Comprehensive Separations

Chromatographic and electrophoretic separations are truly orthogonal, which makes them excellent techniques to couple in a multidimensional system. Capillary electrophoresis separates analytes based on differences in the electrophoretic mobilities of analytes, while chromatographic separations discriminate based on differences in partition function, adsorption, or other properties unrelated to charge (with some clear exceptions). Typically in multidimensional techniques, the more orthogonal two methods are, then the more difficult it is to interface them. Microscale liquid chromatography (p.LC) has been comparatively easy to couple to capillary electrophoresis due to the fact that both techniques involve narrow-bore columns and liquid-phase eluents. 

---

These authors clarify these criteria but the essential operation is that the comprehensive separation takes a one-dimensional data representation and through the use of a second separation mechanism converts this to a two-dimensional presentation of the data, as seen in most of the chapters of this book. 

In general, a comprehensive separation strategy implies the desire to resolve/analyze all components within a sample. In the specific context of a multidimensional chromatographic method, the term is more narrowly applied to indicate that all analytes introduced to the first-dimension separation are also subjected to a second-dimension separation. There are two basic configurations used by our laboratory to carry out comprehensive multidimensional (IEX/RP) protein separations—IEX— Dual Column RP system and IEX—Dual Trap RP system (Figs. 13.1 and 13.2), respectively. 

Most of the methods described for spirolides have been developed for rapid monitoring of these toxins in phytoplankton or shellfish matrices. Due to mass selectivity, a baseline separation of the different spirolides is not achieved in most cases. For research and toxin profiling purposes, a comprehensive separation method for a vast suite of spirolides may be of interest. Recently, several isobaric spirolides, which coelute and thus remain unidentified under standard chromatographic conditions, were detected (Krock et al., unpublished data). Only the attempt at a baseline-separation of all compounds revealed these variants. This was achieved by the following liquid chromatographic conditions for fractionation of spirolides ... 

It is not possible to devise a comprehensive separation scheme for anions however, it is possible to detect them individually in most cases, after perhaps a 1-2 stage separation. It is advantageous to remove all heavy metals from the sample by extracting the anions through boiling with sodium carbonate solution heavy metal ions are precipitated out in the form of carbonates, while the anions remain in solution accompanied by sodium ions. The selected tests are listed in order of increasing complexity it is advisable to do them in the order they are described. 

In GC, retention times (or retention indices) correlate to chemical information and are used to provide supporting evidence for peak identification. GCxGC can comprehensively separate a sample using two different stationary phases, yielding a retention time in each separation dimension for each peak, and with retention information from both separation mechanisms, more confident identification and more chemical information can be obtained than from ID GC. 

Even after comprehensive separation and isolation of a PAH fraction from an environmental sample, although it consists mainly of parent PAH (and isomers) and alkyl substituents, it is still likely to be complex and may contain hundreds of compounds spread over wide volatility and concentration ranges. Gas and liquid chromatographic techniques are complementary in the characterization of PAHs in environmental samples. For high molecular mass and nonvolatile PAHs, LC is superior to gas chromatography (GC). High resolution capability of... 

Shibusawa, Y. Yamakawa, Y. Noji, R. Yanagida, A. Shindo, H. Ito, Y. Three-phase solvent systems for comprehensive separation of a wide variety of compounds by high-speed counter-current chromatography. J. Chromatogr. A, 2006,1133, 119-125. 

LC is the most comprehensive separation technique allowing for the determination of non-polar, polar and ionic analytes... 

Comprehensive separation of complex mixtures is a difficult challenge due to the presence of several components that vary from polar to nonpolar and Irom very low to high concentrations and that show diverse physicochemical properties (acid-base... 

---