Analytical chemistry is the science of obtaining, processing, and communicating information about the composition and structure of matter. In other words, it determines what the matter is and how much of it exists. To address environmental concerns, the results of chemical analyses are usually the main argument for setting restrictions and establishing laws.

How It Relates to Green Chemistry

Analytical chemistry is an efficient tool to assess whether green chemistry is practiced. At the same time, chemical analysis needs solvent, reagents and energy, therefore the principles of green chemistry can be applied. The directly related principles are

• Prevention
• Safer Solvents and Auxiliaries
• Real-time Analysis of Pollution Prevention
• Inherently Safer Chemistry for Accident Prevention

Green Analytical Chemistry

The general steps of chemical analysis include Sample Preparation, Separation, Detection, and Identification. As the chemicals involved in the last two steps, usually require a just small amount of sample and chemicals, green analytical chemistry can be focused on the first two steps.

Sample Preparation

The development of instrumental methods in sample preparation decreases the sample amount needed, therefore reducing the amount of energy used and waste generated. Another green approach is to use photochemistry for sample preparation. Generally, green analytical chemistry can be promoted by using less toxic compounds and selecting processes that are more cost-effective and generate minimal wastes.

The potential environmentally friendly sample preparation techniques include:

Solvent micro-extraction

• In-vial liquid–liquid extraction (in-vial LLE)
• Single-drop micro-extraction (SDME)
• Liquid-phase micro-extraction (LPME)
• Liquid–liquid–liquid micro-extraction (LLLME)

Sorption micro-extraction and liquid desorption

• Solid-phase extraction (SPE)
• In-tube solid-phase micro-extraction (in-tube SPME)
• Fiber-in-tube solid-phase extraction (fiber-in-tube SPE)
• Single short column (SSC)
• Solid-phase micro-extraction (SPME)

Thermal desorption

• Solid-phase micro-extraction (SPME)
• Stir-bar-sorptive extraction (SBSE)

Matrix solid-phase dispersion

• Matrix solid-phase dispersion (MSPD)

Enhanced fluid/solvent extraction

• Supercritical-fluid extraction (SFE)
• Pressurized-liquid extraction (PLE)
• Subcritical-water extraction (SWE)
• Microwave-assisted extraction (MAE)
• Sonication-assisted solvent extraction (SASE)

Thermal desorption from solids

• Direct thermal desorption (DTD)

Among these, SPE is the primary choice for liquid samples. An online (and automated) solid-phase extraction–liquid chromatography (SPE-HPLC) is a fully mature approach and can easily be miniaturized. 

Separation

Separation methods should be developed to use less sample, consume less solvent, achieve high selectivity, reduce analysis time and be performed by mechanically simpler instrument.

Electrical-driven separation methods are qualified for green analytical chemistry, which consume less solvent and sample compared with many chromatographic methods. As such, more laboratories are starting to consider the capillary electrophoresis (CE) as a standard procedure for the separation of complex samples.

Solvents, the main source of waste, are commonly used in sample preparation and separation methods. Therefore, the use of alternative solvents is crucial in green analytical chemical. Supercritical fluids (like water, CO₂) used for extraction can not only save analysis time and reduce organic solvent consumption, but also expand the overall spectrum of solubility, polarity, and volatility properties of solvents and mobile phases. Ionic liquids’ physical properties are promising for green chemistry, including low volatility and appropriate melting point. Moreover, these alternative solvents have the ability of chemical tunability. More discussion about the green solvents is covered in “Green Solvent Selection Guide”.

The development of analytical methods is rapid, and it should be a natural trend to take into consideration the principles of green chemistry to minimize safety, health, and environmental impact.

Reference:

1. American Chemical Society
2. Koel, M. and Kaljurand, M., 2006. Application of the principles of green chemistry in analytical chemistry. Pure and applied chemistry, 78(11), pp.1993-2002.