There are good reasons to reflect on the “greenness” of liquid-phase separation methods.
(Evaporated) organic solvents may impact the environment, the working conditions in the laboratory and the health of analysts. Organic solvents with sufficient purity also tend to be expensive and proper disposal may incur additional costs.
The figure below categorizes a number of common liquid-phase separation methods in terms of the amounts (vertical axis) and nature (horizontal axis) of the organic solvents used. Greenness increases from the top left to the bottom right of the diagram. Low volumes of carrier liquid with low concentrations of toxic solvents are most “green”.
Size-exclusion chromatography (SEC) with organic mobile phases, such as tetrahydrofuran, is very much “not green”. Miniaturization of SEC (indicated by the downward arrow labelled “m” has gained little traction. In part this may be due to the slow diffusion of high-molecular-weight analytes, which threatens to increases the contributions of external band broadening. Dedicated detectors for SEC (see Section 4.6) do not allow a significant reduction of the internal diameter of SEC columns. When possible, reducing the diameter of the packing material (even down to sub-2-µm particles) does lead to an increase in solvent volumes required.
Hydrophilic-interaction liquid chromatography (HILIC) is the least “green” of the separation methods based on a balance of interactions of the analytes with the mobile and stationary phases. The most selective mobile phases contain high concentrations – often > 90% – of acetonitrile (ACN). Miniaturization by reducing the column internal diameter appears the best way forward here.
The latter approach is increasingly successful in reversed-phase liquid chromatography (RPLC). Especially in combination with mass spectrometry, miniaturization is advantageous. Applications of micro-LC with narrow-bore (£ 1.5 mm i.d.) or (£ 530 µm i.d.) capillary columns and flow rates of 1 to 100 µL/min, or nano-LC with column internal diameters (£ 150 µm) and flow rates below 1 µL/min are receiving increased attention.
RPLC may also use less-toxic solvents than ACN, such as small aliphatic alcohols. This is also one of the advantages of subcritical-fluid chromatography (SFC), where methanol (MeOH) is the most common organic modifier. Often relatively low concentrations of MeOH suffice, so that despite relative large columns (typically 3 or 4.6 mm i.d.) and relatively high flow rates (> 1 mL/min) SFC is still considered a “green” technique.
Arguably, capillary electrophoresis (CE, or capillary zone electrophoresis, CZE) is the “greenest” liquid-phase-separation method, with very low (electro-osmotic) flow rates of aqueous buffers. The low total costs of consumables (capillaries and solvents) are a definite advantage of electromigration techniques.
For more information on SEC see Module 4.2
For more information on dedicated detectors for SEC see Module 4.6
For more information on HILIC see Module 3.5
For more information on RPLC see Section 3.6.2
For more information on electromigration techniques, including CZE, see Chapter 5
