Since the development of liquid-phase microextraction (LPME), different LPME modes depending on the experimental set-up to carry out the extraction have been described. Dispersive liquid-liquid microextraction (DLLME), in which a small amount of the water-insoluble extraction solvent is dispersed in the sample, is the most successful mode in terms of number of applications reported. Advances within DLLME have been mainly shifted to the incorporation of green, smart and tunable materials as extraction solvents to improve the sustainability and efficiency of the method. In this sense, hydrophilic media represent a promising alternative since the water-miscibility of these substances increases the mass transfer of the analytes to the extraction media, leading to higher extraction efficiencies. Considering the variety of hydrophilic media that have been incorporated in LPME approaches resembling DLLME, this review aims to classify these methods in order to clarify the confusing terminology used for some of the strategies. Hydrophilic media covered in this review comprise surfactants, polar organic solvents, deep eutectic solvents, ionic liquids, water-miscible polymers, and switchable solvents. Different physicochemical mechanisms of phase separation are discussed for each LPME method, including the coacervation phenomena and other driving forces, such as pH, temperature, salting-out effect, metathesis reaction and organic solvents. LPME modes are classified (in cloud-point extraction, coacervative extraction, aqueous biphasic systems, and different DLLME modes depending on the extraction medium) according to both the nature of the water-miscible extraction phase and the driving force of the separation. In addition, the main advances and analytical applications of these methods in the last three years are described.
Since the introduction of the Green Analytical Chemistry guidelines, there has been an increasing concern on the sustainability of sample preparation approaches, particularly if considering they constitute the most time-consuming step of the analytical method and the main source of laboratory wastes. Among the alternatives explored, it is important to highlight the miniaturization of the extraction methods, which has been accompanied by the seek of greener solvents and sorbents. Biopolymers emerge as potential candidates to be used as sorbents in microextraction schemes given their biodegradability, versatility, and easily functionalization. This review offers an overview on biopolymers (chitosan, cellulose, alginate, and agarose) in sorbent-based microextraction approaches, paying attention to the preparation of the sorbent and the format in which biopolymers are incorporated into the sorbent/device, thus implying a specific microextraction approach, their role in the resulting sorbent material, and the reported analytical applications, covering environmental, food, and bioclinical analysis.
Current trends in incorporating the principles of green chemistry in analytical methods have led to the design and usage of new solvents to replace conventional organic solvents, which characterize by their high volatility, flammability, and toxicity. Among the alternatives that have emerged, amphiphilic solvents, ionic liquids, and deep eutectic solvents are the most explored candidates in this research field. Taking advantage of the solvation properties of these new solvents, together with the synthetic versatility in the case of ionic liquids and deep eutectic solvents, a wide variety of applications of these solvents within green analytical chemistry appear in the recent literature. The aim of this article is to provide a quick summary of the state of the art on the usage of these new green solvents in analytical chemistry, particularly in liquid-phase microextraction methods (within sample preparation) and as additives or pseudostationary phases in liquid chromatography (within analytical separation methods).
This paper proposes a new sustainable and simple strategy for the micro-scale extraction of phenolic compounds from grapevine leaves with analytical purpose. The method is based on a microwave-assisted solid-liquid extraction approach (MA-SLE), using an aqueous solution of an ionic liquid (IL)-based surfactant as extraction phase. The method does not require organic solvents, nor any clean-up step, apart from filtration prior to the injection in the analytical system. Two IL-based surfactants were evaluated, and the method was optimized by using experimental designs, resulting in the use of small amounts of sample (100 mg) and extraction phase (2.25 mL), low concentrations of the selected 1-hexadecyl-3-butyl imidazolium bromide IL (0.1 mM), and 30 min of extraction time. The proposed methodology was applied for the determination of the polyphenolic pattern of six different varieties of Vitis vinifera leaves from the Canary Islands, using high-performance liquid chromatography and photodiode array detection for the quantification of the compounds. The proposed MA-SLE approach was greener, simpler, and more effective than other methods, while the results from the analysis of the leaves samples demonstrate that these by-products can be exploited as a source of natural compounds for many applications.
