The Potential of Automated Strategies in Microextraction Procedures Coupled to Chromatographic Techniques

Automation of microextraction approaches is often challenging to implement. However, significant improvements have been obtained toward affordable and reliable systems to enhance sample throughput. The development of cost-effective laboratory-made devices, in addition to more sophisticated electronic platforms, have allowed for straightforward workflows. In this report, recent developments and trends in automated and semiautomated microextraction-based approaches prior to chromatographic separations are briefly discussed, with a focus on solvent microextraction.

Formidable progress has been achieved in the development of chromatographic instrumentation related to sensitivity enhancement and separation capacity in both liquid and gas phase separations. Improvements in separation columns, proposal of more robust and efficient components, and the development of selective and sensitive detection systems, provide important tools to deal with complex mixtures. However, the development of accurate chromatographic methodologies generally depends on satisfactory sample pretreatment. In most cases, extraction techniques are used to overcome some issues associated with matrix complexity, low analyte concentration, and compatibility of the samples with analytical instrumentation. Particularly, microextraction approaches have gained significant attention because these techniques exhibit environmentally-friendly aspects, satisfactory extraction efficiency and high preconcentration capacity (1).

Liquid-phase microextraction (LPME) consists of numerous techniques in which small volumes of solvents are employed. These techniques permit safer methodologies because they alleviate some drawbacks associated with traditional liquid–liquid extraction (2). Moreover, increasing interest has been directed in the development of automated microextraction procedures coupled to liquid or gas chromatography to offer high sample throughput with low human manipulation.

Most of automated or semi-automated solvent microextraction approaches are based on online flow analysis (generally microfluidic devices), robotic autosamplers, and 96-well plate platforms. Some of these instruments are commercially available, including robotic autosamplers that exhibit impressive performance coupled to liquid or gas chromatography. However, the high cost of these systems is a significant limitation for some laboratories. Therefore, alternative and cost-effective approaches for automating sample preparation techniques associated with chromatographic separations consist of important developments in analytical chemistry.

Strategies Involving Membrane-Based Techniques

Since early this century, semi- and fully-automated sample preparation methodologies using membrane-based techniques have been successfully developed (3–5). The complete integration of these methodologies with autosamplers can be challenging because of the dimensions of the membranes generally used. However, successful attempts to automate hollow-fiber liquid phase microextraction (HF–LPME) and electromembrane extraction (EME) have been reported (6,7).

More recently, a semi-automated approach based on 96-well plate system using hollow-fiber renewable liquid membrane extraction (HFRLM) coupled to liquid chromatography coupled with electrospray ionization and quadrupole time of flight mass spectrometry (LC–ESI-QToF-MS) has been proposed for the determination of cocaine and metabolites in urine samples (8). This approach consists of a series of polypropylene membranes (1 cm length) attached to the pins of an extraction blade combined with a 96-well plate platform. A mixture of hexane:dichloromethane:ethyl acetate (1:1:1 v/v/v) is used to fill the membrane pores and also added to the samples to create the renewable liquid membrane. This affordable, high-throughput configuration permits the possibility of performing multiple extractions and desorptions simultaneously, and the use of LC–ESI-QToF-MS allows accurate and sensitive results. However, the experimental workflow is not completely automated because the injection was performed offline after the evaporation and resuspension steps.

Moreover, a fully automated workflow consisting of a peristaltic pump integrated with a commercial autosampler has been proposed and coupled with ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC–MS/ MS) (9). This system permits sampling, extraction, and analysis of glucocorticoids in water samples. The technique, termed membrane bag assisted liquid-phase microextraction, combines n-octanol and sodium dodecyl sulfate to extract the analytes using cylindrical polypropylene membrane bags. Benefits of this fully automated approach include short extraction time, reduced solvent consumption, and satisfactory extraction efficiency. On the other hand, some limitations of the extraction device at 45 °C or higher have been observed.

Development of Open-Source Robotic Autosamplers

A remarkable alternative to enhance sample throughput consists of developing an open-source multipurpose platform based on a cartesian robot (10). This system can be hyphenated online with liquid chromatography, mass spectrometry, and other analytical techniques. In this work, a cost-effective syringe-based robot capable of performing all steps involved in sample preparation was designed, constructed, and validated. The instrument was successfully employed for the extraction of polycyclic aromatic hydrocarbons using microextraction in packed sorbent (MEPS), headspace dynamic single drop microextraction (SDME), and dynamic HF-LPME coupled to LC–MS. This innovative platform offers new trends in sample preparation techniques coupled to chromatographic instruments, because different geometries and varied possibilities of hyphenation can be examined. In addition, the cost of this prototype is much lower compared to commercial autosamplers. However, this system requires some knowledge involving automation design and Arduino platform. A similar system has also been examined for the determination of sulfonamides and fluoroquinolones in water samples by direct-immersion SDME coupled to LC–MS/MS (11).

Configurations Based on 96-Well Plate Platforms and Magneto-Active Solvents

Recently, the integration of 96-well plate systems with magneto-active solvents such as magnetic ionic liquids (MILs) has been proposed to provide affordable methodologies (12,13). These alternative solvents permit numerous applications because they exhibit unique features of conventional ionic liquids, in addition to responding to magnetic fields that can significantly facilitate phase separation. Moreover, the possibility of performing direct analyses of these solvents using liquid chromatography equipped with diode array and ultraviolet-visible (UV-vis) detectors allow for straightforward procedures.

A semi-automated configuration termed parallel-single drop microextraction (Pa-SDME) has been recently proposed for the determination of environmental pollutants (12). In this configuration, the magnetic solvents are placed on the tips of a series of neodymium magnetic pins and exposed to the sample solution for extraction of analytes. This experimental setup was integrated with a 96-well plate system allowing the analysis of multiple samples. Photos of the device are shown in Figure 1.

FIGURE 1: Pa-SDME platform using magneto-active solvents, reproduced with permission from reference (12).

This experimental workflow exhibited high-throughput, low-cost, and additional drop stability compared to conventional SDME approaches. On the other hand, this system is not fully automated because the injection in LC was performed offline. It is worth mentioning that MIL absorbance should be careful examined to avoid any overlap with chromatographic peaks of the analytes.

Conclusions

The development of cost-effective and automated microextraction strategies capable of being integrated with chromatographic techniques exhibits enormous applicability in the field of analytical chemistry. Despite the achievements briefly discussed in this article, important contributions have also explored 3D-printing technologies to create automated online microfluidic chip-based platforms mostly coupled to liquid chromatography.

In the next years, it is expected that studies devoted to miniaturized lab-made devices, including robot prototypes, will allow for online and fully automated workflows. In addition, more comprehensive studies involving the complete automation of devices based on 96-well plates coupled with magneto-active solvents consist of trends that will be exploited. Although there have been numerous improvements in automated systems, efforts are still required to develop affordable alternatives that can also be adopted for routine analysis in a large number of laboratories. In addition, there is still room for the development of fully automated and low-cost systems able to be integrated with gas chromatography.

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Josias de Oliveira Merib is with the Departament of Pharmacosciences at the Federal University of Health Sciences of Porto Alegre, in Porto Alegre, Brazil. Direct correspondence to: josias@ufc-spa.edu.br