Applied Water Science

Figure 1.1The chemical structures of PAEs. Adapted from [1]. PAEs, phthalic acid esters.

Figure 1.2DEHP biodegradation pathways to obtain MEHP, DBP, and DEP. Reprinted from [14] with permission from Elsevier. DBP, dibutyl phthalate; DEHP, di-2-ethylhexyl phthalate; DEP, diethyl phthalate; MEHP, mono-2-ethylhexyl phthalate; PA, polyacrylate.

PAEs have been analyzed in water samples using gas chromatography (GC) coupled to flame ionization detectors (FIDs) [20], mass spectrometry (MS) [21] and tandem MS (MS/MS) [22], or highperformance liquid chromatography (HPLC) coupled to diode array detectors (DADs) [23], ultraviolet (UV) [24], and MS [25]. Among them, GC is normally the preferred technique since most PAEs are nonpolar and thermostable. It is important to notice that, in all these analytical methods, it has been necessary to include previous sample preparation steps before instrumental analysis to achieve accurate and sensitive results. These steps consist on the isolation and pre-concentration of PAEs since they can be found in water samples at extremely low concentrations. However, since PAEs are not ionizable in water, these samples are normally analyzed directly or after a simple filtration without pH adjustment regardless of the sample preparation technique used in each case [26].

In this context, special attention should be paid to the risk of sample contamination during their analysis, which would result in false positives and/or over-estimated concentrations. As it has already been said, PAEs are ubiquitous contaminants and this includes their possible presence in any laboratory since they can be found in solvents, reagents, filters, etc. Consequently, previous washing steps using PAE-free solvents, if possible (since most organic solvents also contain some PAEs), subsequent heating of non-volumetric glassware at high temperatures (450–550°C) for several hours (4–5 h), washing volumetric or any glassware material with strong oxidizing agents, and, in some cases, even wrapping in heat-treated aluminum foil to avoid adsorption of PAEs from the air are carried out, among others [27–29]. Despite all these precautions, residues of PAEs may finally appear, and the analysis of blanks should be developed on a daily basis in every batch of samples so that background levels can be suitably subtracted [21, 25, 30].

Until very recently, the most widely used sample preparation methods, also for the analysis of PAEs in water samples, have been based on the use of liquid-liquid extraction (LLE) and solid-phase extraction (SPE) [31, 32]. The need for developing quicker, simpler, and miniaturized extraction procedures able to maintain or even to improve the required sensitivity of the analysis has resulted in the development of new sample preparation techniques. In this sense, microextraction techniques have gained notoriety since the extraction is carried out using amounts of extracting phase much smaller than the sample amount (extraction of analytes is not always exhaustive). Microextraction techniques have inherent advantages such as exceptionally high enrichment factors, simplicity, time saving, and the generation of small amounts of organic solvent or reagents wastes, without affecting reproducibility, and compatibility with most analytical instrumentation [33–36]. Among these new alternatives, sorbent-based microextraction techniques have been widely used due to the great diversity of commercially available sorbents, as well as new extraction sorbents (in particular nanomaterials) that are constantly being proposed for their direct use or after a previous functionalization to enhance their selectivity [35–37].

As a result of the above-mentioned issues, the aim of this book chapter is to provide a general overview of the sorbent-based microextraction techniques applied to the analysis of PAEs in water samples, which mainly include solid-phase microextraction (SPME), dispersive SPE (dSPE), and magnetic dSPE (m-dSPE), among others. The extraction ability to quantitatively and selectively extract these target analytes will be commented and discussed.

SPME has been the sorbent-based microextraction technique most used for the analysis of PAEs in water samples (see Table 1.1) probably, among other reasons, because it allows to reduce the risk of PAEs contamination during sample extraction with respect to other conventional extraction techniques. On the one hand, the absence of organic solvents and additional steps reduces PAEs background levels. On the other, water is in many occasions a simple and clean matrix that contains few interferences, so the direct immersion (DI) mode can be used without hardly any impairment of its lifetime (except for waste waters or marine water). Moreover, in SPME, extraction, pre-concentration and direct desorption into analytical instruments can be easily integrated in most cases.

