Ultrasound-assisted emulsification–microextraction of emergent contaminants and pesticides in environmental waters

https://doi.org/10.1016/j.chroma.2008.02.091Get rights and content

Abstract

The analytical use of ultrasound-generated emulsions has recently found a growing interest to improve efficiency in liquid–liquid extraction since they increase the speed of the mass transfer between the two immiscible phases implied. Thus, dispersed droplets can act as efficient liquid–liquid microextractors in the continuous phase, and later they can be readily separated by centrifugation. A novel method based on ultrasound-assisted emulsification–microextraction (USAEME) and gas chromatography coupled to mass spectrometry (GC/MS) has been developed for the analysis of synthetic musk fragrances, phthalate esters and lindane in water samples. Extraction conditions were optimized using a multivariate approach. Compounds were extracted during 10 min in an acoustically emulsified media formed by 100 μL chloroform and 10 mL sample (enrichment factor = 100). The method performance was studied in terms of accuracy (recovery = 78–114%), linearity (R2  0.9990) and repeatability (RSD  14%). Limits of detection (LODs) were at the pg mL−1 level for most of compounds, and at the sub-ng mL−1 level for the most ubiquitous phthalate esters. USAEME is proposed as an efficient, fast, simple and non-expensive alternative to other extraction techniques such as SPE, SPME and LPME for the analysis of environmental waters including bottled, tap, river, municipal swimming pool, sewage and seaport water samples. Since no matrix effect has been found for any of the water types analyzed, quantification could be carried out by using conventional external calibration, thus allowing a higher throughput of the analysis in comparison with other microextraction techniques based on equilibrium such as solid-phase microextraction.

Introduction

Sample preparation represents a major challenge and a very important step in the development and application of an analytical method. In general, this step consists of an extraction and preconcentration procedure of target compounds from a sample matrix.

Liquid–liquid extraction (LLE) [1], [2] and solid-phase extraction (SPE) [3], [4] are commonly used liquid sample pretreatment methods. LLE is among the oldest and more widespread techniques for the extraction of a wide range of organic pollutants from water samples. Nevertheless, LLE is time-consuming, requires large amounts of organic solvents that are potentially toxic, and is difficult to automate. SPE uses much less solvent than LLE but can be relatively expensive.

Over the last 10 years, with the developing interest in miniaturization in analytical chemistry with resultant solvent and sample savings, some newer miniaturized approaches to liquid extraction have been reported. These approaches have resulted in more efficient sample enrichment, faster sample preparation and lower solvent consumption. An attractive alternative pretreatment method to the traditional techniques is solid-phase microextraction (SPME) [5], [6]. SPME is a solvent-free extraction technique that incorporates sample pretreatment, concentration and sample introduction into a single procedure. But the extraction fiber is expensive, fragile and has a limited lifetime, and in addition, sample carry-over can be a problem [7]. Liquid–liquid microextraction (LLME) is a single-step extraction with a very high sample-to-solvent ratio which leads to a high enrichment factor of analytes. So, conventional LLME has been proposed in several US EPA methods as an efficient alternative to LLE [8], [9].

In the past few years, a novel liquid–liquid microextraction system, termed liquid-phase microextraction (LPME) or solvent microextraction (SME), was developed [10], [11]. This approach is based on analyte partitioning between a drop of organic solvent (extractant phase) and the aqueous sample matrix. Different configurations of this technique have recently emerged, including static LPME, dynamic LPME, continuous-flow LPME, headspace LPME (HS-LPME) and hollow fiber LPME [12], [13], [14]. This technique has attracted increasing attention in recent years because of the simple experimental setup, short analysis time and minimum use of solvent. However, several disadvantages such as instability of microdrop and relative low precision are often encountered.

Very recently, a novel microextraction technique, dispersive liquid–liquid microextraction (DLLME), based on dispersion of tiny droplets of the extraction liquid within the aqueous solution has been developed [15]. It is based on a ternary component solvent system like homogeneous liquid–liquid extraction (HLLE) [16] and cloud point extraction (CPE) [17]. The advantages of the DLLME method are rapidity, low cost and high enrichment factors. Its main drawbacks are the difficulty to automate and the necessity of using a third component (disperser solvent), which usually decreases the partition coefficient of analytes into the extractant solvent.

On the other hand, the application of ultrasonic radiation is a powerful aid in the acceleration of various steps of the analytical process, therefore ultrasound assisted liquid–liquid extraction (USALLE) has been used as an alternative to conventional LLE by Luque de Castro's research group [18], [19], who also successfully applied ultrasound assisted emulsification (USAE) for the first time to simultaneously determine polar and non-polar compounds in solid plant samples [20]. They demonstrated high extraction efficiency in a very short time using an emulsion of methanol/water in hexane formed in the presence of ultrasound radiation.

