Multi-dimensional Separations

Martin, M. (1995). On the potential of two- and multi-dimensional separation systems. Fresenius J. Anal. Chem. 352, 625. 

Although is has been in use for over 50 years and has become one of the most widely used routine analysis techniques, GC research remains vibrant and challenging. There are two key areas in which dramatic advancements are being made sampling and sample introduction methods and multi-dimensional separations. A summary of sampling techniques in use with GC is shown in Table 14.9, including the basic principle of the technique and some key applications. These techniques have become critical in extending the use of GC into the diverse fields described in the Applications section. 

Jia et al. (2005) developed a two-dimensional (2-D) separation system of coupling chromatography to electrophoresis for profiling Escherichia coli metabolites. Capillary EC with a monolithic silica-octadecyl silica column (500 x 0.2 mm ID) was used as the first dimension, from which the effluent fractions were further analyzed by CE acting as the second dimension. Multi-dimensional separations have found wide applications in biomedical and pharmaceutical analysis. 

Multi-dimensional separations, in which more than one mechanism of separation is applied to a sample with each mechanism considered as an independent separation dimension, have a long history as described in a recent review (Dixon 2006). It is characteristic of human nature that, while modem developments in chromatography have greatly increased separation power, this achievement has moved the objective to separations of ever more complex mixtures. However, multi-dimensional separation techniques are not only employed in analyses of samples containing overwhelming numbers of analytes, but are also of use when a critical separation of two or more... 

Tarr, P., Ossher, H., Harrison, W., Sutton Jr., S.M. N Degrees of Separation Multi-Dimensional Separation of Concerns. In ICSE 1999 Proceedings of the 21st International Conference on Software Engineering, pp. 107-119. ACM, New York (1999)... 

In the analysis of complex PAH mixtures obtained from environmental samples, reversed-phase LC-FL typically provides reliable results for only 8-12 major PAHs (Wise et al. 1993a). To increase the number of PAHs determined by LC-FL, a multidimensional LC procedure is used to isolate and enrich specific isomeric PAHs that could not be measured easily in the total PAH fraction because of interferences, low concentrations, and/or low fluorescence sensitivity or selectivity. This multi-dimensional procedure, which has been described previously (Wise et al. 1977 May and Wise 1984 Wise et al. 1993a, 1993b), consists of a normal-phase LC separation of the PAHs based on the number of aromatic carbon atoms in the PAH, thereby providing fractions containing only isomeric PAHs and their alkyl-substituted isomers. These isomeric fractions are then analyzed by reversed-phase LC-FL to separate and quantify the various isomers. 

Dixon, S.P., Pitfield, I.D., Perrett, D. (2006). Comprehensive multi-dimensional liquid chromatographic separation in biomedical and pharmaceutical analysis a review. Biomed. Chromatogr. 20, 508-529. 

Recently, multi-dimensional GC has been used for highly complex separations, especially in analysis of fuels, environmental samples and flavors. Most recently, comprehensive two-dimensional GC, in which samples are continuously taken form the effluent of the (long) first... 

Interferences caused by inadequate chromatographic separation during the final determination (e.g., no prior group separation of PCBs or OCPs). This can be improved by multi-dimensional GC or multi-dimensional preparative LC. 

Increasing the number of peaks resolved within a fixed time of separation (increasing the peak capacity in single- or multi-dimensional LC)... 

Multi-dimensional Chromatography. Multi-dimensional chromatography is the term used to describe a variety of methods where fractions from one chromatographic system are each transferred to another for further separation. Combinations of SEC with thin-layer chrcmatography have been shown to enable separation of copolymers by composition in a "cross-fractionation". OC utilizes a combination of two SECs in a cross-fractionation approach. 

Figure 6. Schematic diagram of instrumentation used to perform the multi-dimensional 1C. (a) 10-Port valve positions for detection of 2nd dimension separation (i.e. effluent from column 2 diverted through conductivity detector 2). (b) 10-Port valve positions for injection of 2 dimension cut fractions onto 3 dimension column with subsequent detection using conductivity detector 2 [12],...

More importantly insoluble proteins, such as membrane-bound and nuclear proteins, are under-represented within 2DE analysis. However, membrane proteins can be identified by MS following a IDE separation. Unfortunately, hydrophobicity is the most common reason for poor representation of abundant proteins and seems set to be with us for quite some time (Santoni et al., 2000 Rabilloud, 2002). Emerging proteomic methods without the use of 2DE are being developed, such as isotope-coded affinity tagging (ICAT) and multi-dimensional protein identification technology (MuDPIT). 

In experimental impedance spectroscopic studies, however, several factors may complicate the interpretation of the spectra and a few of these complications will briefly be touched upon i) If high conductivities are considered (a > 10-3 S cm-1), then the corresponding relaxation frequencies are well above the measurement range of a conventional impedance set-up (frequencies up to ca. 10 MHz). Hence, processes with high conductivites cannot be separated by conventional impedance spectroscopy. ii) The assumption of a quasi-one-dimensional current flow, which is the basis of the above presented brick layer model, is often violated [203, 211-214]. Some complications due to multi-dimensional potential distributions will be discussed in Sec. 3.2.1. iii) Highly conductive regions perpendicular to the electrodes (e.g. highly... 

B. Gammelgaard, A. D. Madsen, J. Bjerrum, L. Bendhal, O. Jons, J. Olsen, U. Sidenius, Separation, puriPcation and identiPcation of the major selenium metabolite from human urine by multi-dimensional HPLC-ICP-MS and APCI-MS, J. Anal. Atom. Spectrom., 18 (2003), 65D70. 

The first volume concentrates on separation techniques. H. Pasch summarizes the recent successes of multi-dimensional chromatography in the characterization of copolymers. Both, chain length distribution and the compositional heterogeneity of copolymers are accessible. Capillary electrophoresis is widely and successfully utilized for the characterization of biopolymers, particular of DNA. It is only recently that the technique has been applied to the characterization of water soluble synthetic macromolecules. This contribution of Grosche and Engelhardt focuses on the analysis of polyelectrolytes by capillary electophore-sis. The last contribution of the first volume by Coelfen and Antonietti summarizes the achievements and pitfalls of field flow fractionation techniques. The major drawbacks in the instrumentation have been overcome in recentyears and the triple F techniques are currently advancing to a powerful competitor to size exclusion chromatography. 

Usually bi- or multi-dimension separations must be run individually since they utilize different physico-chemical separation principles and differ greatly in selectivity. Ion-pairing and electrophoretic mobility provide two compatible separation models that may be run simultaneously in the same single separation device to achieve a bi-dimensional technique. Since they are respectively based on the complex hydrophobicity and on the analyte change, experiments demonstrated that ion-pairing... 

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