The first studies in which SPME was applied for PAEs extraction from water samples dealt with the direct application of commercial fiber coatings, including polydimethylsiloxane (PDMS), polyacrylate (PA), PDMS-divinylbenzene (DVB), carboxen (CAR)-PDMS, and carbowax (CW)-DVB. As examples, Cao [21] demonstrated the better performance of PDMS-DVB fibers compared to PDMS and DVB-CAR-PDMS fibers for the headspace (HS) SPME extraction of nine PAEs (DMP, DEP, DIBP, DBP, BBP, DHXP, DEHA, DEHP, and DNOP) from bottled water samples, while Polo et al. [28] found that PDMS-DVB fibers also give higher extraction efficiency than PDMS, PA, CAR-PDMS, and CW-DVB fibers for DBP, BBP, and DNOP, but CAR-PDMS and PA fibers show a better extraction performance for DMP and DEP, and for DEHP, although the first one provided better results for simultaneous analysis of the target PAEs from bottled, industrial harbor, river, urban collector, and influent and effluent waste water samples. As expected, the optimal SPME fiber for the extraction of a particular phthalate depends on both the properties of the coating and the PAEs since these compounds differ from each other in terms of polarity and volatility and, therefore, on their distribution between the fiber coating and the matrix. In addition, low-molecular PAEs are more volatile than those of high-molecular weight [38]. As a result, low-molecular PAEs would be expected to be more efficiently extracted when HS mode is used [38]. However, they have a certain solubility in water and, as consequence, they volatilize very slowly from this kind of matrices. Contrary, although high-molecular PAEs are less volatile, they have a lower water solubility and they volatilize faster at higher temperatures than it could be expected [38]. Accordingly, it has been observed that DEHP and DNOP are extracted from different water samples more efficiently than BBP, DEP, and DMP using HS-SPME [28, 39]. Nevertheless, most of the works published on this topic are based on DI-SPME instead of HS-SPME.

Table 1.1Some examples of the application of SPME and SBSE for the analysis of PAEs in water samples.

μ-ECD, micro-electron capture detector; AC, activated carbon; ACN, acetonitrile; BBP, benzylbutyl phthalate; BMA, butyl methacrylate; BMPP, bis(4-methyl-2-pentyl) phthalate; CAR, carboxen; CE, capillary electrophoresis; COFs, covalent organic frameworks; CW, carbowax; 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; DHXP, dihexyl phthalate; DI, direct immersion; DIBP, diisobutyl phthalate; DIDP, diisodecyl phthalate; DINP, diisononyl phthalate; DIPP, diisopentyl phthalate; DMEP, di(2-methoxyethyl) phthalate; DMP, dimethyl phthalate; DMPP, dimethylethyl phthalate; DNOP, di-n-octyl phthalate; DNPP, di-n-pentyl phthalate; DPhP, diphenyl phthalate; DPP, dipropyl phthalate; DVB, divinylbenzene; EDMA, ethylene dimethacrylate; FID, flame ionization detector; G, graphene; GC, gas chromatography; GO, graphene oxide; HPLC, high-performance liquid chromatography; HS, headspace; IT-SPME, in tube-solid-phase microextraction; LOQ, limit of quantification; MeOH, methanol; MIP, molecularly imprinted polymer; MS/MS, tandem mass spectrometry; MS, mass spectrometry; MWCNTs, multi-walled carbon nanotubes; NPs, nanoparticles; PA, polyacrylate; PAE, phthalic acid ester; PDA, poly(dopamine); PDMS, polydimethylsiloxane; PET, polyethylene terephthalate; PPy, polypyrrole; PS, polystyrene; PVC, polyvinylchloride; SBSE, stir bar sorptive extraction; SPE, solidphase extraction; SPME, solid-phase microextraction; TPB, 2,4,6-triphenoxy-1,3,5-benzene; UHPLC, ultra-performance liquid chromatography; UV, ultraviolet.