Chemical vapor deposition of MOFs (MOF-CVD) has been used to coat solid-phase microextraction (SPME) fibers with ZIF-8, by exposing ZnO layers to the linker vapor (2-methylimidazole). This ZIF-8 coating has been used as a seed layer in a following solvothermal MOF growth step in order to increase the ZIF-8 thickness. The combined MOF-CVD and solvothermal growth of ZIF-8 on the fibers result in a thickness of ~3 μm, with adequate thermal stability, and mechanical integrity when tested with methanol and acetonitrile ultrasonic treatments. The fibers have been evaluated in direct immersion mode using gas chromatography and flame ionization detection (GC-FID), for a group of target analytes including three polycyclic aromatic hydrocarbons (PAHs) and five personal care products (PCPs). The optimized conditions of the SPME-GC-FID methods include low amount of aqueous sample (5 mL), stirring for 45 min at 35 °C, and desorption at 280 °C for 5 min. The method presents limits of detection down to 0.6 μg L⁻¹; intra-day, inter-day and inter-batch relative standard deviation values lower than 16%, 19%, and 23%, respectively; and a lifetime higher than 70 cycles.
Solid‐phase extraction (SPE) is a sample preparation technique and clean‐up procedure widely used in analytical laboratories worldwide. The procedure requires relatively large amounts of samples, which pass through a small amount of sorbent immobilized into a device (mainly cartridges or disks) previously activated. The analytes and in some cases other interfering species experience retention in the stationary phase of the device. After proper washing to remove interfering species, analytes experience desorption using low amounts of a proper elution solvent (usually amounts lower than 3 mL). Although there is a wide number of commercial materials available as sorbents, the current trends on miniaturization, the increasing need of more selective materials, and the requirement of higher sensitivities for target analytes despite analyzing complex samples, undoubtedly shift to the development of novel and high efficient sorbents for SPE, greener if possible. Therefore, recent trends focus their efforts on the development of metal– organic frameworks (MOFs) and their derived materials as novel extractant materials, thus emerging a powerful alternative material into analytical applications. The interest relates to their outstanding properties such as high porosity, astonishing surface areas, tunability, and the possibility of designee highly specific materials by reticular chemistry, among others. This chapter describes main recent analytical applications of SPE reported for MOFs and their derived materials, including covalent organic frameworks (COFs), while covering the dispersive and the magnetic‐assisted operational modes of SPE.
Ionic liquids (ILs) are a group of non-conventional salts with melting points below 100 °C. Apart from their negligible vapor pressure at room temperature, high thermal stability, and impressive solvation properties, ILs are characterized by their tunability. Given such nearly infinite combinations of cations and anions, and the easy modification of their structures, ILs with specific properties can be synthesized. These characteristics have attracted attention regarding their use as extraction phases in analytical sample preparation methods, particularly in liquid-phase extraction methods. Given the liquid nature of most common ILs, their incorporation in analytical sample preparation methods using solid sorbents requires the preparation of solid derivatives, such as polymeric ILs, or the combination of ILs with other materials to prepare solid IL-based composites. In this sense, many solid composites based on ILs have been prepared with improved features, including magnetic particles, carbonaceous materials, polymers, silica materials, and metal-organic frameworks, as additional materials forming the composites. This review aims to give an overview on the preparation and applications of IL-based composites in analytical sample preparation in the period 2017–2020, paying attention to the role of the IL material in those composites to understand the effect of the individual components in the sorbent.
