Abstract
Current society is living in a world in which it is exposed to a broad spectrum of contaminants that can pose different risks for health. In this sense, we are daily bombarded with news related to pollution by plastic residues (especially in the oceans), being one of the main issues that humans must face today, not only because of the direct effects of plastics but also because of the variety of contaminants they can release to the environment. Probably, the most important ones are phthalic acid esters (PAEs), since they easily migrate from the polymeric matrix to the surrounding media, acting as endocrine disruptors in human organisms and resulting in multiple diseases. Their occurrence in water matrices is of especial importance, since it is essential for life, and the presence of PAEs, even at very low levels, can cause serious health problems. This book chapter aims at providing a general and critical overview of the different analytical methodologies that have been developed for the analysis of PAEs in water samples and which are based on the application of sorbent-based microextraction techniques, which is one of the current trends in the Analytical Chemistry field.
Keywords: Phthalic acid esters, analytical methods, sample preparation, microextraction techniques, water samples, sorbents
1.1 Introduction
Phthalic acid esters (PAEs) are a group of dialkyl or alkylaryl esters of phthalic acid (see Figure 1.1), commonly known as phthalates, which are widely used as additives in the polymer industry but also added to paints, adhesives, lubricants, and cosmetics, among others [2]. As an example, low-molecular PAEs such as butyl benzyl phthalate (BBP), dibutyl phthalate (DBP), and diethyl phthalate (DEP) are widely used as solvents and emulsifiers to maintain color and fragrance mainly in beauty products and pharmaceuticals, while high-molecular PAEs such as di(2-ethylhexyl) phthalate (DEHP) are highly used as plasticizers to make polymeric materials more workable and flexible. As a result of the extremely high production of such products, especially plastics, PAEs are exorbitantly present in the daily life. Among them, DEHP is the most currently used. In fact, its production as plasticizer is estimated to be a quarter of the total [3, 4]. Due to these widespread applications and intensive production, together with the fact that they are only retained in the polymer structure through weak secondary molecular interactions and not covalently, PAEs can easily migrate to the environment. As a result, PAEs have become ubiquitous contaminants in the environment, in particular, they can be found in natural waters such as lake, river, sea, and ground waters [5, 6], especially those adjacent or downstream from industrial locations [5]. In addition, their possible migration to drinking waters that are in contact with plastic materials like mineral and tap waters must also be taken into account, as well as their final presence in waste waters [5, 7].
It has already been demonstrated that many PAEs act as endocrine disruptors and that they can be toxic for reproduction, even at extremely low concentrations [8–11]. Even more worrying is the fact that certain PAEs can be easily degraded in the environment by bacteria and fungi and their degradation products can also have an important toxicity. Such is the case of DEHP that can be degraded to DBP, DEP, and especially to mono-2-ethylhexyl phthalate (MEHP), which has shown to be more toxic than DEHP [12, 13] (see Figure 1.2). As a result of the high human exposure to PAEs and their metabolites, their potential risks for health and their persistence, several organizations have established an increasingly broad and restrictive legislation. As examples, the European Union has listed several PAEs as compounds suspected to produce endocrine abnormalities [15] and the International Agency for Research on Cancer has classified DEHP in the group 2B (possibly carcinogenic to humans) [16]. Moreover, the US Environmental Protection Agency (EPA) has included several PAEs (BBP, DBP, DEHP, DEP, dimethyl phthalate (DMP), and di-n-octyl phthalate (DNOP)) in its priority list of pollutants and has established limits of 6 μg/L and 400 μg/L for DEHP and di(2-ethylhexyl) adipate (DEHA) in drinking water, respectively [17], while this maximum allowed concentration has been established in 8 μg/L for DEHP by the World Health Organization [18] and in 1.3 μg/L in surface waters by the European Union [19]. Considering all the above mentioned, it is clear that there is an increasing need to develop highly sensitive and reliable analytical methods for monitoring trace amounts of PAEs in different samples and, especially, in water.
Figure 1.1 The chemical structures of PAEs. Adapted from [1]. PAEs, phthalic acid esters.
Figure 1.2 DEHP 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
