Green Chemistry Perspectives on Analytical Extractions

May 02, 2014

The growing interest in green chemistry requires fresh perspectives on analytical extractions. Reduced solvent consumption, alternative safer solvents, and reasonable energy demands must be balanced with traditional analytical considerations such as extraction yield and selectively. This article introduces some of the concepts behind green chemistry, and discusses green solvent selection and extraction techniques. An overview of alternatives to conventional solvents, new green solvents, ionic liquids, and other solvent options are also described.

The history of modern analytical extractions mirrors the development of green chemistry. For nearly a century Soxhlet extraction, combined with shake-flask methods, was the standard method for the isolation of analytes from solid samples, while multiple liquid–liquid extractions (LLEs) using separatory funnels was the method of choice for liquid samples. In the mid-1980s, new forms of analytical extractions were developed and popularized — such as supercritical fluid extraction (SFE), pressurized-fluid extraction (PFE), solid-phase extraction (SPE), solid-phase microextraction (SPME), microwave-assisted extraction (MAE), single-drop microextraction (SDME), and ultrasonic extraction. These new extraction techniques had benefits such as faster times, lower cost for each extraction, improved yield and reproducibility, and lower solvent volumes. Lower solvent volumes provide significant environmental advantages. Extraction solvents generally provide the bulk of the waste encountered in any analytical method and often have health and safety concerns, such as toxicity or flammability.

Table 1: Principles of green chemistry. Adapted from reference (1).
In parallel with these developments, the concept of green chemistry emerged. This culminated in the statement of the 12 principles of green chemistry in 1998, which are shown in Table 1 (1). The goals of green chemistry are to address environmental, health, and safety concerns when planning a chemical process rather than after it has been performed. Despite the fact that modern extraction technologies and green chemistry are contemporaries, the two fields have advanced independently of each other. The green advantages of newer extraction methods are widely promoted, but they are rarely placed in the context of the green chemistry principles.

Green chemistry in the chromatography laboratory was recently reviewed and this article described ways to save on solvent consumption, alternatives to using acetonitrile in liquid chromatography (LC), and how to assess the "greenness" of analytical methods (2). These topics will not be repeated in this review.