A pH-sensitive polymer based on the poly(styrene-alt-maleic anhydride) co-polymer serves as basis to develop a microextraction method (pH-HGME) in direct combination with high-performance liquid chromatography (HPLC) and fluorescence detection (FD) for the determination of seven organic compounds, including three polycyclic aromatic hydrocarbons (PAHs), three monohydroxylated PAHs and one alkylphenol, in urine. The method bases on the structural modification of the pH-sensitive polymer in the aqueous sample at a high pH value, followed by the formation and insolubilization of a hydrogel containing the preconcentrated analytes by decreasing the pH, and the direct injection of the hydrogel-rich phase in the HPLC-FD system. The optimization of the main variables permitted the selection of low amounts of aqueous sample (10 mL), which was mixed with 10 mg of co-polymer also present in a low volume (150 µL) of concentrated NaOH. The method further requires the addition of 200 µL of concentrated HCl, 3 min of stirring, and 15 min of centrifugation. This pH-HGME-HPLC-FD method presented low limits of detection, ranging from 0.001 µg L-1 to 0.09 µg L-1 in ultrapure water, average relative recoveries of 96.9% for the concentration level of 0.60 µg L-1, and enrichment factors between 1.50 and 17.7. The proposed method also exhibited high precision, with intermediate relative standard deviations lower than 16% for a concentration level of 0.60 µg L-1. The developed pH-HGME-HPLC-FD method performed adequately when analyzing two human urine samples provided by a non-smoker male and a smoker female, respectively. One of the target analytes (2-hydroxynaphthalene) was quantified in both samples using the standard addition method, with a predicted concentration of 7.3 ± 0.4 µg L-1 in the non-smoker male urine and 19.3 ± 0.6 µg L-1 in the smoker female urine.
In this review, we summarize the most recent analytical developments aimed at employing Ionic liquids (ILs) in dispersive liquid-liquid microextraction (DLLME). Four main operation modes can be distinguished: (1) conventional IL-DLLME; (2) temperature-controlled IL-DLLME; (3a) ultrasound-assisted, (3b) microwave-assisted or (3c) vortex-assisted IL-DLLME; and, (4) in-situ IL-DLLME. In these modes, the dispersive solvent can be an organic solvent, a surfactant, or a hydrophilic IL. In some cases, a dispersive solvent is not even necessary. We discuss practical applications of IL-DLLME to determine metals and organic compounds in a variety of samples.
Metal–organic frameworks (MOFs) have attracted recently considerable attention in analytical sample preparation, particularly when used as novel sorbent materials in solid-phase microextraction (SPME). MOFs are highly ordered porous crystalline structures, full of cavities. They are formed by inorganic centers (metal ion atoms or metal clusters) and organic linkers connected by covalent coordination bonds. Depending on the ratio of such precursors and the synthetic conditions, the characteristics of the resulting MOF vary significantly, thus drifting into a countless number of interesting materials with unique properties. Among astonishing features of MOFs, their high chemical and thermal stability, easy tuneability, simple synthesis, and impressive surface area (which is the highest known), are the most attractive characteristics that makes them outstanding materials in SPME. This review offers an overview on the current state of the use of MOFs in different SPME configurations, in all cases covering extraction devices coated with (or incorporating) MOFs, with particular emphases in their preparation.
One of the current research lines in Analytical Chemistry is the design and utilization of novel materials with higher selectivity and improved analytical performance in both sample preparation and separation science. In this sense, metal‐organic frameworks (MOFs) have attracted attention as a potential alternative to current commercially available materials. MOFs are crystalline materials composed by metal ions and organic linkers. These coordination polymers present an interesting set of properties, such as high thermal and chemical stability, ease of synthesis, high synthetic tuneability, robust crystal structures with defined pore topologies, which provides them the highest surface areas known. Given the outstanding properties of MOFs and their ability to interact with different compounds, an increasing number of studies is reported in the literature showing their usefulness as sorbents in solid‐based microextraction schemes. Most applications of MOFs within this area focus on the preparation of highly stable solid‐phase microextraction (SPME) coatings, and the development of miniaturized solid‐phase extraction (μSPE) methods, particularly in the following sub‐modes: the dispersive approach (D‐μSPE) using bare MOFs, and the magnetic‐assisted approach (M‐D‐μSPE) using magnetic composites or heterogeneous materials based on MOFs. The structural diversity of MOFs and the possibility of fabricating task‐specific materials by the functionalization of the organic ligands makes them suitable as stationary phase in different separation techniques, and particularly for the separation of chiral compounds. Thus, MOF‐based packed and capillary columns have been developed for gas chromatography applications taking advantage of their high thermal stability. Packed MOF‐based columns, composed of either porous or core‐shell particles, have been used in liquid chromatography, while MOF‐coated capillaries have also been explored in capillary electrochromatography.