Developments in Green Chromatography

Aug 01, 2014

Green analytical chemistry is a widely recognized concept that has led to the development of new analytical methods with reduced environmental impact and minimized analyst occupational exposure. Achievements include the development of microextraction, ultrasound-assisted extraction (UAE), and microwave-assisted extraction (MAE) techniques. Research towards greener separation processes focuses on the elimination of toxic solvents in liquid chromatography (LC) and the reduction of separation time. The recent developments in the labelling and categorization of analytical procedures are also presented. The National Environmental Methods Index (NEMI), Eco-Scale, and grouping with multivariate statistics are discussed together with their advantages and limitations. This article presents the most recent progress in the development of greener sample preparation and chromatographic separation techniques.

Green analytical chemistry is now a widely recognized term and is defined in the literature as clean, benign, or environmentally friendly analytical chemistry (1). Green analytical chemistry means performing reliable analysis while applying quality assurance/quality control (QA/QC) and minimizing any environmental impacts resulting from the analysis. When one thinks of chromatographic procedures that meet the standards of green analytical chemistry principles, green chromatography comes to mind first (2). Chromatographic procedures are usually a series of operations that involve sample collection, transportation, preparation, proper chromatographic separation, and analysis. Greening chromatography concerns all stages of analysis, but the steps with the greatest environmental impact are sample preparation and the chromatographic separation. These stages are considered to be the most environmentally unfriendly and therefore have a lot of potential for "greening" (3).

Green chromatography originates from the 12 green chemistry principles developed to minimize the environmental impact of chemical synthesis. When compared to chemical synthesis methods, the amount of pollutant waste products generated during chromatographic analysis appears small. However, it has been estimated that one liquid chromatograph can generate one litre of liquid waste daily (2). Three years ago, a series of three articles were published in LCGC Europe addressing the different aspects of green chromatography. The first article provided an introduction to green chromatography, describing its origin and the potential of green liquid chromatography (LC) (4); the second article proved that gas chromatography (GC) is often beneficial compared to LC (5); and the third article reviewed the importance of sample preparation before chromatographic analysis and how green chromatography principles could be applied (6). This article summarizes the achievements and developments over the last three years that have advanced the field of green chromatography.

Sample Preparation

In a chromatographic analysis, sample preparation has the most detrimental effect on the environment and the question of whether or not it is really needed should always be asked. Unfortunately in most cases the answer is "yes" but there are chromatographic procedures that can omit this procedural step, as reviewed in reference 7. Direct procedures can be used to analyze samples with simple matrices such as water, petroleum products, and alcoholic beverages. Such approaches reduce environmental impact as well as saving money.

Figure 1: Solutions in sample preparation that meet green chromatography requirements.
Figure 1 presents the options available to implement green sample preparation including miniaturization (8), solvent elimination (9), and application of extraction assisting or enhancing agents (such as ultrasounds or microwaves) (10). It can be difficult to automate microextraction techniques; however, advances have been made including the development of dispersive liquid–liquid microextraction (DLLME); liquid-phase microextraction (LPME), and single drop microextraction (SDME) (11).

In recent years, a lot of work has focused on new solutions and applications that improve the "greenness" of sample preparation techniques. One technique that has been intensively worked on is DLLME. DLLME utilizes a tiny amount of nonpolar solvent (usually below 100 µL) and approximately 1 mL of less toxic dispersive solvent. Procedures based on DLLME have been proposed to determine UV filters (12), antibiotics (13), polycyclic aromatic hydrocarbons (PAHs) (14), or oestrogens (15) in water samples; and pesticides in honey, after sample dissolution in water (16).

Performing DLLME extraction can be assisted with ultrasounds that eliminate or reduce the need for disperser solvent. The application of dispersive solvent increases the solubility of the extraction solvent. Without the dispersive solvent, extraction solvent volume can be reduced, because less of it is dissolved in water; and despite low initial solvent volume it is easy to recover enough extraction solvent volume to handle.

For example, ultrasound-assisted extraction with 13 µL of chlorobenzene can be applied to determine siloxanes in water samples (17). In LC analysis, ionic liquids are considered as green solvents and can be applied as extraction agents in DLLME.

