Magnetic Sample Preparation Methods Prior to Liquid Chromatography

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1		Victoria Samanidou	--	1958	2023-07-12 08:03:25

This entry is adapted from the peer-reviewed paper 10.3390/magnetochemistry8030029

Magnetic nanomaterials and nanostructures compose an innovative subject in sample preparation. Most of them are designed according to the properties of the target analytes on each occasion. The unique characteristics of nanomaterials enhance the proficiency at extracting and enriching due to their selective adsorption ability as well as easy separation and surface modification. Their remarkable properties, such as superparamagnetism, biocompatibility and selectivity have established magnetic materials as very reliable options in sample preparation approaches.

magnetic preparation chromatography nanomaterials

1. Introduction

While the upgrowth of advanced analytical instrumentation remains at a high level and most of the analytes can be detected, some compounds are still beyond the detection thresholds due to the restriction of instrumental detection limit and interferences of matrices. It is a vital goal to introduce sample preparation techniques prior to an analytical process so as to achieve accurate quantification and lower detection limits. The objective of sample preparation is to isolate the analyte(s) of interest from the sample matrix in the most possible concentrated form and enhance analytes’ separation and/or detection ^[1]. Over the past several decades, considerable time has been devoted to present some widely used sample preparation techniques, such as liquid–liquid extraction (LLE) and solid phase extraction (SPE); each of these techniques bare their own advantages and disadvantages. The classical methods of sample preparation include complicated, time-consuming steps that consume high amounts of toxic organic solvents and require large sample sizes ^[2]. Therefore, novel microextraction protocols have been introduced to be either solvent-less or minimalize the possible amount of organic solvents. According to studies, these methods have shown great outcomes as possible replacements to the classical methods for many different applications. Recent trends are driven by the Green Analytical Chemistry (GAC), which not only encourages the minimization of the volumes used but also the use of green(er) solvents that will have a minimum impact on the environment ^[3]^[4]. Classical SPE-based approaches where the adsorbents are packed into cartridges have been applied in many effective cases; however, it is not suitable for samples containing suspended solid or fouling components, because problems of column blocking, and high pressure are frequently detected. A technique, during which the adsorbents are incubated directly with the samples, can solve the abovementioned problems ^[4].

Magnetic solid phase extraction (MSPE) overcomes problems such as column packing and phase separation. In this approach, magnetic materials are dispersed into an aqueous sample and after incubation for a suitable time until the target analytes are adsorbed on the adsorbent material, an external magnetic field is applied to separate the magnetic materials from the solution in an easy way. In addition, the extraction time is shortened as suspended magnetic particles facilitate mass transfer by offering increased interfacial area between the solid adsorbent and sample solution. Overall, MSPE is a low cost, easily instrumented and quick extraction technique that has captured much attention in the field of sample preparation because of its huge upside potential and very little downside risks ^[5].

2. Magnetic Sample Preparation Methods

The separation mechanism using magnetic nanoparticles (MNPs) depends on the type of sorbent, and is related to the interaction of analyte molecules with the surface functional groups, as in the classical solid phase extraction (SPE). Magnetic nanomaterials used in MSPE are usually designed according to the molecular structures and properties of the targets ^[6].

2.1. Magnetic Solid Phase Extraction (MSPE)

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.

Ye Z. et al. ^[10] reported the synthesis of an efficient highly fluorinated and boron-rich adsorbent (FBA) for the determination of fluoroquinolones (FQs) in environmental water and milk samples. The study revealed satisfactory extraction capability for FQs through fluorophilic and B–N coordination interactions. Moreover, the synthesized FBA also exhibited strong magnetic responsiveness and a good lifespan. This novel MSPE performed prior toto high-performance liquid chromatography at optimal conditions (HPLC-DAD) to quantify trace levels of FQs in the examined matrices. Low limits of detection which ranged from 0.0049 to 0.016 μg/L in water samples and 0.010–0.046 μg/kg in milk samples were reported, while the respective recovery rates with regard to the analysis of target FQs in real samples were in the ranges of 80.1–120% and 78.9–119% for water and milk samples accordingly ^[10].

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 ([C₆mim]) 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 [C₄mim][PF₆] was employed for the extraction of lead-pyrrolidine-dithiocarbamate (Pb-PDC) complex. Following the IL-DMME step, an amount of 50 mg of Fe₃O₄ 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[MnCl₄²⁻]) 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].

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Georgios Antoniou

Victoria Samanidou

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Update Date: 12 Jul 2023

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