device was washed with MeOH and this volume was combined with the desorption portion. In this case, three materials were evaluated as possible sorbents in terms of enrichment factors, MIL-101(Cr), MIL-100(Fe) and powdered activated carbon. MIL-101(Cr) showed the strongest adsorption ability for the extraction of the six PAEs selected from bottled water samples, providing suitable extraction recovery.
1.6 Conclusions
The use of sorbent-based microextraction techniques is a current trend in sample preparation that will surely continue its consolidation during next years as a result of their inherent advantages related with Green Analytical Chemistry principles. Such techniques have also been applied with success for the extraction of PAEs from different types of water samples. Among them, SPME has been mostly used, followed by dSPE. SBSE, as a variation of SPME, has also been applied but in extremely few occasions together with other small variants of each of the technique. Though commercial sorbents can also be used for such purpose, the latest achievements in this field have been done using laboratory made coatings, mainly using nanomaterials such as MWCNTs, graphene, GO, coated m-NPs, etc., as well as new polymers or nanoporous materials such as MOFs. In this last case, a suitable characterization of them using different surface characterization techniques is necessary as well as clear demonstration of their stability and batch-to-batch reproducibility. In all probability, this will constitute an active research area in the near future.
Once suitably optimized, the application of all these extraction procedures has demonstrated that PAEs are present in nearly any type of water sample as a result of their capacity to migrate from plastics, which is probably their main source. Further studies should also continue to be assessed in order to continue monitoring their presence in all environmental compartments.
Acknowledgements
J.G.S. would like to thank “Cabildo de Tenerife” for the Agustín de Betancourt contract at the Universidad de La Laguna.
References
1. Feng, C.-H., Jiang, S.-R., Micro-scale quantitation of ten phthalate esters in water samples and cosmetics using capillary liquid chromatography coupled to UV detection: effective strategies to reduce the production of organic waste. Microchim. Acta, 177, 167, 2012.
