Du Q. et al. described a novel MSPE utilizing magnetic molecularly imprinted polymers (MMIPs) using triallyl isocyanurate as functional monomer. The study revealed that the monomer was successfully used for the enrichment and determination of sterigmatocystin (STG) in wheat samples
[7]. MMIPs use magnetic nanoparticles as the core covered by MIPs shell. The MMIPs can be separated from the solvent by external magnetic field due to the magnetic properties of magnetic nanoparticles
[8]. It is worth reporting that their low toxicity combined with the skills above make MMIPs a qualified option in the fields of purification and separation
[9]. Magnetic solid phase extraction, based on the MMIPs, combined with high-performance liquid chromatography (MSPE-HPLC) at optimal conditions was successfully used for the extraction of STG. The recovery of this method gave very satisfactory results at 87.6–96.9% and the limit of detection (LOD) was 0.63 ng⋅g
−1.
2.2. MIL-DLLME
Simplicity, low-cost, rapidity, reduced amount of chemicals solvents, good recoveries and high enriching ability are the main benefits that attained the attention of researchers. The majority of MIL-based microextractions protocols are conducted utilizing DLLME and the goal is to describe an illustrative point of view at this section.
A recently found class of magnetic ionic liquids (MILs) made of a single component was discovered and is now in the forefront of research in MIL-DLLME. These chemicals owe their magnetic property to complex ions of metals overcoming the need for external magnetic supports. In sample preparation MILs are important media due to their physiochemical properties that results in a strong counter force to external magnetic fields. Typically, for the extraction of the target analytes ultrasound irradiation can be employed for the uniform dispersion of MILs in the sample
[11]. The majority of the initially synthesized MILs exhibited hydrophilicity and were expected to result in good extraction performance when used in hydrophobic media. MILs are composed of a plethora of functional groups including esters and protonated primary amines and as a result they are water miscible. Moreover, MILs are miscible with polar solvents after a very staminal shake which restricts their applications, while they are not miscible with non-polar solvents (e.g., n-hexane)
[12]. The first application of MILs in DLLME was introduced in 2014 by Yuanpeng Wang et al. In this study, a MIL-based DLLME was proposed for the extraction of triazine herbicides from vegetable oils prior to their analysis by liquid chromatography. An aliquot of 1-hexyl-3-methylimidazolium tetrachloroferrate ([C
6mim]) was employed as the extractant. In order to ensure the rapid magnetic separation, carbonyl iron powder was added to form carbonyl iron powder (CIP)-MIL. Overall, the method showed better performance followed by good precision and sensitivity, as well as low limits of detection and limits of quantification for the target analytes
[13].
Aiming to expand the applicability of MILs in aqueous media, a demand for the preparation of hydrophobic MILs arose. Thus, it was necessary to take some actions so the hydrophobic character can be urged in MILs. Typical approaches that can be employed to improve the hydrophobic character of MILs include either the replacement of hydrolysis susceptible FeCl4− anion with other transition metal-based anions or the use of long aliphatic alkyl chain-based organic cations. Taking that into account, to avoid the limitations related with FeCl4−, MILs with MnCl42− were introduced in DLLME.
As it was previously mentioned, ILs utilize organic solvents, which are toxic and can affect health and cause environmental problems. That being said, there is a need for the use of safer and greener chemicals for the replacement of conventional solvents that exhibit high toxicity. Recently, an ionic liquid-linked dual magnetic microextraction (IL-DMME) developed by Yilmaz and Sylak with magnetic nanomaterials was proposed as a new extraction media. This novel method demonstrates that IL-DLLME and dispersive μ-magnetic nanoparticle solid phase extraction (D-μ-SPE) is an effective combination that assisted in overcoming the abovementioned limitations. By using vortex mixing, 1-butyl-3-methylimidazolium hexafluorophosphate [C
4mim][PF
6] was employed for the extraction of lead-pyrrolidine-dithiocarbamate (Pb-PDC) complex. Following the IL-DMME step, an amount of 50 mg of Fe
3O
4 MNPs was used for extracting the IL and Pb-PDC complex. Under optimum sample preparation conditions, the method presented low detection limit (0.57 μg L
−1) and good repeatability (<7.5%,
n = 10). The proposed methodology was finally employed for the determination of lead in hair, plant and water samples
[14].
2.3. Single Drop Microextraction (SDME)
SDME is a sample preparation mode based on solvent microextraction (SME), which is often combined with GC or HPLC
[15]. SDME has two separate modes, namely “direct immersion” and “headspace”. A standard execution includes the utilization of a few microliters of an organic solvent microdrop that is kept on the tip of a microsyringe, which is placed directly in the liquid matrix or in the headspace above the sample in order to achieve the extraction of the target analytes. After a certain period of time, the microdrop is withdrawn inside the syringe and further analyzed by an analytical technique
[16]. Limitations of this sample preparation technique includes the instability of the microdrop during the immersion in the liquid sample, as well as the long microextraction times
[17].
A recent study indicates the parallel-SDME/MIL-based (Pa-SDME) analytical methodology that is able to take advantage of the magnetic properties, drop stability and extraction capacity of the trihexyl (tetradecyl) phosphonium tetrachloromanganate (II) ([P
6,6,6,14]
2[MnCl
42−]) MIL. The proposed scheme was coupled with a 96-well plate to provide high throughput and low cost. As proof-of-concept, the proposed analytical strategy was used for the extraction of methylparaben, ethylparaben, propylparaben, bisphenol A, butylparaben, benzophenone and triclocarban. In order to stabilize the magnetic IL droplets during the parallel sample handling, the 96-well plate contained a set of magnetic pins. As such, a sample throughput of less than 1 min per sample was achieved. Among the benefits of this technique over conventional SDME approaches is its ability to maintain a stable solvent microdrop to facilitate high throughput. The extracts were analyzed by HPLC-DAD under optimal conditions. The results were satisfactory and promising with low LODs and good linearity
[18].
2.4. Stir Bar Sorptive Extraction (SBSE)
SBSE can be considered as an alternative to the SPME technique. In this regard, SBSE increases the typical low capacity of conventional SPME fibers and is based on the partitioning of the desired compounds between the stationary phase-coated magnetic stir bar and the sample solution. More specifically, in comparison with SPME coatings, the coating of SBSE occupy a significantly higher volume resulting in increased extraction capacity and extraction efficiency. For years, the only commercially available stir bar coatings were polydimethylsiloxane (PDMS) and a PDMS/Ethylene glycol copolymer limiting the applicability of this technique to the extraction of hydrophobic target analytes. Although the demand for coatings with high affinity towards a bigger group of analytes, especially the polar ones, was the guidance for the fabrication of novel stir bars coated with novel magnetic composites
[19].
A work developed by M. Díaz-Álvarez et al. was based on the entrapment of modified magnetic nanoparticles within an imprinted polymer monolith for creating molecularly imprinted stir bars. As the first step, modification of the surface of the MNPs by oleic acid took place, followed by encapsulation in a silica network. For this purpose, vinyl groups were grafted onto the particles’ surface. Moreover, a glass vial insert was employed as the mold for the subsequent copolymerization. As a result, the obtained imprinted monolith presented magnetic properties allowing its use as magnetic stir bar. The main factors affecting the polymer morphology were optimized. The technique of SBSE utilized theses top notch imprinted stir bars for efficient extraction of triazines from soil sample extracts. The recoveries ranged from 2.4 to 8.7% but despite that observation, high selectivity was obtained allowing the determination of the target analytes with detection limits lower than 7.5 ng g
−1 [20].