Strategies for interfacing solid-phase microextraction with liquid chromatography

Recent Applications of Solid Phase Microextraction Coupled to Liquid Chromatography

Solid phase microextraction (SPME) is one of the most popular sample preparation methods which can be applied to organic compounds allowing the simultaneous extraction and pre-concentration of analytes from the sample matrix. It is based on the partitioning of the analyte between the extracting phase, generally immobilized on a fiber substrate, and the matrix (water, air, etc.), and has numerous advantages such as rapidity, simplicity, low cost, ease of use and automation, and absence of toxic solvents. Fiber SPME has been widely used in combination with various analytical instrumentation even if most of the work has been done coupling the extraction technique with gas and liquid chromatography (GC and LC). This manuscript presents an overview of the recent works (from 2010 to date) of solid phase microextraction coupled to liquid chromatography (SPME-LC) relevant to analytical applications performed using commercially available fibers or lab-made fibers already developed in previous papers, and to improved instrumental systems and approaches.

In-tube solid-phase microextraction: Current trends and future perspectives

In-tube solid-phase microextraction (IT-SPME) was developed about 24 years ago as an effective sample preparation technique using an open tubular capillary column as an extraction device. IT-SPME is useful for micro-concentration, automated sample cleanup, and rapid online analysis, and can be used to determine the analytes in complex matrices simple sample processing methods such as direct sample injection or filtration. IT-SPME is usually performed in combination with high-performance liquid chromatography using an online column switching technology, in which the entire process from sample preparation to separation to data analysis is automated using the autosampler. Furthermore, IT-SPME minimizes the use of harmful organic solvents and is simple and labor-saving, making it a sustainable and environmentally friendly green analytical technique. Various operating systems and new sorbent materials have been developed to improve its extraction efficiency by, for example, enhancing its sorption capacity and selectivity. In addition, IT-SPME methods have been widely applied in environmental analysis, food analysis and bioanalysis. This review describes the present state of IT-SPME technology and summarizes its current trends and future perspectives, including method development and strategies to improve extraction efficiency.

Automated needle-sleeve based online hyphenation of solid-phase microextraction and liquid chromatography

A novel approach for the online coupling of solid-phase microextraction (SPME) and liquid chromatography (LC) is introduced. An innovative Si@GO@βCD coated needle-sleeve extractant device was developed and then employed in the automated online SPME-LC-UV determination of estrogen-like isoflavones from human urine samples. The extractant SPME device is easily attachable at the endpoint of an analytical syringe needle and operated by a lab-made autosampler. Fully automated online SPME-LC is accomplished by proper autosampler programming to perform the following steps: i) the analytes extraction by direct immersion of the extractant device into the stirred sample, ii) a rinsing step iii) the analytes desorption/enrichment, iv) the online transference of the extract to the LC injection valve. Besides allowing the online SPME hyphenation, this extraction modality efficiently addressed the drawbacks associated with the clogging and dispersion of graphene-based microextraction techniques performed in packed-bed and dispersive formats. The main extraction parameters and the performance of the automated online SPME-LC method developed were carefully studied. The results show a good sensitivity, reliability, and straightforward analytical strategy for the determination of organic compounds in complex samples. The detection limit of the method was 20 μg L¹ for DAI and 10 μg L^-1 for GEN, FOR and BIO. The intra-day RSD was below 10% and inter-day RSD was below 13%. The total analysis time was less than 17 min per sample.

A Review: Sample Preparation and Chromatographic Technologies for Detection of Aflatoxins in Foods

As a class of mycotoxins with regulatory and public health significance, aflatoxins (e.g., aflatoxin B1, B2, G1 and G2) have attracted unparalleled attention from government, academia and industry due to their chronic and acute toxicity. Aflatoxins are secondary metabolites of various Aspergillus species, which are ubiquitous in the environment and can grow on a variety of crops whereby accumulation is impacted by climate influences. Consumption of foods and feeds contaminated by aflatoxins are hazardous to human and animal health, hence the detection and quantification of aflatoxins in foods and feeds is a priority from the viewpoint of food safety. Since the first purification and identification of aflatoxins from feeds in the 1960s, there have been continuous efforts to develop sensitive and rapid methods for the determination of aflatoxins. This review aims to provide a comprehensive overview on advances in aflatoxins analysis and highlights the importance of sample pretreatments, homogenization and various cleanup strategies used in the determination of aflatoxins. The use of liquid-liquid extraction (LLE), supercritical fluid extraction (SFE), solid phase extraction (SPE) and immunoaffinity column clean-up (IAC) and dilute and shoot for enhancing extraction efficiency and clean-up are discussed. Furthermore, the analytical techniques such as gas chromatography (GC), liquid chromatography (LC), mass spectrometry (MS), capillary electrophoresis (CE) and thin-layer chromatography (TLC) are compared in terms of identification, quantitation and throughput. Lastly, with the emergence of new techniques, the review culminates with prospects of promising technologies for aflatoxin analysis in the foreseeable future.

