Orthogonal chromatography

L. B. Fay, A. Newton, H. Simian, F. Robert, D. Douce, P. Hancock, M. Green, and I. Blank, Potential of gas chromatography-orthogonal acceleration time-of-flight mass spectrometry (GC-oaTOFMS) in flavor research, J. Agric. Food Chem., 2003, 51, 2708-2713. 

Gas chromatography-orthogonal acceleration time-of-flight mass spectrometry (GC-oaTOFMS) facilitated the identification of 1,3,5-dithiazine 33 from extracts of marine natural products <2003JAF2708>. Similarly, GC coupled to MS allowed identification of dihydro-2,4,6-triethyl-(42/)-l,3,5-dithiazine 34 as one main component of seed oil in Azaridachta indica <2005JWS185>. By the same token, GC-MS techniques made possible the identification of each isomer of 1,3,5-dithiazine 35 (thialdine) <1997FST411>. 

Combination of different separation techniques into a single experiment (multi-dimensional chromatography also called 2D chromatography, orthogonal chromatography, and cross-fractionation) has shown to overcome such types of limitations (see Section 9.5). 

Schematic diagram of an orthogonal Q/TOF instrument. In this example, an ion beam is produced by electrospray ionization. The solution can be an effluent from a liquid chromatography column or simply a solution of an analyte. The sampling cone and the skimmer help to separate analyte ions from solvent, The RF hexapoles cannot separate ions according to m/z values and are instead used to help confine the ions into a narrow beam. The quadrupole can be made to operate in two modes. In one (wide band-pass mode), all of the ion beam passes through. In the other (narrow band-pass mode), only ions selected according to m/z value are allowed through. In narrow band-pass mode, the gas pressure in the middle hexapole is increased so that ions selected in the quadrupole are caused to fragment following collisions with gas molecules. In both modes, the TOF analyzer is used to produce the final mass spectrum.

Py/GC/MS. pyrolysis, gas chromatography, and mass spectrometry used as a combined technique Py/MS. pyrolysis and mass spectrometry used as a combined technique oa-TOF. orthogonally accelerated time of flight Q. quadrupole field or instrument... 

Multidimensional chromatography brings together separations often based on different selectivity mechanisms. Although the forms of the mobile phase are not required to be different in the individual steps of a multidimensional separation, we usually strive to achieve orthogonal selectivity of these individual separation steps (1). 

This chapter will first cover the nature of electrophoretic separations, especially those concerning capillary electrophoresis. Comprehensive multidimensional separations will then be defined, specifically in terms of orthogonality and resolution. The history of planar and non-comprehensive electrodriven separations will then be discussed. True comprehensive multidimensional separations involving chromatography and capillary electrophoresis will be described next. Finally, the future directions of these multidimensional techniques will be outlined. 

Other groups have also used EC and CE to perform non-comprehensive multidimensional separations (15, 16). A three-dimensional separation was performed by Stromqvist in 1994, where size exclusion chromatography (SEC), reverse-phase HPLC, and CZE were used in an off-line manner to separate peptides (17). The most useful information gained from all of these non-comprehensive studies was knowledge of the orthogonality and compatibility of EC and CE. 

In particular, for copolymers this required an orthogonal coupling of one GPC to another to achieve the desired cro fractionation before application of dual detectors. This method is really a new polymer analysis member of a family of approaches developed in the literature which we are now terming "Orthogonal Chromatogr hy . It not only provides both a cro fractionation approach for copolymers and a new way of determining the GPC s "imperfect resolution" it also enables separation mechanisms previously reserved for the liquid chromatography of small molecules to be used for polymer analysis. 

Figure 15. Cross fractionation by orthogonal chromatography arrangement of...

An important by-product of the development of this approach is that Orthogonal Chromatography provides a direct method of estimating the shape of the chromatogram for extremely narrow molecular weight distributions. This shape function is fundamental information for axial dispersion evaluation and is not otherwise easily obtained. Even commercially available monodisperse standards synthesized by anionic polymerization are too polydisperse. 

D. Recommendations. Cross-Fractionation using Orthogonal Chromatography has high potential in the analysis of complex polymers and even polymer latices (with Hydrodynamic Chromatography). Multi-detector analysis, particularly utilizing spectrofluorometry, should be very useful in developing the technique. 

Two unconventional modes of GPC operation utilizing "Orthogonal Chromatography were utilized to circumvent th problems ... 

The objective of combined analytical separations is to obtain nonredundant information from independent systems. The success of all multidimensional methods in chromatography is dependent on the creation of complementary separation mechanisms, applied in a sequential manner, to enhance the separation capacity of the system. For techniques to be complementary to each other, the acquired data should be orthogonal. A multidimensional system is commonly defined as a system in which ... 

Gilar, M., Olivova, P., Daly, A.E., Gebler, J.C. (2005). Orthogonality of separation in two-dimensional liquid chromatography. Anal. Chem. 77, 6426-6434. 

Venkatramani, C.J., Xu, J., Phillips, J.B. (1996). Separation orthogonality in temperature-programmed comprehensive two-dimensional gas chromatography. Anal. Chem. 68, 1486. 

Marriott, RJ. (2002). Orthogonal GC-GC in multidimensional chromatography, in Multidimensional Chromatography. Mondello, L., Lewis, A.C., Bartle, K.D. editors. John Wiley Sons, Inc., New York. 

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