Cleanup multidimensional techniques

There are two general types of multidimensional chromatography separation schemes those in which the effluent from one column flows directly on to a second column at some time during the experiment, and those in which some type of trap exists between the two columns to decouple them (off-line mode). The purpose of a trap is often to allow collection of a fixed eluate volume to reconcentrate the analyte zone prior to the second separation step, or to allow a changeover from one solvent system to another. The use of offline multidimensional techniques (conventional sample cleanup) with incompatible mobile phases, is common in the literature, and replacing these procedures with automated on-line multidimensional separations will require continuous development efforts. 

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. 

Applications If an extract needs further cleanup, it is possible to couple it with multidimensional chromatographic techniques such as LC-LC or LC-GC. The first chromatographic step can then be used for the on-line cleanup and concentration of the extract, and the second one for the final separation. Large-volume, on-column injection (LVI-COC) is particularly useful for coupled LC-GC in which 100-350 xL fractions of eluent from the NPLC cleanup separation step are transferred on-line to the GC column. For example, on-line removal of high-MW interfering material, such as polymers from a polymer/additive dissolution, can be achieved easily by using SEC before the fraction containing additives is transferred to the GC. 

Applications Multidimensional SEC techniques can profitably be applied to soluble polymer/additive systems, e.g. PPO, PS, PC - thus excluding polyolefins. A fully automated on-line sample cleanup system based on SEC-HRGC for the analysis of additives in polymers has been described, as illustrated for PS/(200-400ppm Tin-uvin 120/327/770, Irgafos 168, Cyasorb UV531) [982], In this process, the high-MW fractions are separated from the low molecular masses. SEC is often used as a sample cleanup for on-line analysis of additives in food extracts these analyses are usually carried out as on-line LVI-SEC-GC-FPD. 

Multidimensional and multihyphenated techniques may become increasingly useful, particularly for the analysis of drugs in biological fluids where LC-GC interfacing has a great deal of promise with respect to sample cleanup and preparation timeJ GC-MS applications continue to grow in number from the qualitative structural identification point of view, for quantitative analysis that uses and for other... 

Multidimensional separation approaches can be incorporated on-line via CS or off-line via fraction collection. Developed in the early 1980s, this technique has been mainly used to isolate multiple components that are contained in a complex mixture [29,30], particularly for natural product, petrochemical, and environmental analyses. To date, the most important applications involve trace enrichment and sample cleanup with environmental and pharmaceuhcal... 

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