In a heterogeneous system of two immiscible liquid phases the effect of ultrasound radiation is the concurrent result of several partial phenomena with complex interrelationships and dependent on a considerable number of variables [21], [22], [23], [24]. Regarding liquid–liquid extraction, the main effects of ultrasounds can be summarized as follows: the fragmentation of one of the phases to form emulsions with submicron droplet size [25] that enormously extend the contact surface between both liquids; the inverse effect (coalescence) occurring in certain conditions [22], [26] as a result of Langevin or Rayleigh pressures; the homogenization of the external phase by the action of acoustic flows [22], [27]; the increment of temperature [22], [23]; and the momentary and localized strong increments of pressure and temperature in the proximity of the cavitational collapses [25], [26] that, when originated near to the liquid–liquid interface, may selectively affect the exchange of components without scarcely altering the whole of the emulsion.

The application of a miniaturized approach to this technique by using a microvolume of extracting organic phase, provides the advantages of both DLLME [15] and USALLE [18] and some more, mainly derived from the low concentration of inner phase drops: decrease of the coalescence effect [26], [28], decrease of the radiation absorption and the resultant warming [23], and the acoustic flow facilitation [29] with the result of an homogenization speed increase. The consequence is a very efficient and fast analyte extraction. After mass transfer, the two phases can be readily separated by centrifugation. In this way, ultrasound-assisted emulsification–microextraction (USAEME) can be employed as a simple and efficient extraction and preconcentration procedure for organic compounds in aqueous samples.

Phthalic acid esters are a group of chemical compounds that are mainly used as plasticizers and some of them also as carriers or solvents for synthetic musks fragrances in many personal care products (PCPs). Significant migration into the environment is demonstrated during their production, manufacture, use and disposal [30], [31]. Synthetic musks are fragrance additives used in a wide range of consumer products. Nowadays, polycyclic musks like galaxolide and tonalide form part of most fragrance formulations for household and cosmetic products. Nitromusks are currently present in about 5% of the products that were not reformulated, and although nitromusks are being phased out in Europe, completion is expected in 2008 [32], [33]. Since musk compounds are continuously introduced into the environment mainly via urban wastewater effluents [34], [35], their environmental persistence associated with their lipophilicity makes their routine monitoring in wastewaters and other environmental samples still necessary today. Gamma-hexachlorocyclohexane (γ-HCH), commonly known as lindane, is a pesticide that has been used in a broad range of applications including agriculture, horticulture and forestry. Because of its ubiquity, lindane is still found in environmental samples [36]. The development of simple, sensitive and reliable analytical methods to analyze these compounds in different water samples is hence necessary.

The aim of the present work is to propose a novel method based on USAEME and gas chromatography coupled to mass spectrometry (GC/MS) for the analysis of synthetic musk fragrances, phthalate esters and lindane in water samples. To the best of our knowledge, this paper describes the first application of ultrasound-assisted emulsification microextraction for the determination of organic compounds in water samples without the addition of an emulsifier. Optimization of the extraction conditions is achieved using a multifactorial experimental design approach. The method performance is studied in terms of accuracy, linearity, repeatability and limits of detection (LODs). To demonstrate the applicability of the proposed method, several types of water samples including plastic-bottled water, tap water, river water, municipal swimming pool water, sea harbour water and sewage waters, are analyzed.

Section snippets

Reagents and materials

The musk compounds, 6,7-dihydro-1,1,2,3,3-pentamethyl-4-(5H)-indanon (DPMI, cashmeran) and 4-acetyl-1,1-dimethyl-6-tert-butylindan (ADBI, celestolide) were kindly supplied by Ventos (Cornella de Llobregat, Barcelona, Spain). 1,3,4,6,7,8-Hexahydro-4,6,6,7,8,8-hexamethylcyclopenta-(γ)-2-benzopyran (HHCB, galaxolide), 7-acetyl-1,1,3,4,4,6-hexamethyltetraline (AHTN, tonalide), 6-acetyl-1,1,2,3,3,5-hexamethylindane (AHMI, phantolide) and 5-acetyl-1,1,2,6-tetramethyl-3-isopropylindane (ATII,

Performance of the GC/MS analysis

First experiments were conducted to achieve good chromatographic separation of the target compounds. The mass spectrum for each compound was obtained in the experimental conditions, and the most adequate ions for quantification were selected. Retention times at the optimized chromatographic conditions, as well as the identification and quantification ions are shown in Table 2.

Linearity was tested for most of the target compounds between 1 ng mL−1 and 500 ng mL−1 by injecting standards prepared in

Conclusions

A method based on ultrasound-assisted emulsification–microextraction (USAEME) coupled to GC/MS has been developed for the analysis of synthetic musk fragrances, phthalate esters and lindane in water samples. USAEME is proposed as an efficient, simple, rapid and non-expensive alternative to other extraction techniques such as SPE, SPME and LPME. Besides, it is environmentally friendly because of the low organic solvent consumption and easy to automate. Using the optimized conditions established

Acknowledgements

This research was supported by FEDER funds and projects CTQ2006-03334 (CICYT, Ministerio de Ciencia y Tecnologia, Spain), PGIDT06PXI3237039PR and PGIDIT05RAG50302PR (Xunta de Galicia). J. Regueiro would like to acknowledge his FPU doctoral grant to Ministerio de Ciencia y Tecnologia.

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