Opportunities for green microextractions in comprehensive two-dimensional gas chromatography / mass spectrometry-based metabolomics - A review

Microextractions have become an attractive class of techniques for metabolomics. The most popular technique is solid-phase microextraction that revolutionized the field of modern sample preparation in the early nineties. Ever since this milestone, microextractions have taken on many principles and formats comprising droplets, fibers, membranes, needles, and blades. Sampling devices may be customized to impart exhaustive or equilibrium-based characteristics to the extraction method. Equilibrium-based approaches may rely on additional methods for calibration, such as diffusion-based or on-fiber kinetic calibration to improve bioanalysis. In addition, microextraction-based methods may enable minimally invasive sampling protocols and measure the average free concentration of analytes in heterogeneous multiphasic biological systems. On-fiber derivatization has evidenced new opportunities for targeted and untargeted analysis in metabolomics. All these advantages have highlighted the potential of microextraction techniques for in vivo and on-site sampling and sample preparation, while many opportunities are still available for laboratory protocols. In this review, we outline and discuss some of the most recent applications using microextractions techniques for comprehensive two-dimensional gas chromatography-based metabolomics, including potential research opportunities.

Development of a Microfluidic Open Interface with Flow Isolated Desorption Volume for the Direct Coupling of SPME Devices to Mass Spectrometry

Technologies that efficiently integrate the sampling and sample preparation steps with direct introduction to mass spectrometry (MS), providing simple and sensitive analytical workflows as well as capabilities for automation, can generate a great impact in a vast variety of fields, such as in clinical, environmental, and food-science applications. In this study, a novel approach that facilitates direct coupling of Bio-SPME devices to MS using a microfluidic design is presented. This technology, named microfluidic open interface (MOI), which operates under the concept of flow-isolated desorption volume, consists of an open-to-ambient desorption chamber (V ≤ 7 μL) connected to an ionization source. Subsequently, compounds of interest are transported to the ionization source by means of the self-aspiration process intrinsic of these interfaces. Thus, any ionization technology that provides a reliable and constant suction, such as electrospray ionization (ESI), atmospheric-pressure chemical ionization (APCI), or inductively coupled plasma ionization (ICP), can be hyphenated to MOI. Using this setup, the desorption chamber is used to release target compounds from the coating, while the isolation of the flow enables the ionization source to be continuously fed with solvent, all without the necessity of employment of additional valves. As a proof of concept, the design was applied to an ESI-MS/MS system for experimental validation. Furthermore, numerical simulations were undertaken to provide a detailed understanding of the fluid flow pattern inside the interface, then used to optimize the system for better efficiency. The analytical workflow of the developed Bio-SPME-MOI-MS setup consists of the direct immersion of SPME fibers into the matrix to extract/enrich analytes of interest within a short period of time, followed by a rinsing step with water to remove potentially adhering proteins, salts, and/or other interfering compounds. Next, the fiber is inserted into the MOI for desorption of compounds of interest. Finally, the volume contained in the chamber is drained and moved toward the electrospray needle for ionization and direct introduction to MS. Aiming to validate the technology, the fast determination of selected immunosuppressive drugs (e.g., tacrolimus, cyclosporine, sirolimus, and everolimus) from 100 μL of whole blood was assessed. Limits of quantitation in the subppb range were obtained for all studied compounds. Good linearity (r² ≥ 0.99) and excellent precision, with (8%) and without (14%) internal standard correction, were attained.

A digital microfluidic interface between solid-phase microextraction and liquid chromatography-mass spectrometry

We introduce a method to couple solid-phase microextraction (SPME) with HPLC-MS using digital microfluidics (DMF). In the new system, SPME fibers are used to extract analytes from complex sample solutions, after which the analytes are desorbed into solvent droplets in a DMF device. The open geometry of DMF allows straightforward insertion of SPME fibers without requiring a complicated interface, and automated droplet manipulation enables multiplexed processing of the fibers. In contrast to other multiplexed SPME elution interfaces, the low volumes inherent to DMF allow for pre-concentration of analytes prior to analysis. The new SPME-DMF-HPLC-MS method was applied to the quantification of pg/mL-level free steroid hormones in urine. We propose that this new method will be useful for a wide range of applications requiring cleanup and pre-concentration with convenient coupling to high-performance analytical techniques.

