Including Simplicity and Sustainability Concepts in Sample Preparation

Sample preparation is a common step in any analytical process. Therefore, analytical scientists are familiar with sample preparation and are aware of its impact on the quality of the results. Although much effort is being devoted to designing analytical processes that allow raw samples to be analyzed, many methodologies require the incorporation of chromatographic separation steps. In these cases, sample preparation is unavoidable, if only for reasons arising from the compatibility of the sample matrix with the instrumental requirements. The consideration of basic analytical properties (sensitivity, selectivity, and precision) must be accompanied by other parameters, such as method transferability and minimal environmental impact, to make sample preparation both simple and sustainable.

The past century has witnessed an evolution of measurement processes incorporating automation and miniaturization. Simplification was incorporated more recently and has had a great impact on the measurement processes developed in the first decades of the 21st century. However, many of these approaches have not considered the complex synthesis processes of the materials used for isolating analytes. Indeed, the inclusion of highly efficient materials in the preconcentration process means that the overall process is simpler, but this does not necessarily mean that the steps needed to prepare these sensitive and selective tools are simple.

Along with these trends in measurement processes, a global concern about the impact human beings have on the environment and the progressive deterioration and contamination of environmental compartments has emerged. Chemistry has not been unaffected by this scrutiny, being in many cases blamed for environmental pollution. The principles of green chemistry (1) and green analytical chemistry (GAC) (2) have become guides for developing new measurement processes to reduce their environmental impact. Using materials from renewable sources is identified in both the green chemistry and GAC principles that highlight how chemistry can improve its environmental friendliness.

The new strategies for sample preparation must therefore be oriented to the design of analytical processes that preferably consider the simplification of the workflows and the inclusion of novel extract- ant materials obtained after a sustainable synthetic route.

Making Sample Preparation Simple

One of the negative connotations of sample preparation is that it is the main source of errors in any analytical process. Precision is the analytical property that is most affected by sample preparation because of all the errors that can be made by the analyst when preparing the sample. As a result, a lack of precision often contributes to greater dispersion in the results. Because eliminating this stage is practically impossible, and even less so when it is followed by a chromatographic separation process, it is best to keep sample preparation as simple as possible.

By focusing the discussion on the isolation of analytes from the sample matrix, the miniaturization of this step has rendered the design of microextraction units that allows for the development of novel extraction formats, such as pipette-tips and in-tube (or stirred) units. The design of microextraction units also results in better compatibility (integration) with instrumental techniques (chromatography coupled to different detectors). Direct coupling of the microextraction units with instrumental techniques is also a simplification of the analytical process. The level of errors associated with these simplified approaches is clearly lower than that associated with conventional procedures because the number of steps required is reduced. However, it is also evident that success in applying these miniaturized approaches relies on the enhanced performance of the sorptive phases.

The combination of conventional polymeric phases with nanomaterials is also a clear trend. The resulting material synergically combines the extraction capacity of the polymer with the specific characteristic of the nanomaterial (hydrophobic interactions, hydrogen bonds, magnetism).

The possibility of obtaining magnetic extractants is advantageous because they permit the elimination of tedious steps, such as filtration or centrifugation, simplifying the whole analytical process. These hybrid composite materials can be prepared following a synthetic route that renders a) core-shell magnetic nanoparticles covered by a layer of polymeric phase; b) polymer microparticles decorated with magnetic nanoparticles; or c) a polymer network with magnetic nanoparticles embedded, which gives rise to composites that combine nano and micrometric structures. This last approach is much simpler than the other two because the composite is easily synthesized by a solvent changeover, playing with the different solubility of the polymeric network in two liquid phases. Following this approach, our research group has prepared magnetic polymer composed by nylon-6 by dissolving it in formic acid. Next, a dispersion on magnetic nanoparticles was added and once homogenized, the dispersion was added with a syringe to a beaker containing water. This step resulted in the gelation of the polymer around the magnetic nanoparticles because of the insolubility of the nylon-6 in the aqueous phase (3). In addition to the simplicity of the synthesis, the sorbent phase can be tailored to the analytical problem that needs to be solved. For example, the nylon-6 was appropriate for the isolation of PAHs from waters whereas a polymeric network containing aromatic moieties can be used to boost the extraction of beta-blockers, propranolol and carvediol, from urine samples (4).

Making Sample Preparation Sustainable

Sample treatment is also responsible for the highest consumption of organic solvents, reagents, and auxiliary energies. Consuming these components in sample treatment is against GAC principles. To complete the current scenario, the 2030 Agenda for Sustainable Development also impacts the guidelines for analytical research, which needs to contemplate one or more of the 17 sustainable development goals (SDGs).

The sustainability of sample preparation can be attained by using raw materials for the preparation of new extractant phases (5). Cork, cotton, paper, or wood can be used as raw materials or with minimal modifications as sorbents. However, sometimes they can be thermal (6) or chemically (7) modified to provide the solid with better selectivity towards the target analytes. The additional advantage of these materials is that they converge with the circular economy, contributing to the GAC.

