Green Solvents for Analytical Chemistry
Résumé
Food and natural products are complex mixtures of primary (sugars, proteins and lipids) and secondary (aromas, pigments, antioxidants) metabolites and other organic and mineral compounds. Before such substances can be analysed, they have to be extracted from the plant matrix in order to be introduced in a liquid form into the analytical detector. Different methods can be used for extraction, e.g. Soxhlet or Kumagawa extraction, maceration, decoction, elution and simultaneous distillation–extraction. Nevertheless, all these extraction methods are generally a form of solvent extraction, using petroleum solvents, such as hexane or mixtures of hydrocarbons. From the point of view of environmental protection and the development of greenchemistry, these flammable and toxic petroleum solvents will have to be replaced with alternative solvents in the future. For example, in the perfume industry, extraction of aromas was considered ‘‘clean’’ when compared with heavy chemical industries, but researchers and professional specialists found that its environmental impact is far greater than first appeared. The overall environmental impact of an industrial or analytical extraction cycle is not easy to estimate; however, it is known that it requires at least 50% of the energy of the whole industrial process. In spite of the high energy consumption and the large amounts of solvents (mainly hexane) used, often the yield is very low. For example, just 1 mL of rose absolute (used in well-known perfumes), which weighs less than 1 g, requires more than 1–2 kg of fresh roses as raw material (which becomes a chemical waste) and also large amounts of solvents (n-hexane, alcohol), energy (fossil)and water as cooling and cleaning agents (wastewater). This chapter gives an overview of available green solvents that could be applied in extraction as a sample preparation method prior to analysis. Examples of replacements of petroleum solvents are given to illustrate successes in using solvent-free systems, water, super- or subcritical fluids, ionic liquids, deep eutectic solvents and bio-based solvents. A survey is also presented of existing tools used as decision-making aids for the selection of one or more solvents according to the target molecules, especially HSPiP software programs (based on Hansen solubility parameters) and COSMO-RS (Conductor-like Screening Model)