A new strategy to eliminate sample mixing during in-tube solid phase microextraction

During in-tube solid phase microextraction, sample mixing with mobile phase contained in the autosampler tubing during extraction may result in some amount of sample becoming entrained in the mobile phase rather than returning to the sample vial or being directed to waste after extraction. In cases where target analytes have relatively low affinity for the sorbent on the wall of the capillary, mixing can impact data quality. Where the sample contains components that may interfere with either the separation (e.g. proteins) or detection (e.g. ions with MS detection), additional difficulties can arise. In the current research, the magnitude of the sample mixing effect was illustrated by analyzing ranitidine and a series of polycyclic aromatic hydrocarbons (PAH). The sample volume equivalent of mixing was calculated as 37 μL for ranitidine and 20 μL for PAHs using the same inner diameter of capillary. To address this issue, a novel approach involving adding a switching valve located between the metering pump and the capillary was developed. Capillary flush conditions, draw/eject speed and extraction time were optimized for ranitidine with the result that in the final method, no mixing of sample with mobile phase was apparent in the detected amounts. To provide information on a compound class with intermediate polarity, two -blockers were also extracted using the optimized washing conditions respectively. The results indicated that the issue of sample mixing had been resolved for these as well. Finally, in-tube SPME calibration of these three analyte classes was shown to be highly linear, providing further indication that sample mixing was not impacting data quality. Available literature on the subject was surveyed, and a discussion on the rational selection of conditions to guide method development was also provided.

Mussel inspired polydopamine functionalized poly(ether ether ketone) tube for online solid-phase microextraction-high performance liquid chromatography and its application in analysis of protoberberine alkaloids in rat plasma

A novel and simple poly(ether ether ketone) (PEEK) tube-based solid-phase microextraction (SPME)-HPLC method was developed in this work. A mussel inspired polydopamine method was used to functionalize the chemically resistant surface of PEEK tube and a poly(acrylamide-ethylene glycol dimethacrylate) monolith was synthesized inside the PEEK tube and chemically bonded with its surface. The stability of the polydopamine layer was investigated and found to be well enduring to most of commonly used organic solvents and strong acidic conditions. The PEEK tube-based monolith was coupled with HPLC system through a six-port valve, and the performance of the online SPME-HPLC system was demonstrated by analyzing three protoberberine alkaloids in aqueous samples, namely jatrorrhizine (Jat), palmatine (Pal) and berberine (Ber). These analytes were well extracted and separated within 10 min, and enrichment factors of about 400 were obtained. The limit of detection of the proposed method was decreased to 0.01 ng/mL by using UV detection. Finally, the online SPME-HPLC method was applied for determining Jat, Pal and Ber in rat plasma samples. Reproducibility was obtained by evaluating the intra- and inter-day precisions with RSDs less than 6.1% and 8.5%, respectively, and recoveries of the analytes in spiked plasma samples ranged from 89.8% to 96.7%. The proposed method was also applied to determine the concentration of Jat, Pal and Ber in rat plasma after oral administration of Cortex Phellodendri Chinensis.

Improvement of solid phase microextraction fiber assembly and interface for liquid chromatography

Modifications were made on commercial SPME fiber assembly and SPME-LC interface to improve the applicability of SPME for LC. Polyacrylonitrile (PAN)/C18 bonded fuse silica was used as the fiber coating for LC applications because the fiber coating was not swollen in common LC solvents at room temperature. The inner tubing of SPME fiber assembly was replaced with a 457 μm outside diameter (o.d.) solid nitinol rod. And the coated fiber (o.d. 290 μm) was installed onto the nitinol rod. The inner diameter (i.d.) of the through hole of the ferrule in the SPME-LC interface was enlarged to 508 μm to accommodate the nitinol rod. The much larger inner rod protected the fiber coating from being stripped when the fiber was withdrawn from the SPME-LC interface. The system was evaluated in term of pressure test, desorption optimization, peak shape, carryovers, linear range, precision, and limit of detection (LOD) with polycyclic aromatic hydrocarbons (PAHs) as the test analytes. The results demonstrated that the improved system was robust and reliable. It overcame the drawbacks, such as leak of solvents and damage of fiber coatings, associated with current SPME fibers and SPME-LC interface. Another sealing mechanism was proposed by sealing the nitinol rod with a specially designed poly(ether ether ketone) (PEEK) fitting. The device was fabricated and tested for manual use.

Recent advances in column switching sample preparation in bioanalysis

Column switching techniques, using two or more stationary phase columns, are useful for trace enrichment and online automated sample preparation. Target fractions from the first column are transferred online to a second column with different properties for further separation. Column switching techniques can be used to determine the analytes in a complex matrix by direct sample injection or by simple sample treatment. Online column switching sample preparation is usually performed in combination with HPLC or capillary electrophoresis. SPE or turbulent flow chromatography using a cartridge column and in-tube solid-phase microextraction using a capillary column have been developed for convenient column switching sample preparation. Furthermore, various micro-/nano-sample preparation devices using new polymer-coating materials have been developed to improve extraction efficiency. This review describes current developments and future trends in novel column switching sample preparation in bioanalysis, focusing on innovative column switching techniques using new extraction devices and materials.