An alternative solution to DLLME is solidification of floating organic drop (SFO). The extraction solvents used in SFO are higher alcohols such as nonanol, decanol, or undecanol. After extraction, the sample is subjected to a lower temperature and the solvent solidifies and can be easily mechanically separated from the sample matrix. The technique can be used in sample preparation before GC and LC analysis. It is suitable for the extraction of compounds with log kow in the range 3–7 (18). For example: Triazines in water samples can be determined using 10 µL of 1-undecanol and 100 µL of acetonitrile as dispersive solvent (19). A similar procedure was designed for the determination of amphetamines in urine samples (20), and for the determination of pesticides in vegetables and fruits after extraction with acetone (21). In 2013, Dong et al. applied DLLME to the analysis of soil samples (22). The authors extracted phytocides from soil with a water–methanol mixture and evaporated to dryness before the residue was redissolved in methanol. The samples were subjected to ionic liquid-based ultrasound-assisted DLLME before high performance liquid chromatography (HPLC) analysis.

A derivatization step can be easily combined with DLLME. It is particularly practical when the derivatizing agent easily hydrolyzes in the water sample (23). In 2014, Casado et al. performed acetylation with simultaneous extraction of benzotriazoles on a mixture of microvolumes of acetic anhydride, acetonitrile, and toluene (24). In 2013, Farajzadeh et al. applied butylchloroformate as a derivatizing agent in DLLME for the determination of amantadine, a pharmaceutical prescribed for influenza treatment, in human plasma samples (25).

The main trends in solid-phase microextraction (SPME) are automation, in situ sampling, development of in vivo applications, and the development of new coatings (non-bonded, bonded, or cross-linked polymers) (26). Polydimethylsiloxane (PDMS) has traditionally been used as a SPME fibre coating, but fibres based on polyacrylate or polyglycol polymers are now available for the extraction of polar analytes. The support for coatings has traditionally been fused-silica, but recently metal-based fibres have been developed for increased durability and improved mechanical properties. Increasing the coating film thickness to increase fibre capacity and therefore sensitivity is another trend; however, it can increase the extraction time required to obtain equilibration (27).

Ionic liquids have recently been demonstrated as fibre coatings, opening up the possibility to design and modify coatings to obtain greater selectivity towards analytes (28). In 2012, Feng et al. applied a polymerized ionic liquid 1-vinyl-3-octylimidazolium bromide as a coating in SPME to isolate alcohols from water (even methanol) and produced good recoveries (29). In 2012, Ho et al. extracted chlorophenols from water with 1-butyl-3-methylimidazolium hexafluorophosphate-coated SPME fibre (30).

Novel applications of SPME include the determination of contaminants or bioactive compounds directly in tissues of living organisms, or in their habitat, as demonstrated by Bessonneau et al. in the case of corals (31). In vivo SPME sampling has also been developed to determine emerging contaminants in fish (32), and volatile halogenated organic compounds in trees (33), insects, higher animals, and even humans (34).

Stir-bar sorbent extraction (SBSE) is another rapidly developing technique, where the majority of applications use PDMS coatings. Recently, alternative polar coatings have been introduced and intensively investigated for the extraction of analytes of a wide range of polarities including polyacrylate and polyglycol coatings (35). For the extraction of chlorophenols and chloroanizoles from wine samples, ethylene glycol with silicone in the form of a copolymer was found to be more effective than a polyacrylate coating (36). This is because it allows for simultaneous extraction and preconcentration of both polar phenols and nonpolar chloroanisoles. Another coating for the extraction of polar compounds from water samples is methacrylic acid and divinyl benzene copolymer (37). This SBSE coating gives much better recoveries (around 100% for the most of investigated compounds) for polar pharmaceuticals than commercial PDMS coating-based stir bar (less than 20%).

The stir bar has also been applied as a passive sampler for the in situ extraction of nitroorganic pesticides (38). Such an approach seems to be a promising technique for the collection of integrated samples. The in situ exposition and sample preparation steps are both straightforward. Selection of the most appropriate stir bar coating can result in sampling of polar analytes, which are usually not collected with more traditional passive samplers.

Stir bars can also be coated with molecularly imprinted polymers (MIPs) (39) or chiral imprinted polymers (40) for extraction of certain groups of analytes. The time needed to perform analysis is relatively long; however, the limit of detection (LOD) obtained are very low. For example, three hours of SBSE extraction followed by thermal desorption, two-dimensional GC separation and time-of-flight mass spectrometry (TOF-MS) allowed for the determination organochlorine pesticides at pg/L concentrations (41).

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