Review ArticleHomogenous liquid-liquid micro-extraction of pollutants in complex matrices
Graphical abstract
Introduction
Environmental contamination by organic and inorganic pollutants has received superior attention from researchers due to their toxic nature [1], [2], [3], [4]. These pollutants have negative impacts to the environment. Long-term exposure to these organic and inorganic pollutants can cause asthma, decreased cardiac output, heart and lung diseases, and allergic reactions [5], [6], [7], [8]. Thus, it is critically important to determine their concentrations in food and environmental samples. Close monitoring of these pollutants would enable corrective measures to be taken before their concentrations exceed the maximum allowable limits in food and environmental sample.
There are many analytical techniques that can be employed to monitor organic and inorganic pollutants in food and environmental samples. These include the adsorptive removal of the pollutants using adsorbents such as optical composite materials [9] and functionalised composite materials [10] as well as electrochemical detection [11], [12] and the use of sensors [13]. In addition, chromatographic techniques such as gas chromatography [14], [15], [16], supercritical chromatography [17], [18], [19], micellar electrokinetic chromatography [20], [21], [22] and high performance liquid chromatography [23], [24], [25] can be used during analysis of pollutants. Spectroscopic techniques such as inductively coupled plasma-mass spectroscopy [26], [27], [28] and atomic absorption spectroscopy [29], [30], [31] are also useful for the analysis of pollutants in complex matrices. Analysis of organic and inorganic pollutants using these techniques is very effective if their concentrations are above the instrument’s limits of detection and quantification. The contaminants in food and environmental samples, however, are usually found in trace concentrations. Thus, it is difficult to directly analyse trace amounts of pollutants by instrumental methods since their concentrations will be usually lower than the limits of detection of the analytical instruments. For these reasons, the use of pre-concentration methods prior to instrumental analysis becomes an imperative to cause improvement of sensitivity as well as for the enhancement of selectivity during their detection.
Pollutants in food and environmental samples can be pre-concentrated using traditional methods such as liquid-liquid extraction [32], [33] and solid-phase extraction [34], [35]. These traditional pre-concentration techniques use large amounts of toxic organic solvents and have a long extraction time. In addition, these traditional pre-concentration techniques are usually associated with the production of large amounts of secondary wastes [36], [37], [38]. As a result, the attention of most researchers is now focused on miniaturised pre-concentration techniques. Among these we have miniaturised sorbent-based techniques such as QuEChERS [39], [40], solid phase micro-extraction [41], [42] and stir bar soptive extraction. They are eco-friendly since they use very small amounts of organic solvents, and they are sometimes solvent-free. In addition, they are highly selective, especially when composites with tailor-made functionalities are used as adsorbents. However, fabrication of some these functionalised composite adsorbents occur for long periods at elevated temperatures risking pollution of the environment by toxic by-products. Alternatively, liquid phase micro-extraction (LPME) techniques such as single-drop micro-extraction [43], hollow fibre liquid-phase micro-extraction [2], [44], [45], [46], dispersive liquid–liquid micro-extraction [47], [48], [49], [50] and homogeneous liquid–liquid micro-extraction (HLLME) [51], [52], [53] can be used. These LPME techniques have an edge over the traditional methods since they use lower volumes of organic solvents, low operation cost and they have relatively higher extraction recoveries.
DLLME and HLLME are closely related pre-concentration techniques. The DLLME technique is, however, widely used by researchers as compared to HLLME [1], [54], [55]. It is based on the use of a ternary solvent system composed of aqueous sample, a disperser and extraction solvent (the disperser and extraction solvents are usually organic) [54], [56]. Its use during pre-concentration depends on the availability of an extraction solvent with a high affinity of the analyte, low solubility in water, and can form tiny droplets in the disperser solvent [47], [57]. The advantages of DLLME include that it uses less organic solvents, short extraction time, high enrichment factors and less operation cost. One of its limitations, however, is that it uses two different organic solvents and is, therefore, bound to cause relatively high pollution of environments from solvents [57], [58]. A variant of DLLME, HLLME can be used to minimise this problem since it uses a binary solvent system (aqueous sample and extraction solvent). Thus, it causes less pollution of the environment by solvents as compared to DLLME. Its choice during pre-concentration of pollutants depends on the availability of an extraction solvent whose hydrophilicity can be easily changed by an external trigger such as pH or temperature changes.
HLLME is a relatively simple, fast and efficient sample pre-concentration technique [59]. Some researchers have successfully used it to pre-concentrate contaminants in complex matrices (Fig. 1). It involves the extraction of the desired analyte, existing in the homogeneous aqueous solution, into the water-immiscible sediment phase. The initial condition during this pre-concentration technique is a homogeneous solution, where there is no interface between the water phase and the extraction solvent phase. Consequently, it has the advantage of extremely fast extraction speed due to the absence of obstacles from the surface contact between the aqueous phase and the organic phase during the extraction procedure.
The efficiency of the HLLME during pre-concentration of pollutant residues largely hinges on the proper choice of the extraction solvents. The solvent used as the extractant should have functionalities that will enable it to form strong interactions with target pollutant. During conventional HLLME, organic solvents are usually used as extraction solvents [59], [60]. The use of organic solvents, however, is not very appealing from an environmental point of view since most of them are toxic to living organism. Thus, some researchers are now replacing organic solvents with emerging green solvents such as ionic liquids [61], switchable solvents [51] and deep eutectic solvents (DES) [62] during HLLME pre-concentration of pollutants in food and environmental samples. Centrifugation is usually used to cause phase separation during HLLME and other pre-concentration techniques. However, this time-consuming step can be by-passed by using air floatation to cause phase separation during HLLME [63], [64].
This paper explores the application of HLLME during the pre-concentration of pollutants in complex matrices. Special emphasis is placed on the use of emerging green solvents such as ionic liquids, switchable solvents and DESs during the HLLME methods. In addition, the application air floatation to cause phase separation during HLLME is treated in detail in this paper. To the best our knowledge, this is going to be among the first papers to give a detailed review of the application of HLLME during pollutant pre-concentration in food and environmental samples. This is going to be an informative paper to those in the food industry as they engage in quality control as well as the agricultural sector since HLLME is a green pre-concentration technique that can be very useful during monitoring of agro-chemicals in the environment.
Section snippets
Deep eutectic solvent-based HLLME
Deep eutectic solvents (DESs) are eutectic mixtures of two or three non-toxic, biodegradable components that can form intermolecular hydrogen bonds between them [65], [66], [67]. The mixture should be composed of hydrogen bond donors and hydrogen bond acceptors, which interact resulting in the formation of a eutectic mixture, with a melting point that is lower than the components mixed [68], [69]. Depending on the components used during synthesis, DESs can be either hydrophilic or hydrophobic
Challenges and future prospects
HLLME is a sensitive and effective technique for the pre-concentration of pollutants in food and environmental samples. Its use, however, is not free of challenges. The challenges encountered during the HLLME technique are usually associated with the solvents used for the extraction of the pollutant residues in complex matrices. Organic solvents are usually used during conventional HLLME techniques as extractants [52], [59], [80]. The use of organic solvents poses a serious environmental
Conclusion
HLLME is a relatively fast, simple and cost-effective technique for the pre-concentration of trace amounts of pollutants in food and environmental samples. It uses very small amounts of solvents that can be easily recovered after the pre-concentration procedure. The environmental footprint of HLLME can be reduced by replacement of toxic organic solvents with emerging green solvents such switchable solvents, ionic liquids and DES during the pre-concentration of pollutants in complex matrices.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
Authors are grateful to the Research centre, University of Venda, for financial support.
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