The use of paper is particularly interesting. Its planar format differentiates it from their counterparts and permits its use in thin film microextraction. Moreover, paper can be easily adapted (cut by means of scissors) to the geometry of the microextraction device. The preparation of a paper-based microextration unit can be as simple as the deposition of a thin layer of sorptive phase by dip coating (8). For this purpose, the polymeric phase is dissolved in a volatile solvent. Next, the piece of paper with the desired geometry is immersed in the solution. The evaporation of the liquid phase generates a polymeric thin film over the paper surface. The thickness of this layer is determined by the concentration of the polymer in the solution and the number of dips optimized in the procedure. In cases of more than one dip, it is recommended to change the dipping direction of the paper between immersions to favor the homogeneous distribution of the solid. Polystyrene recycled from yogurt containers (to make the method greener) can be used to extract a hydrophobic drug (methadone) from saliva samples (9). However, other sorbent phases, like carbon nanohorns (10), polymeric ionic liquids (11) or molecularly imprinted polymers (12), can also be deposited over the paper surface to isolate antidepressants or nonsteroidal anti-inflammatory drugs from urine or quinine from soft drinks, respectively.

The versatility of paper in microextraction is huge in both composition of the sorbent phases and applicability. A recent approach developed by our research group is a magnetic paper, which synergically combines the sorption capacity of a polyamide with the magnetic behavior of magnetic nanoparticles (13). The paper is immersed in a dispersion of magnetic nanoparticles prepared in a nylon-6 formic acid solution. A step forward of this application is that the magnetic paper can be easily integrated into a drill-based sampler that can be used for in situ sampling of environmental water samples. The portability of the device and the simplification of the sample delivery to the laboratory are among its main favorable features. It has been successfully applied to the isolation of four parabens and triclosan from swimming pool water samples. The excellent sensitivity achieved can be explained not only by the affinity of the analytes to the polymeric phase (nylon), but also to the stirring of the sample during the sampling step, which improves the extraction efficiency minimizing the diffusion boundary layer.

Final Remarks

The selected examples described in this article highlight the relevance of simplicity and sustainability in the development of new sample preparation procedures. The benefits they provide include its commitment to green principles and SDG. Analytical chemists are aware of the clear contribution of microextraction techniques to develop environmentally friendly analytical processes. The simplicity of the synthetic routes of sorbent phases, and the use of renewable material, both as extraction media or support for them, are no doubt key trends. The approaches briefly described here are fully compatible with chromatographic separations. Using highly selective detectors, such as tandem mass spectrometers, no doubt increases the analytical properties of the methodologies, which cannot be forgotten if high-quality information is to be delivered. Finally, the simplification inherent to the new microextraction formats clearly contributes to the expansion of the proposals. The easier the workflow, the faster its transference. We should work to facilitate the update of official or standard methods where robustness is one of the best valued features.

References

(1) P.T. Anastas and J.C. Warner, Green Chemistry: Theory and Practice (Oxford University Press, New York, New York, 1998), pp. 30.

(2) A. Gałuszka, Z. Migaszewski, and J. Namiésnik, TrAC Trends Anal. Chem. 50, 78–84 (2013).

(3) E.M. Reyes-Gallardo, R. Lucena, S. Cárdenas, and M. Valcárcel, J. Chromatogr. A 1345, 43–49 (2014).

(4) T. Ballesteros-Esteban, E. M. Reyes-Gallardo, R. Lucena, S. Cárdenas, and M. Valcárcel, Bioanalysis 8, 2115–2123 (2016).

(5) G. Mafra, M.T. García-Valverde, J. Millán-Santiago, E. Carasek, R. Lucena, and S. Cárdenas, Separations 7, 2–22 (2020).

(6) M.T. García-Valverde, C. Ledesma-Escobar, R. Lucena, and S. Cárdenas, S. Molecules 23, 1026 (2018).

(7) M.T. García-Valverde, M.L. Soriano, R. Lucena, and S. Cárdenas, Anal. Chim. Acta 1126, 133–143 (2020).

(8) J. Ríos-Gómez, R. Lucena, and S. Cárdenas, LCGC Europe 33, 60–66 (2020).

(9) J. Ríos-Gómez, R. Lucena, and S. Cárdenas. Microchem. J. 133, 90–95 (2017).

(10) J. Ríos-Gómez, B. Fresco-Cala, M.T. García-Valverde, R. Lucena, and S. Cárdenas, Molecules 23, 1252 (2018).

(11) J. Ríos-Gómez, M.T. García-Valverde, A.I. López-Lorente, C. Toledo-Neira, R. Lucena, and S. Cárdenas. Anal. Chim. Acta 1094, 47–56 (2020).

(12) M.C. Díaz-Liñán, A.I. López Lorente, S. Cárdenas, and R. Lucena, Sensors and Actuators B: Chemical 287, 138–146 (2019).

(13) F.A. Casado-Carmona, R. Lucena, and S. Cárdenas, Talanta 228, 122217 (2021).

Soledad Cárdenas Aranzana is with the Affordable and Sustainable Sample Preparation (AS2P) Research Group in the University Institute for Research in Fine Chemistry and Nanochemistry (IUNAN) at the University of Córdoba, in Córdoba, Spain. Direct correspondence to: scardenas@uco.es