Green Chromatographic Techniques

This chapter addresses some practical and theoretical aspects of counter current chromatography (CCC), highlighting the specific advantages of this support-free liquid stationary phase purification technique. The focus is on the latest instrumental developments which demonstrates that the poorly known CCC technique may exhibit a high, unrealised potential for greener purification processes.

The review considers a new preparative method of separating concentrated solutions of mineral electrolytes into individual components by size exclusion chromatography on neutral nanoporous hypercrosslinked polystyrene sorbents NanoNets (Purolite International Ltd., UK and USA). Basic principles of the method as well as factors determining the selectivity of separations are discussed. Unprecedented effect of a spontaneous increase in the concentration of separated components is explained on the basis of the concept of ideal separation process. The unprompted partial resolution of inorganic salts into parent acids and bases is a logical consequence of the size exclusion mechanism of ion separation; at the same time, this resolution proves the correctness of our understanding of the separation mechanism. The review discusses briefly previous works in this field and true reasons for well-known “acid retardation” process, the process of separating mineral acids from their salts on anion exchange resins under conditions excluding ion exchange.

‘Supercritical fluid’ describes a gas or liquid at conditions above its critical point. A greater range of solvent properties can be achieved with Supercritical fluid as a single solvent by careful manipulation of temperature and pressure at the supercritical state. Supercritical fluids are attractive media for several chemical reactions having better control over the reaction rates in different areas of biochemistry, polymer chemistry and environmental science. Supercritical fluid extraction (SFE), a rapid, convenient, efficient, and selective method has been used successfully for the separation of analytes prior to supercritical fluid chromatography (SFC), which is a relatively recent chromatographic technique and is commercially available since 1982. SFC significantly reduces the usage of organic solvents and wastes by using supercritical CO₂ as the mobile phase. The important principles of green chemistry that are applicable to green chromatography includes prevention of waste, use of safer solvents and increasing energy efficiency. All these factors are taken care of in SFC which combines some of the best features of HPLC as well as GC. Analytes that cannot be vaporized for analysis by gas chromatography or have no functional groups for detection by the usual liquid chromatography techniques, can be separated and detected using SFC. By now SFC has been applied to wide variety of materials including natural products, pharmaceuticals, foods, pesticides, herbicides, surfactants, polymers and polymer additives, heavy metals, fossils fuels, petroleum, explosives and propellants. SFC has now become an attractive alternative for chiral drug separation.

The current chapter emphasizes the use of high-performance thin-layer chromatography (HPTLC) as a green analytical separation alternative. The chapter provides historical development of thin-layer chromatography towards becoming modern, automated, high resolution technique in the form of high-performance thin-layer chromatography, and their further advances in miniaturization of chromatographic beds in the form of ultra-performance thin-layer chromatography (UPTLC). Also, following the three R philosophy (reduce, replace, reuse), and twelve principles of green chemistry, the chapter provides the short review of green sample pretreatment techniques coupled with the HPTLC. In addition, the use of green separation modalities such as reversed-phase chromatography, hydrophilic interaction chromatography and salting out thin-layer chromatography is emphasized. The use of thin-layer chromatography as a simple screening technique and a powerful sample pretreatment method is highlighted.

Effective green chemistry, green reversed-phase high-performance liquid chromatography (RP-HPLC), and green thin layer chromatography (TLC) methods used for preparing and purifying allicin, a garlic-derived organosulfur compound, are described here. A greener version of the acidic oxidation reaction of diallyl disulfide (DADS) is used to produce allicin with high yield. Green RP-HPLC eliminates the liquid/liquid extraction step from either the DADS acidic oxidation reaction mixture or from garlic extract, allowing the single-step purification of allicin. The proposed method involved the use of eco-friendly ethanol as the alternative eluent for acetonitrile. The pure allicin HPLC fraction prepared this way is quite stable and can be used directly for chemical and biological applications. In addition, by changing silica gel TLC plate to RP-C₁₈ TLC plate, 50 % aqueous ethanol, instead of a solvent blend of hexane : isopropanol (92:8) can be used to identify the allicin on TLC plate. Here, the traditional usage of toxic organic solvents has been avoided and a more efficient chemical reaction scheme is employed, which permits the classification of the present method as green. These green methodologies are used successfully to prepare pure allicin, investigate the thermal, pH, and vacuum drying decomposition of the allicin, and analyze various preparations of garlic extract.

This chapter will describe some of the main separation techniques, considered environmentally correct, with special emphasis on sample preparation. To illustrate aspects of green analytical chemistry, techniques for sample preparation and hyphenation capable of miniaturization will be presented, which reduce, replace or eliminate the use of hazardous organic solvents focusing on organic analytes in complex matrices, mainly based on supercritical fluid extraction, membranes, solid phase extraction, matrix solid phase dispersion as well as sorptive extraction techniques, including some examples of their uses.

Since the main pathway for animal food contamination by OCPs is the ingestion of polluted feed, the determination of the contamination degree of the fodders is extremely important. In case of complex fodders in whose composition there are various materials, the potential contamination sources are multiple. The determination of organochlorine pesticides was made through the gas chromatography method using capillary columns and detector with electrons capture. For the separation step, a method specific to non-fat aliments was performed. This method requires the use of three subsequent elutions with 200 ml ethylic ether/light petroleum, in variable proportions. For the purpose of optimizing the method, there were determinations performed on the same samples through extraction in the normal variant and in reduced ones. This way, we could verify the level up to which the solvent consumption can be reduced without affecting the accuracy of the determination. The practical determinations proved that only the 1:2 reduced method has fully qualitative application and specific quantitative applications.

Size exclusion chromatography (SEC) is one of the most popular methods for separation and molecular characterization of natural or synthetic macromolecules mixtures. This chapter describes the retention mechanisms for SEC as ideal size-exclusion, which separates molecules primarily on the basis of their hydrodynamic volume, and non-ideal size-exclusion, which depends on entropic and enthalpic retention mechanisms and applies when adsorption occurs. Furthermore, recent applications of SEC in the biomedical and pharmaceutical sciences are shown. Finally, the use of SEC as a technique for speciation analysis of polydimethylsiloxanes (PDMS) is presented. PDMS belong to polysiloxanes, with the common name silicones, and are used in a wide variety of applications, such as in the medical (e.g. implants), pharmaceutical (e.g. the active pharmaceutical substances dimeticone and simeticone) and food (e.g. the food additive E 900) areas. Speciation analysis of this polymer is very important, because the degree of polymerization and the particle size, (or molecular weight) have experimental impacts on the toxicity, absorption and migration in living organisms.