(NA)DES: Delivering on a Green Promise?

The 12 principles of Green Chemistry were introduced by Anastas and Warner in 1998 (1). The aim of these principles is to reduce the chemical-related impact on human health and the environment by designing customized, more efficient, and more sustainable chemical products and processes. However, only the 11th principle, aimed at developing real-time processes, was directly related to analytical chemistry, which led to the definition of 12 Green Analytical Chemistry (GAC) principles in 2013 (2). 

Most of the GAC principles refer directly or indirectly to sample preparation, pointing towards the minimization or, whenever possible, complete elimination of this step of the analytical process. Yet, despite efforts over the past few decades, direct determination of specific sample components (by spectroscopy determination combined with chemometrics, for example) is only feasible for a rather limited number of applications. In practice, most sample-analyte combinations still require sample treatment before instrumental analysis – regardless of the technique selected for final separation and detection.

As a result, a plethora of novel miniaturized techniques have been developed in recent years – some of which have become well-accepted strategies for robotized and automatic treatment of gaseous and liquid matrices. Although equivalent developments for solid samples are still lacking, advances achieved in this research field over the years have been rapidly adopted by the analytical laboratories and commercial companies, thus contributing to the incorporation of GAC principles into analytical practice.

Nevertheless, other issues highlighted by the GAC principles have yet to be adequately addressed; for example, the need to simplify waste management (principle 7), the (already pressing) demand to avoid using hazardous chemicals in analytical chemistry (principle 11), and related to this, the need to minimize analyst exposure (principle 12). In the late 1990s, research in this area benefited from the synthesis of room-temperature ionic liquids (ILs). ILs were introduced as a green alternative to conventional volatile organic solvents (VOSs) from fossil sources because of their negligible volatility, chemical and thermal stability, and low flammability over a relatively wide range of temperatures. Their tunable physico-chemical properties and tailored selectivity led to their rapid acceptance as green solvents in a variety of research areas, including the analytical field. Later on, their inherent toxicity and limited degradability led to their exclusion from the category of green solvents, thus promoting the development of a new generation of solvents, the deep eutectic solvents (DESs). 

DESs were introduced in 2001 as eutectic mixtures prepared by mixing two/three accessible bulk, cheap, and non-toxic chemicals. Similarly to ILs, DESs are chemically and thermally stable, have low vapor pressures and flammability, while also exhibiting high viscosities, which limit their practical implementation in certain application areas. Additional concerns regarding the safety of some of these mixtures prompted a shift to the use of natural components as alternative ingredients for the synthesis of eutectic mixtures. 

The first “natural deep eutectic solvent” (NADES) was synthesised in 2011. NADESs were initially used for the extraction of bioactive compounds from natural sources. However, during the last decade, the practicality of these green and non-toxic solvents has been gradually demonstrated in a variety of research areas, including pharmaceutical- and bio-orientated applications, food and environmental studies, and the development of new materials – again thanks to their tunable properties. The introduction of hydrophobic water-stable NADESs (hNADEs) in 2015 enabled application to non-polar analytes not amenable by previously described DES – and promising results are increasingly published involving these solvents. 

It is evident from these considerations that (NA)DESs are opening up a wide variety of new analytical possibilities that should be carefully considered and evaluated. More importantly, the results also point to (NA)DESs as an invaluable alternative to address the largely overlooked GAC principles concerning the nature and toxicity of the reagents used in the analytical practice and, in particular, for sample preparation. 

Now, it is our responsibility as analytical chemists to increase the greenness of analytical methodologies and processes as much as possible, and to demonstrate our commitment to reducing our potential impact on human health and the environment.

Sample Preparation Study Group and Network belongs to the Division of Analytical Chemistry of the European Chemical Society (DAC-EuChemS) and includes three working groups (WG): 1. Science and Fundamentals, 2. Automation, Innovation and Entrepreneurship, 3. Information Exchange and Networking.

The Sample Preparation Network welcomes new European and non-European regular members. Membership is open to individuals who subscribe to the objectives of the network and who are professionally engaged in or associated with sample preparation.

For more information please visit:  https://www.sampleprep.tuc.gr/en/home