Multidimensional chromatography interfaces

If simple sample pretreatment procedures are insufficient to simplify the complex matrix often observed in process mixtures, multidimensional chromatography may be required. Manual fraction collection from one separation mode and re-injection into a second mode are impractical, so automatic collection and reinjection techniques are preferred. For example, a programmed temperature vaporizer has been used to transfer fractions of sterols such as cholesterol and stigmasterol from a reversed phase HPLC system to a gas chromatographic system.11 Interfacing gel permeation HPLC and supercritical fluid chromatography is useful for nonvolatile or thermally unstable analytes and was demonstrated to be extremely useful for separation of compounds such as pentaerythritol tetrastearate and a C36 hydrocarbon standard.12... 

Can be readily interfaced to LC outlet, permitting multidimensional chromatography (MuDPIT analysis of whole proteomes). 

Figure 1 Schematic diagram of a multidimensional chromatography system. The two systems correspond to the methods employed, e.g., the GC or LC system. The separation channel may be an elution column or separation plane. Between the two systems is the process for transferring solute from the first to second dimensions. It may be a heart-cut process (GC-GC), a fraction collection step (LC-GC), a modulation process (GC X GC), or a plate positional change for planar TLC. Some systems will allow or require a detection step between the two systems, such as a monitor detector in MDGC. In the figure, step 1 refers to the sample application/injeclion step 2 to the interface or intermediate sample-processing device and step 3 to the elution of the separated sample into a detection system.

Multidimensional chromatography is a very attractive technique for the analysis of complex mixtures where a mono-dimensional separation cannot be sufficient to resolve all the components of interest. Obvious advantages are the higher peak capacity and resolution offered by these systems. Typically, one part of the chromatogram from the first column is transferred to another column via a suitable interface. 

Multidimensional or hyphenated instmments employ two or more analytical instmmental techniques, either sequentially, or in parallel. Hence, one can have multidimensional separations, eg, hplc/gc, identifications, ms/ms, or separations/identifications, such as gc/ms (see CHROMATOGRAPHY Mass spectrometry). The purpose of interfacing two or more analytical instmments is to increase the analytical information while reducing data acquisition time. For example, in tandem-mass spectrometry (ms/ms) (17,18), the first mass spectrometer appHes soft ionization to separate the mixture of choice into molecular ions the second mass spectrometer obtains the mass spectmm of each ion. 

The possibiHties for multidimensional iastmmental techniques are endless, and many other candidate components for iaclusion as hyphenated methods are expected to surface as the technology of interfacing is resolved. In addition, ternary systems, such as gas chromatography-mass spectrometry-iafrared spectrometry (gc/ms/ir), are also commercially available. 

Figure 12.19 Schematic diagram of the interface system used for supercritical fluid cliromatography-gas chromatography. Reprinted from Journal of High Resolution Chromatography, 10, J. M. Levy et al., On-line multidimensional supercritical fluid clrromatogi a-phy/capillary gas chromatography , pp. 337-341, 1987, with permission from Wiley-VCH.

Figure 12.23 SFC-SFC analysis, involving a rotaiy valve interface, of a standard coal tar sample (SRM 1597). Two fractions were collected from the first SFC separation (a) and then analyzed simultaneously in the second SFC system (h) cuts a and h are taken between 20.2 and 21.2 min, and 38.7 and 40.2 min, respectively. Peak identification is as follows 1, tii-phenylene 2, chrysene 3, henzo[g/ i]perylene 4, antliracene. Reprinted from Analytical Chemistry, 62, Z. Juvancz et al, Multidimensional packed capillary coupled to open tubular column supercritical fluid chromatography using a valve-switcliing interface , pp. 1384-1388, copyright 1990, with permission from the American Chemical Society.

Figure 12.24 Schematic diagram of the multidimensional packed capillary to open tubular column SFC-SFC system. Reprinted from Analytical Chemistry, 62, Z. Juvancz et al., Multidimensional packed capillary coupled to open tubular column supercritical fluid chromatography using a valve-switching interface , pp. 1384-1388, copyright 1990, with permission from the American Chemical Society.

Z. Juvancz, K. M. Payne, K. E. Markides and M. E. Eee, Multidimensional packed capillary coupled to open tubular column superaitical fluid chromatography using a valveswitching interface , Awa/. Chem. 62 1384-1388(1990). 

Multidimensional liquid chromatography encompasses a variety of techniques used for seunple separation, cleanup and trace enrichment [12,279-289]. A characteristic feature of these methods is the use of two or more columns for the separation with either manual or automatic switching by a valve interface of fractions between columns. These techniques require only minor modification to existing equipment, and of equal importance, enable the sample preparation and separation procedures to be completely automated. 

Willis, J. N. and Wheeler, L., Use of a gel permeation chromatography-Fourier transform infrared spectroscopy interface for polymer analysis, in Chromatographic Characterization of Polymers, Hyphenated and Multidimensional Techniques, Provder, T., Barth, H. G., and Urban, M. W., Eds., American Chemical Society, Washington, D.C., 1995, chap. 19. 

Principles and Characteristics Multidimensional gas chromatography (MDGC) is widely used, due to the mobile-phase compatibility between the primary and secondary separating systems, which allows relatively simple coupling with less-complicated interfaces. In its simplest form, 2DGC can be carried out in the off-line mode. The most elementary procedure involves manual collection of effluent from a column, followed by reinjection into another column of a different selectivity (e.g. from an apolar to a polar column). Selecting proper GC-column combinations is critical. In on-line mode, the interface in MDGC must provide for the quantitative transfer of the effluent from one column... 

There are a few commercially available systems that are loosely or tightly integrated for 2DLC. There are commercial systems for multidimensional gas chromatography as the interface between columns can be accomplished without valves using thermal modulation (Marriott, 2002) and the thermal modulator is tightly coupled to the software. 

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. 

Authenticity evaluation has recently received increased attention in a number of industries. The complex mixtures involved often require very high resolution analyses and, in the case of determining the authenticity of natural products, very accurate determination of enantiomeric purity. Juchelka et al. have described a method for the authenticity determination of natural products which uses a combination of enantioselective multidimensional gas chromatography with isotope ratio mass spectrometry (28). In isotope ratio mass spectrometry, combustion analysis is combined with mass spectrometry, and the 13C/12C ratio of the analyte is measured versus a C02 reference standard. A special interface, employing the necessary oxidation and reduction reaction chambers and a water separator, was used employed. For standards of 5-nonanone, menthol and (R)-y-decalactone, they were able to determine the correct 12C/13C ratios, with relatively little sample preparation. The technical details of multidimensional GC-isotope ratio MS have been described fully by Nitz et al. (29). A MDGC-IRMS separation of a natural ds-3-hexen-l-ol fraction is... 

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